Fluorescence and Phosphorescence from Higher Excited States of

Review pubs.acs.org/CR

Fluorescence and Phosphorescence from Higher Excited States of Organic Molecules Takao Itoh Graduate School of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima City, 739-8521 Japan experimental interest. Further, it is of importance to investigate emission of this kind in conjunction with the mechanisms of intramolecular electronic relaxation processes. There have already been several partial reviews on the present subject. In 1978 Turro et al. presented a review on organic photoreactions, where they surveyed emission from upper electronic states.2 A brief review on S2 → S0 fluorescence (S2 fluorescence) is given also in standard textbooks such as the CONTENTS one by Birks first edited in 1973.3,4 In 1993 Maciejewski and 1. Introduction A Steer presented a review on the photochemistry and photo2. Radiative and Nonradiative Processes in Organic physics of thiocarbonyl compounds, which typically exhibit S2 Molecules and Kasha’s Rule B fluorescence.5 Ermolaev also presented a review in 2001 on 3. Identification of Emission from Higher Excited ultrafast nonradiative transitions between higher excited states States C in organic molecules, where he summarized the results of the 4. Fluorescence from Higher Excited States E determination of nonradiative rate constants using a range of 4.1. Azulenes and Analogues E methods for various molecules.6 Although molecules such as 4.2. Aromatic Acenes G azulenes, thiones, and pyrene are known to be typical systems 4.3. Polyenes I that exhibit emission from higher excited states, there are still a 4.4. Thioketones M number of other lesser-known molecules that show various 4.5. Metalloporphyrins O types of such anomalous emission.7 During the past three or 4.6. Other Fluorescent Molecules Q four decades, spectroscopic data on such molecules have been 5. Phosphorescence from Higher Excited States R accumulated extensively. To date, however, there are no 5.1. Aromatic Carbonyl Compounds R systematic review articles that survey all classes of organic 5.2. Quinones T molecules showing the various types of emission from higher 5.3. Halogenated Aromatic Compounds T excited singlet and triplet states. Information on the emission 5.4. Other Phosphorescent Molecules U properties reported so far in numerous papers allows 6. Mechanisms of Appearance of Emission from generalizations of the mechanism of the appearance of such Higher Excited States U emission. Although SciFinder or Chemical Abstracts searches 7. Summary V for a particular molecule or property can be carried out without Appendix A V much difficulty, it is not always applicable for the systematic Appendix B W search for molecules exhibiting such anomalous emission, Author Information W which is often expressed in the title or in the abstract of the Corresponding Author W paper as “S2”, “dual”, “triple”, or “anomalous” emission. Notes W Furthermore, the notations dual, triple, or anomalous emission Biography W do not always correspond to emission from upper excited References X states. For example, the triple fluorescence of aminosalicylates consists of normal S1 fluorescence in addition to proton transfer and twisted intramolecular charge transfer fluorescence.8 In 1. INTRODUCTION such a case, the observed emission does not involve upper Photoemission from organic molecules originates normally excited states, because it originates from different chemical from only the lowest excited state of a given spin multiplicity, species. irrespective of the photon energy used for the electronic It is the goal of this review to survey this specific subject excitation. The generalization of this observation is referred to comprehensively and systematically as a milestone for future as Kasha’s rule.1 Indeed, this rule has been found to be true for works. The chemical species treated in the present article a number of organic molecules, such as aromatic compounds include organic molecules of medium to large size such as that exhibit detectable emission. Although Kasha’s rule applies aromatic carbonyl compounds, quinones, polyenes, and aromatic acenes reported up to 2011, but polymers and to most of the emissive organic molecules, there are exceptions to this rule. Some organic molecules are known to show emission from higher excited states. Anomalous emission or Received: May 11, 2011 emission from higher excited states is of theoretical and © XXXX American Chemical Society

A

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molecules include fluorescence and phosphorescence emission, but infrared emission occurring between vibrational levels in a given electronic state is often treated as a nonradiative process. Fluorescence is defined as a radiative transition between electronic states of the same multiplicity, while phosphorescence is defined as that between states of different multiplicity. The emission quantum yield of all emissive organic molecules is 1 as well as T j → S 0 phosphorescence with j = 1 and 2. Throughout the present review, Si → S0 fluorescence and Tj → S0 phosphorescence are expressed simply by Si fluorescence and Tj phosphorescence, respectively, and S0 → Si absorption is expressed simply by absorption to Si. Further, the energy separation between the Si and Sm states or Tk and Tl states is expressed by ΔE(Si − Sm) or ΔE(Tk − Tl) with m ≥ 0.

2. RADIATIVE AND NONRADIATIVE PROCESSES IN ORGANIC MOLECULES AND KASHA’S RULE Explanation of the molecular radiative and nonradiative processes of polyatomic molecules has been given in a number of standard textbooks such as the one by Birks or by Turro3 or

Figure 1. Scheme showing the radiative and nonradiative processes of organic molecules: IC32, IC21, and IC20 indicate internal conversion from S3 to S2, from S2 to S1, and from S1 to S0, respectively. ISC1t, ISCt0, and ISC00 indicate intersystem crossing from S1 to T1, from the high vibrational levels of T1 to S0, and from the zero vibrational level of T1 to S0, respectively. F2, F1, and P1 indicate the radiative processes from S2, S1, and T1, respectively, and Vib is the vibrational relaxation in the S1 or T1 manifolds.

in reviews.9,10 Therefore, in this section these processes will be described only briefly. A molecule excited electronically by a photon loses its energy through different relaxation processes (Figure 1). These processes can be divided into radiative and nonradiative (or radiationless) processes. Radiative processes of organic B

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condensed phases. The nonradiative transition rate of collisionfree molecules is normally excitation-energy dependent, and thus the excitation spectrum does not always agree exactly with the corresponding absorption spectrum. Even in such a situation, however, the excitation spectrum shows vibrational features that are nearly identical with those observed in the absorption spectrum. Hence, the measurement of the excitation spectra is also a powerful tool for confirming the emitting entity even under collision-free conditions. Another key technique for the identification of the emitting states is to investigate the mirror-image relationship between the emission and the S0 → Sn absorption spectra with n ≥ 2. Because of the Franck−Condon principle, the emission and absorption spectra normally exhibit an approximate mirrorimage relationship. Thus, by confirming the mirror-image relationship, one can make sure that the emission in question originates from the electronic state responsible for the optical absorption. Measurements of the emission lifetime and quantum yield are also of importance for confirmation of the emitting state. Because of the relationship between the intrinsic and observed radiative lifetimes, τint = τobs/Φ, the intrinsic lifetime, τint, calculated from the observed emission lifetime, τobs, and quantum yield, Φ, is close to that deduced from the oscillator strength of the emitting state. If the τint value is different from that deduced from the oscillator strength of the emitting state, the assignment of the emitting state may be incorrect. An example of this is the discovery of the forbidden S1(21Ag) excited state located below the allowed S2(1Bu) state of 1,4diphenyloctatetraene for which the intrinsic lifetime deduced from the observed fluorescence lifetime and quantum yield is much longer than that calculated from the oscillator strength of the S2(11Bu) state.23,24 When the intensity of the emission from the higher excited state is weak, the emitted photon may be reabsorbed by the absorption band of the molecule or the emission signals may be masked by strong Raman scattering from the solvent. An example of this is also seen for diphenyloctatetraene, for which a significant overlap between the weak S2 fluorescence and the strong S0 → S2 absorption makes it difficult to observe the former emission.25 In such a case, use of low-optical density samples is essential for the unambiguous observation of the S2 fluorescence, although lowering of the sample concentration weakens the fluorescence signals. In some cases, S2 and S1 fluorescence overlap and we may encounter difficulties in distinguishing them. A simple experimental method based on a combination of polarized absorption and emission measurements was suggested for the separation of overlapping S2 and S1 fluorescence spectra.26 Although this method may not be applicable to fluid-solution and vapor samples, a time-resolved method can be used for samples in which each of the overlapping emissions shows a different decay time. Detection of phosphorescence from higher excited states is often accompanied by difficulties and may require considerable experimental skills. Degassing of the sample is essential to avoid the quenching of the long-lived phosphorescent states by molecular oxygen in the ground state, 3Σg−, when the sample form is vapor or fluid solution. In some case, even after degassing of the sample by means of repeated freeze−pump− thaw, water molecules contained in the sample crystals remain. This may increase the sample pressure or influence the phosphorescence properties after sealing off the cell.27

the emission from higher excited states is often observable for small molecules. In general, the dominant nonradiative process taken by the molecules excited into the Sn state (n ≥ 2) is considered to be the internal conversion to the nearest and lower singlet state Sm (n > m). For example, the energy of a molecule in the S3 state is considered to be transferred almost exclusively to the S2 state, and not to the S1 or T1 state. With medium- or large-size organic molecules such as benzene, there exist the following general concepts concerning the relative rates of the nonradiative transitions: (i) In most cases, the rate of internal conversion from a given excited state to another excited state is faster than that of intersystem crossing from that state. (ii) Internal conversion or intersystem crossing from a given excited state to a lower excited state dominates over conversion directly to the ground state. (iii) The nonradiative rate between the two states decreases with increasing energy separation between the two states (the energy gap law).17 (iv) The rate of intersystem crossing is relatively large when that nonradiative transition accompanies a change of orbital type, e.g., the intersystem crossing rate from the 1(π, π*) to 3(n, π*) state is faster than that from 1(π, π*) to 3(π, π*), and also the rate from 1 (n, π*) to 3(π, π*) is faster than that from 1(n, π*) to 3(n, π*) (El-Sayed’s rule for intersystem crossing).18−20 The energy gap law for nonradiative processes (iii) is an extension of the weak coupling case in the nonradiative transition theory, which is an application of time-dependent perturbation theory in quantum mechanics.17 El-Sayed’s rule for intersystem crossing (iv) is related to the relative magnitude of the spin−orbit interaction between the triplet and singlet states. Although there are some exceptions, the emission properties of many organic molecules can be understood in terms of these concepts. A review on nonradiative electronic relaxation under collision-free conditions was published as early as 1977 by El-Sayed and coworkers.21

3. IDENTIFICATION OF EMISSION FROM HIGHER EXCITED STATES Whenever an abnormal or unidentified emission is detected from a molecule, the possibility of impurity emission should be considered first. Extreme care to ensure the sample’s purity should be taken before anomalous emission is reported with confidence. Typical examples of dual emission as a result of impurities are those of fluoranthene and phenylnaphthalene, for which incorrectly assigned “S2” fluorescences were eliminated by use of pure synthetic samples.22 Measurement of excitation spectrum is one of the most useful and effective tools to confirm that the emission in question actually originates from the molecule itself. In condensed phases such as fluid solutions or matrices, a molecule suffers a significant amount of collisions during the lifetime of the excited states. Hence, normally it relaxes to the lower vibrational levels of the emitting state irrespective of the energy of excitation, providing that the excitation spectrum is identical to the absorption spectrum. The situation is almost the same for molecules in the static vapor phase at high total pressure (e.g., ∼102 Torr), where the molecules also suffer many collisions during the lifetime of the excited state. However, this is not always the case for molecules in the vapor phase at low pressure or in a jet. Under low-pressure conditions, molecules often exhibit intrinsic photophysical properties that are somewhat different from those observed in C

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Table 1. Typical Molecules Showing the Fluorescence from Higher Excited Singlet States along with the Emitting States molecule

emitting states

reference

azulene azulene, derivatives pseudoazulenes 2-amino-1,3-diazaazulene azuleno[5,6,7-cd]phenalene s-indacene, derivatives

S2(π, S2(π, S2(π, S2(π, S3(π, S2(π,

π*) π*), π*), π*), π*), π*)

S1(π, S1(π, S1(π, S2(π,

π*) π*) π*) π*)

29 72−85 65, 66 68 71 69, 70

pyrene pyrene, derivatives coronene 1,2-benzanthracene 20-methylcholanthrene 1,12-benzperylene 3,4-benzpyrene chrysene 3,4-benzophenanthrene ovalene

S3(π, S2(π, S3(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π,

π*), π*), π*), π*), π*), π*), π*), π*), π*), π*),

S2(π, S1(π, S2(π, S1(π, S1(π, S1(π, S1(π, S1(π, S1(π, S1(π,

π*), S1(π, π*) π*) π*), S1(π, π*) π*) π*) π*) π*) π*) π*) π*)

102, 105, 107 108 106, 116 106 106 106 102 103 103, 106 94, 100

diphenylbutadiene diphenylhexatriene diphenylhexatriene, derivatives stiff-5-diphenylhexatriene diphenyloctatetraene diphenyldecapentaene diphenyldodecahexaene diphenyltetradecaheptaene octatetraene decatetraene dodecapentaene tetradecaheptaenes β-carotene α-carotene apo-carotenes sprilloxanthine

S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S3(π,

π*), π*), π*), π*), π*), π*), π*), π*), π*), π*), π*), π*), π*), π*), π*), π*),

S1(π, S1(π, S1(π, S1(π, S1(π, S1(π, S1(π, S1(π, S1(π, S1(π, S1(π, S1(π, S1(π, S1(π, S1(π, S2(π,

π*) π*) π*) π*) π*) π*) π*) π*) π*) π*) π*) π*) π*) π*) π*) π*), S1(π, π*)

156 25, 160, 147 163, 164 161, 162 25, 142 142b, 143 142b, 143 143, 152 136, 138, 139 139−141 139 132 173−176 184 185 197

thiophosgene thiocarbonyl chlorofluoride xanthione Michler’s thione 4H-1-benzopyran-4-thione 4-thiouridine thiketones, derivatives indanethione

S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π,

π*), S1(n, π*), S1(n, π*), S1(n, π*), S1(n, π*), S1(n, π*), S1(n, π*), S1(n, π*)

π*), π*), π*), π*), π*), π*), π*),

205, 210−213 210−212 206−208 245 258 259 207 233

metal tetrabenzoporphyrins (metal = Zn, Mg, Cd, Sm, Eu, Gd, Rb, Dy, Ho, Er, Tm, Yb, and Lu) metal tetraphenylporphyrins (metal = AlCl, GaCl, InCl, Zn, and Cd) metal tetra-para-tolylporphyrins (metal = Sm, Eu, Gd, Tb, Yb, and Lu)

S2(π, π*), S1(π, π*), T1(n, π*)

261, 263, 266, 267, 269, 270, 274

S2(π, π*), S1(π, π*), T1(n, π*) S2(π, π*), S1(π, π*) T1(n, π*)

271, 266 272

fullerene (C70) azobenzene naphthothiadiazine diphenyl-s-tetrazine o-hydroxybenzaldehyde 1,4-anthraquinone isoquinoline silyl ketones germyl ketones diphenylacetylene derivatives [18]annulene

S2(π, S2(π, S2(π, S2(n, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π, S2(π,

D

π*), π*), π*) π*), π*), π*), π*) π*), π*), π*) π*),

T1(n, T1(n, T1(n, T1(n, T1(n, T1(n, T1(n,

π*) π*) π*) π*) π*) π*) π*)

T1(π, π*) S1(n, π*) S1(n, π*), T1(n, π*) T1(π, π*) S1(n, π*), T1(π, π*) S1(n, π*) S1(n, π*) S1(π, π*)

320, 321 313 323 322 316 310 311 303 303 306, 308 317

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Appropriate choice of solvent is also an important factor for the detection of the anomalous phosphorescence from solution samples. In the case of quinones, it is normally difficult to detect phosphorescence in degassed hydrocarbon fluid solutions such as hexane at room temperature due to hydrogen abstraction by the CO group in the excited 3(n, π*) state.7 In some cases, the intensity of the observed anomalous emission may be weak. In solution, the presence of Raman scattering from the solvent often interferes with the observation of such weak emission. In this sense, the use of vapor samples is of particular convenience for the detection of weak anomalous emission, although in the vapor phase the emission signals are often weak due to the low concentrations of the vapor samples. To detect weak emission signals, the photon-counting technique combined with a discriminator is useful, along with the use of a double monochromator, laser excitation, and electric cooling of the photomultiplier. In addition, the use of reflection mirrors mounted beside the sample holder enhances the emission signals by a factor of 3−4.28 The product of the emission quantum yield and the optical density of the sample is a critical value that can be used to determine whether or not the emission in question is detectable. When this value is below 10−7, it is normally difficult to detect the emission, even with a high-sensitivity spectrophotometer equipped with a photoncounting system operating with a long counting time.

4. FLUORESCENCE FROM HIGHER EXCITED STATES During the past four decades, numerous examples of fluorescence from higher excited states have been reported. Molecular species showing fluorescence from higher excited states are listed in Table 1 along with the emitting states.

Figure 2. Azulene and its analogues that show S2 fluorescence.

4.1. Azulenes and Analogues

A number of reviews on the S2 fluorescence of azulenes already exist.2,3 In this section, therefore, the emission properties of azulene and analogues are surveyed briefly, including data reported predominantly after 1977. Azulene, a typical nonalternant aromatic molecule, is the first example of a molecule showing S2 fluorescence. Its derivatives also are known as molecules that exhibit emission of this type. The S2(1A1) fluorescence of azulene (C2v symmetry) was reported first by Beer and Longuet-Higgins in 1955.29 Shortly after that publication, a considerable number of papers were published on the emission properties and electronic states of azulene and its analogues.30−36 The excited states of the azulene family are characterized by a large energy separation between the S1(1B2) and S2 states, ΔE(S2 − S1), of normally over 10 000 cm−1, and the much larger oscillator strength of S2 than that of S1. The large ΔE(S2 − S1) value presumably leads to small horizontal Franck−Condon factors that result in comparatively slow S2 → S1 internal conversion, and as a result, S2 fluorescence can compete favorably with internal conversion. Azulene emits much more strongly from the S2 state than from S1. In solution, the quantum yield of the S2 fluorescence is reported to be about 5 × 10−2, whereas that of the S1 fluorescence is ∼10−6.37 Dual fluorescence from S2 and S1 states was observed for several substituted azulenes with ΔE(S2 − S1) values of 9 000−10 000 cm−l.38 Azulene derivatives with a smaller ΔE(S2 − S1) value tend to show the fluorescence from S1 in addition to that from S2.38 Molecular structures of azulene and analogues showing S2 fluorescence are illustrated in Figure 2. Emission and absorption spectra of azulene in solution are shown in Figure 3.

Figure 3. Absorption (broken-line curves) and emission (solid-line curve) spectra of azulene in ethanol.38

The decay rates of the S2 state of azulenes have been investigated in conjunction with the energy gap law by a number of researchers.37,39−42 Through the spectral measurements of 30 azulene derivatives with different ΔE(S1 − S2) values, the S2 fluorescence yield was shown to decrease almost exponentially with decreasing ΔE(S1 − S2).40,41 Griesser and Wild have examined the decay processes of the S2 and S1 states of azulene and derivatives and shown that the S2 − S1 internal conversion rates obey the energy-gap law.42 Applicability of the energy gap low for S2 fluorescence was investigated also for a particular azulene molecule by changing ΔE(S1 − S2). Steer and co-workers have measured the S2 fluorescence lifetimes and E

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Figure 4. Some pseudoazulenes that exhibit S2 fluorescence.

The S2 → S1 fluorescence was observed for azulene in the near-infrared region (700−1000 nm).55−59 Huppert et al. reported the S2 → S1, S2 → S0, and S1 → S0 fluorescence spectra of azulene vapor with quantum yields of 5 × 10−4, 0.2, and 8 × 10−6, respectively,57,60 but in solution the S2 fluorescence quantum yield is reported to be ∼0.05.37 Klemp and Nickel measured the S2 → S0/S2 → S1 fluorescence quantum yield ratio of 455 ± 100 for azulene in isopentane at 190 K.59 It was reported also that azulene emits Tk → T1 fluorescence close to 500 nm, but detailed spectroscopic data was not provided.61 Although the S2 emission of azulene is prompt fluorescence, azulene exhibits delayed S2 fluorescence due to hetero triplet− triplet annihilation involving azulene (Az) and fluoranthene (Fl) in the T1 states, T1(Az) + T1(Fl) → S2(Az) + S0(Fl).62 S2 fluorescence was observed for trimethylazulene also through homotriplet−triplet annihilation, T1(Az) + T1(Az) → S2(Az) + S0(Az).63 Further, the S2 fluorescence of azulene was observed following electron-impact excitation.64 Pseudoazulenes are defined as heterocyclics in which one −CHCH− group in the heptagonal ring of azulene is replaced by a heteroatom, e.g., oxygen (see Figure 4). These molecules also show S2 fluorescence. Interestingly, with increasing excitation energy, the fluorescence spectra of pseudoazulenes in Shpolskii matrices at low temperature are broadened and red-shifted, as was observed in the fluorescence spectra in the vapor phase at low pressure, indicating extremely short decay times for S2 states.65 Through the spectral measurements of 18 pseudoazulene derivatives with different ΔE(S1 − S2) values, the nonradiative rates from S2 were shown to decrease almost exponentially with increasing ΔE(S1 − S2). This observation indicates that the S2 → S1 internal conversion is the main nonradiative route of the S2 state and that the energy-gap law applies also to pseudoazulenes.65 1,3-Diazaazulene is known to show S1 fluorescence and T1 phosphorescence in condensed phases,67 but recently it was revealed that a derivative of 1,3-diazaazulene (2-amino-1,3diazaazulene) exhibits S2 fluorescence in addition to S1 fluorescence in the vapor phase.68 The ΔE(S1 − S2) value of 2-amino-1,3-diazaazulene is ∼3 200 cm−1, similar to corresponding energy gaps in aromatic acenes such as pyrene rather than parent azulene. Klann et al. and later Gellini et al. reported the S 2 fluorescence from a derivative of s-indacene, 1,3,5,7-tetra-tertbutyl-s-indacene.69,70 The S1 and S2 states of this molecule are

quantum yields of azulene, azulene-d8, 1,3-dichloroazulene, 1,3dibromoazulene, 4,6,8-trimethylazulene, and 1,4-dimethyl-7isopropylazulene in six different solvents, where the ΔE(S2 − S1) values of azulenes change depending on the solvent by ∼500 cm−1.37 The rate of the nonradiative decay process from S2 was shown to decrease almost exponentially with increasing ΔE(S2 − S1) for azulene and azulene-d8, which was interpreted also within the framework of the energy-gap law. Steer and coworkers have shown that intersystem crossing from S2 is important for halogenated derivatives and that factors other than ΔE(S2 − S1) are involved in determining the nonradiative rates for alkyl-substituted azulenes.37 Jet experiments have provided information on the intrinsic photophysical properties of cooled and isolated azulene molecules.43−53 Ito and co-workers first reported the excitation and fluorescence spectra of azulene in a jet in the S0 → S2, S3, and S4 transition regions.43 They discovered the presence of a considerable intramolecular vibrational relaxation occurring above 1600 cm−1 from the S2 origin. Demmer et al. investigated the dynamics of the S2 state of jet-cooled azulene.52 They observed quantum beats in the fluorescence decay curves, which were interpreted in terms of intramolecular vibrational energy redistribution.52 The S0 → S1 absorption or excitation spectrum of jet-cooled azulene was observed, and the lifetime was estimated to be 1 ps based on the bandwidth.44−48 The real-time observation of excited-state dynamics using mass spectroscopy revealed an ultrafast process on the time scale of 900 fs for S1 and 60 ps for the S2 state.49 The vibronic structure of the S2 state was analyzed by measuring the excitation and fluorescence spectra in a jet.43,50 The fluorescence lifetime and quantum yield were measured for individual vibrational levels in the S2 state in a jet, and the S2 fluorescence lifetime at the S2 origin was obtained to be 3 ns.51,52 Recently, Semba et al. measured rotationally resolved excitation spectra of the S0 → S2 origin and origin + 467 cm−1 bands of jet-cooled azulene and showed that rotational constants of the S2 state are almost identical to those of the S0 state.53 This observation suggests that the geometry of azulene in S2 is similar to that in S0. This situation is substantially different from that suggested previously. 2 Hirota et al. measured the time-resolved fluorescence spectra of azulene in the S2 state in compressed gases and in liquids and showed that the vibrational energy relaxation rates in the S2 state are 1−2 times faster than those in the S0 state.54 F

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Figure 5. Medium to large acenes that exhibit fluorescence from higher excited states.

located at approximately 10 000 and 18 000 cm−1, respectively, whereas those of azulene are located at approximately 15 000 and 28 000 cm−1. The observed S2 fluorescence shows a good mirror-image relationship with the S0 → S2 absorption, but the excitation spectrum depends subtly on the observation wavelength, as does the S2 fluorescence spectrum. These observations were interpreted in terms of the fluorescence from different conformers of 1,3,5,7-tetra-tert-butyl-s-indacene, arising from different orientations of the four tert-butyl groups at the 1, 3, 5, and 7 positions of parent s-indacene. It was reported that azuleno[5,6,7-cd]phenalene exhibits S3 and S2 fluorescence in solution at low temperatures.71 A small peak on the red shoulder of the strong S3 absorption band was assigned as the S2 absorption band for azulenophenalene.71 The ΔE(S3 − S2) value of this molecule is only 1 050 cm−1, while the ΔE(S3 − S1) value is 9 840 cm−1.71 The appearance of the S3 fluorescence was interpreted in terms of the slow S3 → S1 and S3 → S2 internal conversions, despite the small ΔE(S3 − S2) value.71 In addition to the molecules mentioned above, there are a number of azulene derivatives and azulene analogues from which S 2 fluorescence has been observed. These are benzazulene, cyclopentaheptaene, cyclopentazulene, pentalenoheptalene, azulenopyrrole, azulenofuran, azulenophenalene, and azulenopyridine.72−85

available and some are commonly used as solvents, detailed information on emission from higher excited states has not been provided. However, it was reported later that the anthracene crystal exhibits weak S2 fluorescence when excited through a two-step process via the lowest excited state, direct excitation, or annihilation of singlet excitons.89−91 Further, naphthalene vapor excited by means of glow discharge is reported to show Tk → T1 fluorescence around 400 nm.92 The excited states of most medium or large acenes are characterized by comparatively small ΔE(S1 − S2) values (normally below ∼4000 cm−1) and larger oscillator strength of S2 than that of S1. In solution or in matrices, S2 florescence is reported to occur through the thermal population from the S1 state for 3,4-benzopyrene, 1,12-benzoperylene, 1,2-benzanthracene and 2′-methyl-1,2-benzanthracene, 3-methylpyrene, 3,4benzotetraphene, and ovalene.93−101 The emission properties of medium or large acenes in the vapor phase are of particular interest, because the intrinsic photophysical property is observed in the absence of environmental effects and because the appearance of the S2 fluorescence is much more significant compared to smaller acenes such as naphthalene. Molecular structures of medium to large aromatic acenes that exhibit fluorescence from higher excited states are illustrated in Figure 5. Hoytink et al. observed S2 fluorescence from pyrene (0.05 Torr at 170 °C) and 3,4benzopyrene (0.1 Torr at 260 °C) in the static vapor phase.102 At almost the same time, the emission properties of some of the aromatic acenes such as pyrene and 3,4-benzopyrene were investigated by a number of researchers in the static vapor phase at low pressure.102−104 In particular, the fluorescence properties of pyrene and its derivatives were investigated in detail in the static vapor phase by Baba and co-workers.105−110 Although pyrene vapor is known to show weak S2 fluorescence in addition to strong S1 fluorescence at high total pressure, this S2 emission was assigned to delayed fluorescence occurring through the thermal activation of the S1 state.102 However, the observation of the S2 fluorescence under collision-free

4.2. Aromatic Acenes

Fluorescence from higher excited states has been reported for a number of aromatic acenes of medium to large size such as pyrene, coronene, 1,12-benzoperylene, 3,4-benzopyrene, 1,2benzanthracene, 3,4-benzotetraphene (benzochrysene), ovalene, 1,2-:3,4-dibenzanthracene, and chrysene as well as their derivatives in a variety of media (vide infra). In addition, extremely weak fluorescence from higher excited states was reported in the 1970s for smaller acenes such as benzene, mesitylene (1,3,5-trimethylbenzene), naphthalene, and pxylene, as well as toluene in solution and in the vapor phase.86−88 Although these smaller acenes are commercially G

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ℏ) V2ρ(S2), respectively, utilizing Fermi’s golden rule, where V is the coupling constant between S1 and S2 and ρ(Si) is the density of the vibronic states in Si at the energy of excitation. It was demonstrated that the excitation-energy dependence of the ratio, k21/k12, agrees qualitatively with that calculated based on Fermi’s golden rule.107 Further, it was shown for pyrene-h10 and -d10 vapors through the investigation of the effect of added buffer gas on the fluorescence that the k12 value increases almost linearly with increasing excitation energy from 33 600 to 42 700 cm−1, but k21 is almost constant regardless of excitation energy.109 The fluorescence and excitation spectra of pyrene vapor were also measured in a jet.111−114 The excitation spectrum in a jet for the S0 → S2 region shows a complicated structure due to the interaction between the S2 and S1 states.112 The dispersed fluorescence spectrum in a jet obtained by excitation into the S4 state is similar to that in the static vapor phase, but the S2/S1 fluorescence intensity ratio is intensified by a factor of 1.7, and is weakened for pyrene-d10 compared to pyrene-h10.113 These observations indicate that the contribution of the thermally activated S2 fluorescence is small under collision-free conditions and that the S2/S1 fluorescence intensity ratio is related to the vibrational-level density. Nakajima measured the fluorescence and excitation spectra of chrysene, 1,2-benzanthracene, 20-methylcholanthrene, coronene, and 1,12-benzoperylene in the static vapor phase at low pressure.106 He compared the fluorescence quantum yield ratios, ΦF(S2)/ΦF(S1), with those calculated based on Fermi’s golden rule.106,115 The values for ΦF(S2)/ΦF(S1) with the excitation into the S3 state are 0.037(0.038) and 0.037(1.0), respectively, for 1,2-benzanthracene and 20-methylcholanthrene, with the calculated values shown in parentheses.106 The reported difference between experimental and theoretical quantum yield ratio, 0.037 and 1.0, is large for 20methylcholantrene, which may arise due to underestimation of the ρ values. With coronene vapor, the value for [ΦF(S3) + ΦF(S2)]/ΦF(S1) obtained by the excitation into the S4 state is 0.11(0.079).106 However, recent investigation based on careful purification of coronene sample revealed that the S2 and S3 fluorescences reported by Nakajima likely originate from impurities and that the real S2 and S3 fluorescences are much weaker than those reported by Nakajima.115,116 Further, the S2 and S3 fluorescences of coronene vapor were analyzed as prompt fluorescence, which does not involve the fluorescence occurring through reverse internal conversion.116 In general, the purification of coronene is tremendously difficult.117 In any case, the occurrence of the S2 fluorescence for most of the medium- to large-size acenes such as pyrene, benzopyrene, benzanthracene, and benzoperylene in the vapor phase at low pressure can be interpreted by the relaxation model involving the reversible internal conversion between S1 and S2. The excitation-energy dependence of the relative S2/S1 fluorescence quantum yield ratio can be interpreted at least qualitatively using Fermi’s golden rule. Kinetic expressions for the S2 and S1 fluorescence quantum yields are derived in Appendix A for pyrene vapor as an example of medium-size acenes.107 Birks also derived similar expressions for S2 and S1 fluorescence quantum yields and lifetimes of isolated aromatic molecules.118 In Figure 7, fluorescence spectrum of coronene in hexane is shown along with the absorption spectrum. Weak S2 fluorescence of pyrene was observed also through triplet−triplet annihilation, T1 + T1 → Sn + S0 (n = 1, 2), along with the strong S1 fluorescence.119 The S2 and S1 fluorescences

Figure 6. Fluorescence and excitation spectra of pyrene vapor measured at 140 °C:107 fluorescence spectra are obtained by excitation into the S2, S3, and S4 origin bands. All the spectra are normalized to a common magnitude. The excitation spectrum was newly measured for this review.

conditions (in the static vapor phase at low pressure or in a supersonic jet expansion) demonstrated that the occurrence of the S2 fluorescence is not due to thermal population alone.102,105 In Figure 6, the fluorescence and excitation spectra of pyrene vapor at low pressure are displayed. Under the lowpressure condition, the relative intensity of the S2 fluorescence of pyrene vapor increases to a certain extent compared to that in solution or under the high-pressure condition (∼100 Torr) in the presence of added buffer gas.107 When the excitation energy is increased, the S2 fluorescence shifts to the red in almost the same way as does the S1 fluorescence.107 These observations indicate that both the S1 and S2 fluorescence originate from the unrelaxed higher vibronic levels of the excited electronic states. Further, the S2/S1 fluorescence quantum yield ratio, Φ F (S 2 )/Φ F(S 1 ), increases almost exponentially with increasing excitation energy.107 Similar fluorescence properties were described for pyrene-d10, 1methylpyrene, and 4-methylpyrene vapors.108 Analyses of the fluorescence spectral data of pyrene vapor revealed that the ΦF(S2)/ΦF(S1) value can be related to the ratio ρ(S1)/ρ(S2), with ρ(S1) and ρ(S2) denoting the vibrational-state densities of S1 and S2 states at the energy of excitation, respectively.107 The pressure dependence of the ΦF(S2) and ΦF(S1) values also were investigated by changing the pressure of added buffer gases.110 It was shown that ΦF(S2) decreases but ΦF(S1) increases with increasing buffer gas pressure.110 These observations have been interpreted kinetically in terms of reversible internal conversion between the S1 and S2 states. Through the investigation of the effect of added buffer gas, including the quenching effects of S2 fluorescence by molecular oxygen, the ΦF(S2) value was shown to consist of fast and slow components.109,110 The fast component was considered to correspond to the S2 fluorescence directly emitted from S2, and the slow component corresponded to the S2 fluorescence, which is emitted from S2 formed from S1 through reverse S1 → S2 internal conversion. The decrease of ΦF(S2) upon increasing buffer gas pressure was interpreted in terms of the decrease of the slow component of S2 fluorescence. In the case of reversible internal conversion between S2 and S1, the rate constants of the forward S2 → S1 (k21) and reverse S1 → S2 internal conversions (k12) can be formulated as k21 = (2π/ℏ)V2ρ(S1) and k12 = (2π/ H

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photon forbidden, it is two-photon allowed through the excitation of an imaginary Bu state. The forbidden 21Ag state is normally not observable in room-temperature absorption spectra and is considered to obtain its one-photon transition intensity mainly through vibronic coupling with the strongly allowed S2(11Bu) state. The reasons for the absence of the S1(21Ag) state in absorption spectra at room temperature are that its origin band is forbidden and that it is masked by the onset of the strong S2(11Bu) absorption band.128 In this sense, the feature of the forbidden S1(21Ag) state of polyenes is different from that of the S1(n, π*) state of many (CO)- or (−N)-containing molecules for which the band origin of the forbidden S0 → S1(n, π*) absorption appears distinctly. It is known that the energy level of the 11Bu state of polyenes decreases significantly with increasing solvent polarizability, (n2 − 1)/(n2 + 2) with n being the refractive index of the solvent, whereas that of the 21Ag state is almost unchanged.128 Therefore, the ΔE(S2(11Bu) − S1(21Ag)) value of polyenes can be changed systematically by changing solvent polarizability. The trend of recent research on the electronic states of polyenes has seemingly shifted from the determination of the location and nature of their 21Ag states to their photophysics and dynamical behavior.140,141 The spectroscopy and excited electronic states of octatetraene and other polyenes were reviewed by Hudson et al. in 1982128 and by Kohler in 1993.129 Typical polyene molecules that exhibit S2 fluorescence are illustrated in Figure 8. Prototypical polyenes would be R−(CHCH)N−R with R = H or alkyl group. For all-s-trans conformers of unsubstituted polyenes (UPs; H−(CHCH)N−H) with N = 3−8, the S1 and S2 states have been assigned as 21Ag and 11Bu, respectively.130−135 In rigid matrices at low temperature and in room-temperature solution, unsubstituted all-s-trans polyene (UP) with N = 4 (octatetraene) shows only S1(21Ag) fluorescence, but in the vapor phase, including in a jet, it shows S2 fluorescence in addition to weak S1 fluorescence.131 In a rigid matrix at low temperature, UPs with N = 4−6 show only S1 fluorescence, but UP with N = 7 shows both S1 and S2 fluorescence.134 In room-temperature ether−isopentane−ethanol (EPA) solution, dimethyl-substituted polyenes (H3C− (CHCH) N −CH 3 ) with N = 4 and 5 show only S 1 fluorescence, but the ones with N = 6 and 7 show both S1 and S2 fluorescence.132 Since the research history of the emission properties of octatetraene (H−(CHCH)4−H) is somewhat complicated, it is described briefly below. Gavin et al. measured the absorption and emission spectra of octatetraene along with the fluorescence quantum yield and lifetime in solution and in the static vapor phase.136 They observed fluorescence from the S2(11Bu) and S1(21Ag) states in solution but observed only the S2 fluorescence in the vapor phase. It seems that they did not recognize the 11Bu fluorescence to be S2 emission.136 In 1979, Granville et al. reported highly resolved one- and two-photon S0(11Ag) → S1 excitation and 21Ag(S1) → S0 fluorescence spectra of octatetraene in octane at 4.2 K, where the vibrationless electronic origin was observed in the twophoton excitation spectrum between the vibronically induced one-photon excitation and fluorescence origins.137 This measurement confirmed the assignment of the S1(21Ag) fluorescence of octatetraene. Heimbrook et al. measured the emission and excitation spectra of octatetraene in a supersonic jet and assigned the emission as the fluorescence from the S2(11Bu) state.138

Figure 7. Fluorescence (solid-line curves) and absorption (dotted-line curves) spectra of coronene in hexane at room temperature.116 Data taken from ref 116.

of pyrene vapor were measured also by low-energy (4−7 eV) electron-impact excitation. 120 Further, weak S 3 and S 4 fluorescences were observed for 1,2-benzanthracene and fluoranthene, respectively, through triplet−triplet annihilation.121 In addition, 5,6,11,12-tetraphenyltetracene (rubrene) and bromoanthracene are reported to show Tk → T1 and T2 → T1 fluorescence, respectively, in solution.122 4.3. Polyenes

Linear polyenes have the chemical structure R1−(CH CH)N−R2, with N ≥ 2 and R1 or R2 being hydrogen, alkyl, aryl, or other groups. These molecules are prototypes of πelectron conjugated systems for which a number of spectroscopic and quantum mechanical studies have been carried out since the 1930s.123,124 Most all-s-trans conformers of symmetrically substituted polyenes (R1 = R2) such as α,ωdiphenylpolyenes belong to the C2h point group. All-s-trans conformers of linear polyenes exhibit an intense absorption band that corresponds to an allowed electronic transition, 11Ag (S0) → 11Bu*(π, π*), in the UV−vis region (see Appendix B). The excitation energy of the 1Bu*(π, π*) state is known to decrease systematically with increasing polyene chain length. The chain-length dependence of the excitation energy and oscillator strength of the strong 11Ag(S0) → 11Bu*(π, π*) absorption bands have been interpreted at least qualitatively with simple molecular orbital theory such as Hückel or Pariser−Parr−Pople (PPP) methods.124,125 The 11Bu*(π, π*) state had been considered to be the lowest excited singlet state (S1) for a long time, but in 1972 the presence of a forbidden excited singlet state, 21Ag, located below the allowed 11Bu state, was pointed out by Hudson and Kohler.23,24 That is, the 11Bu state is not the first singlet excited state, S1, but the second, S2. Schulten and Karplus provided a theoretical rationalization for the forbidden 21Ag state, which is doubly excited in nature, and described it with extensive configuration interaction.127 Since those findings, a number of spectroscopic studies of polyenes have been carried out concerning the forbidden low-lying 21Ag(S1) state.129 At present, there is a general consensus that for unsubstituted all-s-trans linear polyenes (H−(CHCH)N−H) with the number of the polyene double bonds (N) ≥ 3, the S1 state is not 11Bu but 21Ag.129,130 Although the S1(21Ag) state is oneI

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Figure 8. Some all-trans linear polyenes that exhibit S2 fluorescence.

Bouwman et al. observed the structured S2(11Bu) fluorescence and the broad S1(21Ag) fluorescence in the emission spectra of all-s-trans octatetraene and decatetraene (1,8dimethyl-substituted octatetraene) in the static vapor phase.139 Later, Petek et al. reported similar emission for decatetraene vapor in supersonic molecular beams.140 To reveal the relaxation processes of decatetraene, the pressure and excitation-energy dependence of the S1 and S2 fluorescence were investigated in the vapor phase.141 The pressure dependence of the fluorescence yields of decatetraene vapor was interpreted in terms of a relaxation model involving reversible internal conversion between S1 and S2 and the vibrational relaxation in the S1 manifold. The reverse S1 → S2

internal conversion rate was shown to be significantly slower compared to the forward S2 → S1 internal conversion rate.141 That is, the S2 fluorescence can be regarded practically as prompt fluorescence without being accompanied by the reverse S1 → S2 internal conversion for decatetraene. Fluorescence spectra of all-s-trans decatetraene vapor following the excitation at different wavelengths are displayed in Figure 9. All-s-trans-diphenylpolyenes (DPs) are typical C2h model polyenes for which a number of spectroscopic data have been accumulated as described below. The emission properties of DPs with different polyene chain lengths are informative in considering the mechanism for occurrence of fluorescence from higher excited states. Figure 10 shows the fluorescence and J

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and temperatures.149,150 However, this did not influence the analyses of the S2/S1 fluorescence intensity ratio of this molecule significantly.25,147 On the other hand, the S 2 fluorescence of the longer DPs with N = 6 and 7 occurs as the prompt emission due to the comparatively large ΔE(S1 − S2) values (∼3 500−5 000 cm−1). Thus, the relative S1/S2 fluorescence intensity ratio is almost independent of temperature for DPs with N = 6 and 7. For the DP with N = 5, both of the thermally activated and prompt S2 fluorescence emissions were observed depending on the conditions such as solvent

Figure 9. S2 and S1 fluorescence spectra of pure 2,4,6,8-decatetraene [CH3(CHCH)4CH3] vapor at 35 °C obtained by excitation at different wavelengths.141 Data taken from ref 141.

Figure 11. S2/S1 fluorescence quantum yield ratios of α,ωdiphenylpolyenes (DPs) with different N values in CCl4 and CS2 at room temperature.143a Data taken from ref 143a.

polarizability and temperature. 143 Therefore, the S 2 /S 1 fluorescence quantum yield ratio, ΦF(S2)/ΦF(S1) decreases with increasing N for DPs with N = 3−5 but increases for DPs with N = 5−7 when the temperature is kept constant (see Figure 11).143,151 The ΔE(S1 − S2) dependence of the relative S2 fluorescence intensity was investigated in detail for the DP with N = 7.152 As previously mentioned, the ΔE(S1 − S2) value of linear polyenes depends significantly on the solvent polarizability.153,154 The S2/S1 fluorescence intensity ratio of the DP with N = 7 was shown to increase with increasing ΔE(S1 − S2) values, which are varied by changing the solvent polarizability. This observation was interpreted in terms of “intensity borrowing mechanism” by S1 from S2.152 Hirata et al. have measured the S2 → S1 internal conversion rate constants for DPs with N = 3−8 in solution.155 They have shown that the internal conversion rate does not depend significantly on N, indicating that the energy-gap law for the S2 → S1 internal conversion does not apply for DPs. The DP with N = 2 (diphenylbutadiene) shows fluorescence from S2(11Bu) in the static vapor phase at high total pressure in the presence of buffer gas but shows only the S1(21Ag) fluorescence in a jet.156 Since the ΔE(S1 − S2) value of the DP with N = 2 is only 1 100 cm−1 in the vapor phase, the weak S1 fluorescence is considered to be masked by the strong S2 fluorescence in the static vapor phase at high total pressure. Further, it is suggested that the S1 state of the DP with N = 2 is probably assigned to 21Ag in low polarizable solvents such as perfluoropentane at room temperature and that the fluorescence occurs mainly from S2(11Bu) in such solvents.157,158 The 11Bu fluorescence obtained by the two-photon excitation into the 21Ag state also was observed for the DP with N = 2 in a low polarizability solvent.159 DP with N

Figure 10. Fluorescence and absorption spectra of α,ω-diphenylpolyenes (DPs) with different N values in CCl4 at room temperature.143a All spectra are normalized to a common magnitude.

absorption spectra of DPs with N ranging from 3 to 7 in CCl4 solution at room temperature. These diphenylpolyenes exhibit dual fluorescence from the S1(21Ag) and S2(11Bu) states in room-temperature solution,142 but the mechanism for the occurrence of the S2 fluorescence differs depending on N.143 With shorter DPs (N = 3 and 4), the S2 fluorescence occurs as the result of thermal population from the S1 state due to small S1 − S2 energy separations, ΔE(S1 − S2), (∼1 000−2 000 cm−1).25,144−150 Thus, the relative intensity of S2 fluorescence increases with increasing temperature for these shorter DPs. It was later shown that the fluorescence spectrum of DP with N = 3 includes a fluorescence contribution from the s-cis,s-trans conformer, which increases at higher excitation wavelengths K

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= 3 shows only S1(21Ag) fluorescence both in the static vapor phase and in a jet, whereas DPs with N = 4 and 5 show weak S2(11Bu) fluorescence and stronger S1(21Ag) fluorescence in the static vapor phase.143 It is possible to invert the 21Ag and 11Bu levels for some diphenypolyenes and to observe the full 11Bu fluorescence in sufficiently polarizable solvents (in this case, the 11Bu state is S1). The full 11Bu fluorescence was observed for the DP with N = 3 in a highly polarizable solvent at 77 K, although the emission consists of dual fluorescence from the S1(21Ag) and S2(11Bu) states in commonly used solvents such as hexane or benzene at temperatures near room temperature.160 In the case of a rigid analogue of the DP with N = 3 (all-strans 1,4-diindanylidenyl-2-butene), weak thermally activated S2 fluorescence appears only in low polarizability solvents along with strong S1 fluorescence.161,162 The ΔE(S1 − S2) value of this molecule is smaller than that of diphenylhexatriene, and the S1(21Ag) and S2(11Bu) energy levels are inverted when solvent polarizability is increased.161 Weak thermally activated S2 (11Bu) fluorescence has also been observed for derivatives of diphenylhexatriene along with S1 fluorescence.144,163,164 In addition, 1-phenyl-4-(1′-pyrenyl)-1,3-butadiene and isomers of 1-(1′-naphthyl)-6-phenylhexatriene are reported to show thermally activated S2 fluorescence in addition to stronger S1 fluorescence in room temperature solutions.165,166 All-s-trans dithienylpolyenes with N = 2 and 3 are also reported to show thermally activated S2 fluorescence in roomtemperature solution and in the static vapor phase, although these polyenes possess rotational isomers depending on the orientation of the two thienyl groups on the two edges of the polyene chain.167,168 These rotational isomers were identified and separated by the emission, excitation, and hole-burning spectral measurements in a jet.169,170 Carotenoids are organic pigments that occur naturally in plants as well as in some other light-harvesting organisms in photosynthesis. There are over 623 known carotenoids. These can be split into two classes, xanthophylls, which contain oxygen, and carotenes, which are purely hydrocarbons containing no oxygen.171,172 Some of these carotenoids can be extracted from natural sources such as carrot or spinach. A number of carotenoids possess polyene structures with long polyene chain lengths. Emission properties of carotenoids have been intensively investigated, mainly in conjunction with photosynthesis. The strong absorption band in the visible region of carotenoids is caused by the allowed S0 → S2(11Bu+) transition, the energy of which decreases as N increases. The light energy absorbed by carotenoids through the S0 → S2 transition is considered to be transferred to chlorophyll. β-Carotene (Figure 8) is a typical carotenoid on which numerous spectroscopic studies have been carried out. Emission from β-carotene consists mainly of fluorescence from the S2(11Bu+) state, which was once thought to be the S1 state. With carotenoids possessing a long polyene chain such as β-carotene, it seems to be the S1 fluorescence that is anomalous, rather than the S2 fluorescence. The S1 fluorescence of βcarotene was reported first by Bondarev and Knyukshto to appear only weakly at the far red side of the S2 fluorescence in highly polarizable solvents.173,174 Soon after, Anderson et al. reported the fluorescence spectrum from the S1 (21Ag) state of β-carotene appearing at about 13 000 cm−1.175 Careful analyses of the emission spectrum indicated that the S1 state of βcarotene is located at 14 100 cm−1.176 Ultrafast dynamics of the excited states of carotenoids were reviewed by Polivka and

Sundstroem in 2004.177 Extremely rapid decay times of the S2 state ranging from 50 to 300 fs were reported for β-carotene as well as for other carotenoids.177−183 Recently, α-carotene was also shown to emit dual fluorescence from the S1 and S2 states in highly polarizable solvents.184 Christensen et al. measured the absorption and fluorescence spectra of a series of apocarotenes with polyene double bonds N ranging from 5 to 11 in EPA at 77 K.185 Apo- and diapo-carotenes (Figure 8) with N ranging from 7 to 10 exhibit both S1 and S2 fluorescence, for which the relative S2 fluorescence intensity tends to increase with increasing N.185 In general, the S2/S1 fluorescence intensity ratio tends to increase with increasing N or with increasing ΔE(S2 − S1) energy gap, which is achieved by changing the solvent polarizability. This tendency can be seen for a number of different carotenoids.186−191 Among a number of fluorescent linear polyenes, the molecules possessing the longest polyene chain length may be decapreno-β-carotene (N = 15) and dodecapreno-β-carotene (N = 19), which show only weak S2 fluorescence with the quantum yields of 10−4−10−5.189 However, S2 fluorescence spectral data are available for the N = 13 polyene (1′,2′-dihydro-3′,4′,7′,8′-tetradehydrospheroidene) as the molecule having the longest polyene chain length.192 It was predicted theoretically that there is a forbidden singlet excited state, 11Bu−, in addition to the forbidden 21Ag−(S1) state, located below the allowed 11Bu+ state for polyenes with long polyene chain length (N > 9).193,194 Later, the presence of the 11Bu− was indicated through the measurements of the resonance Raman excitation profile and fluorescence spectra of carotenoids.195−197 The observed fluorescence was interpreted as a superposition of the three kinds of fluorescence from S1(21Ag−), S2(11Bu−), and S3(11Bu+), although the measured fluorescence spectra are extremely weak and somewhat noisy.197 More recently, Kosumi et al. measured femto-second time-resolved absorption spectra of β-carotene homologues with N = 7−15.198,199 These spectra showed that the energygap law for the S2 → S1 internal conversion applies for carotenoids with N < 9, but the reverse energy-gap law applies for those with N > 11. The observed reverse energy-gap law was interpreted in terms of the presence of an excited state located between the 21Ag− and 11Bu+ states for carotenoids with N > 11.199 Frank et al. measured the absorption, fluorescence, excitation, and time-resolved absorption spectra of a series of spheroidenes with N ranging from 7 to 13 and showed that the S2 fluorescence quantum yield shows a maximum for spheroidenes with N = 9, which then decreases with increasing N.192 In any case, carotenoids with large N values can be regarded as unique molecules in the sense that these may exhibit triple fluorescence from S1, S2, and S3. The electronic states of carotenoids were reviewed by Christensen in 1999.200 Although there is general agreement that several longer all-strans polyenes show both S1 and S2 fluorescence, the situation of polyene spectroscopy is not as simple as has been explained heretofore. Recently, it was suggested by Christensen et al. that the reported fluorescence from the forbidden S1 state of carotenoids and longer all-s-trans linear polyenes likely originates in part from the s-cis conformer, which is possibly produced photochemically, and that the S1 fluorescence of all-strans polyenes is somewhat weak compared to that of the s-cis conformer.201 The results presented by Christensen et al. may require a reinterpretation of the fluorescence obtained previously for all-s-trans conformers of long polyenes and carotenoids.201 Further, the complexity of the spectroscopy of carotenoids also arises from the observation that the S1 L

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Figure 12. Some thioketones that exhibit S2 fluorescence.

fluorescence spectra of all-s-trans conformers resemble those of the s-cis conformers.201 However, even in such a situation, the observations that many of the longer polyenes, irrespective of which regioisomer is present, show dual fluorescence from S1 and S2 seem to be still valid at present. There are two distinct mechanisms for the occurrence of the S2 fluorescence for all-s-trans linear polyenes. One is the thermally activated delayed S2 fluorescence that occurs as a result of thermal activation from the S1 state and is observable when the ΔE(S1 − S2) value is comparatively small (1 000−2 000 cm−1). The other is the prompt S2 fluorescence that occurs as a result of the fast radiative process from S2 competing with the fast S2 → S1 internal conversion and is observable when the ΔE(S1 − S2) value is large (>∼3 000 cm−1). In the case of the prompt S2 emission, there is a tendency for increasing S2/S1 fluorescence intensity ratio with increasing N or ΔE(S1 − S2). This observation was interpreted in terms of intensity borrowing by S1 from S2 and/or the energy-gap law for the S2 → S1 internal conversion rate.152,202,203,176,187,192 In the case of the intensity-borrowing mechanism, the S2/S1 fluorescence quantum yield ratio, ΦF(S2)/ΦF(S1), increases with increasing ΔE(S1 − S2) for a fixed molecular species, because the S1 fluorescence intensity is expressed approximately by the form f(S2)V2/ΔE(S1 − S2)2, where f(S2) is the oscillator strength for the S0 → S2 transition and V is the coupling constant between S1 and S2.152,202,203 Further, the energy-gap law can also explain the variation of ΦF(S2)/ΦF(S1) with increasing ΔE(S1 − S2), because the relative S2 → S1 internal conversion rate tends to decrease with increasing ΔE(S1 − S2).192,199 At present, a number of polyenes are known to show prompt S2 fluorescence emission that was once thought to be the conventional S1 fluorescence. As mentioned above, the S2/S1 fluorescence intensity ratio tends to increase with increasing polyene chain length. This tendency can be seen clearly when N is changed successively for a series of polyenes, R−(CHCH)N−R.132,175 This observation has been interpreted in terms of the energy-

gap law for the S2 → S1 internal conversion in some cases,176,187 but it cannot be explained simply by the energy-gap law.143,204 4.4. Thioketones

Thioketones, also known as thiocarbonyl compounds or thiones, are molecules with the general formula R1R2CS, with R1 or R2 being alkyl group, aryl group, or halide. The S2 fluorescence of this family of molecules was first reported for thiophosgene (Cl2CS) in 1974 by Steer et al.,205 whereas the S2 fluorescence of alkylthioketones was reported in 1975 independently by Huber and Mahaney and by Ware and coworkers.206−208 These reports were followed by numerous studies on the fluorescence properties of thioketones and their analogues in a variety of media. Photochemistry of thioketones was reviewed by Turro et al. in 1978,2 and the photophysics, photochemistry, and spectroscopy of this family were reviewed in detail by Maciejewski and Steer in 1993.5 However, research on the emission properties of thioketones and related molecules continues up to the present time. In this section, therefore, the emission properties of thioketones and analogues are surveyed concisely, focusing mainly on the emission properties reported after 1993. The excited states of thioketones are characterized by a comparatively large energy separation between the S1(n, π*) and S2(π, π*) states (normally over 6 000 cm−1) with much larger oscillator strength for S2 than for S1. Thus, the excitedstate feature is somewhat similar to that of azulenes, except that the S1 state is 1(n, π*) in nature. Typical thioketones that exhibit S2 fluorescence are displayed in Figure 12. The simplest molecule of this family is thioformaldehyde (H2CS). Unfortunately, however, the S2(1A1) fluorescence has not been observed for H2CS nor H8C5S (thiocyclopentanone), although the S1(1A2) fluorescence and T1(3A2) phosphorescence have been reported.209 With regard to small thioketones (4-atom molecules), thiophosgene and thiocarbonyl chlorofluoride (ClFCS) are known to show S2 M

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fluorescence.210−212 In particular, thiophosgene has been the subject of numerous photophysical studies since the first observation of S1 and S2 fluorescence.205,213 Early works on the photophysics of thiophosgene were reviewed by Steer in 1981.214 Most of the research on the emission properties of thiophosgene was carried out in the vapor phase, but the emission from the three excited states, S2, S1, and T1, has been observed also in solution.215,216 The S1 absorption of thiophosgene is located at ∼550 nm and the much stronger S2 absorption is located at about 260 nm, with a relative S2/S1 absorption intensity ratio of ∼103 (ΔE(S1 − S2) = ∼20 000 cm−1). The quantum yield of the S2 fluorescence of thiophosgene vapor is reported to be as high as 0.5−1.0, indicating the lack of efficient nonradiative processes from S2.217,218 The S2 fluorescence lifetime of thiophosgene vapor is reported to be 6−25 ns, which depends on the excitation energy as well as the pressure.219,220 A weak band at 34 728 cm−1 in the absorption spectrum was assigned as the S2 origin of thiophosgene vapor.221 Excitation of thiophosgene vapor into higher vibronic levels in the S2 state results in the decrease of the S2 fluorescence yield and a shortening of the S2 fluorescence lifetime.205,217,218,222 These observations have been attributed to the crossing of the S2 and S3 potentials.223 Through an optical−optical double resonance-fluorescence depletion experiment in a jet, the invisible S3 state is shown to be located near the S2 state.224,225 Zhang et al. calculated the potential surfaces of the excited states of thiophosgene with a combined complete active space self-consistent field and multireference confidence interval method.226 Their results indicate that both the S2 dissociation and the S2 → S3 internal conversion cause the loss of the S2 fluorescence yield upon photoexcitation at 235−253 nm. Further, they predicted that the S2 → S3 internal conversion followed by the direct internal conversion to S0 results in the formation of Cl and ClCS(2A′) radicals in S0, whereas the S2 dissociation leads to the formation of Cl and ClCS(2A″) radicals in excited electronic states.226 S2 fluorescence resulting from sequential two-photon absorption and singlet−singlet energy pooling also was reported for thiophosgene vapor.227 As mentioned above, the first reports of the S2 fluorescence from larger thioketones such as xanthiones and arylalkyl thioketones in 1975 were followed by numerous investigations of the emission properties of these systems.214,228,229 In the case of 4H-1-benzopyran-4-thione in solution, S2 fluorescence occurs as a broad emission band adjacent to broad S2 absorption, accompanied by T1 phosphorescence and weak thermally activated S1 fluorescence.5 Steer et al. showed that the nonradiative rate of the S2 state decreases exponentially with increasing ΔE(S2 − S1) for thioketones possessing different ΔE(S2 − S1) values in perfluoroalkane at room temperature.230−232 Their observation indicates that the S2 → S1 internal conversion obeys the energy-gap law and that it is the main nonradiative process from S2 for these molecules. They used perfluoroalkanes as solvents, because other commonly used solvents such as hydrocarbons quench the S2 fluorescence.233 Thus, it was of importance to clarify the role of perfluoroalkanes used as the solvent as well as the quenching mechanism of S2 fluorescence. The mechanism of the quenching of the S2 fluorescence by solvents (i.e., the decrease of the S2 fluorescence intensity by the interaction of thioketones with the solvent) has been investigated by several researchers.234−239 In most cases, the quenching data were analyzed using the Smoluchowski−Collins−Kimball (SCK)

model.240,241 For example, Maciejewski et al. investigated the solvent quenching effect on the S2 fluorescence of seven aromatic thioketones, and the results were rationalized using the SCK model.242 Krystkowiak also fitted the S2 fluorescence quenching of xanthione by dimethylpentane in perfluoroalkanes to the SCK model.243 Milewski et al. investigated the emission properties of the complexes of 4H-1-benzopyran-4-thione with β-cyclodextrin and alcohols. They showed that the S 2 fluorescence and the T1 phosphorescence were enhanced due to the shielding effect imposed on the thioketone from water molecules, when the thioketone molecule was trapped in βcyclodextrin.244 Palit et al. investigated relaxation dynamics of the S 2 and S 1 states of 4,4′-bis(dimethylamino)thiobenzophenone (Michler’s thione) in different solvents.245 Weak S2 fluorescence and much weaker S1 fluorescence were observed for Michler’s thione.245 The S2 fluorescence intensity was reduced in polar solvents due to the reduced ΔE(S2 − S1) value and the interaction of Michler’s thione with solvents.245 The S2 fluorescence of Michler’s thione was observed also in polar solvents, where the ΔE(S2 − S1) value is only ∼4 000 cm−1.245 The S2 fluorescence and excitation spectra of van der Waals complexes of arylthioketones were measured in a jet to investigate microscopic solvation effects.246−250 Topp and coworkers measured the S2 fluorescence lifetime of van der Waals complexes of xanthione with seven n-alkanes and perfluorohexane in a jet by means of picosecond time-resolved and holeburning spectroscopy.246 They demonstrated some parallels between the dynamics in a jet and dynamics observed in fluid solutions at room temperature. Steer and co-workers measured the excitation spectra of van der Waals complexes of xanthione and benzopyranthione with He, Ne, Ar, Kr, Xe, N2, and CO in a jet.247,248,250 They identified the structures of the complexes and showed that dipole−induced dipole interactions contribute to the binding energies of the complexes through an analysis of the microscopic solvation shifts of the S2 origin bands in the excitation spectra.250 They reported also the excitation spectra of van der Waals complexes of xanthione with nine n-alkanes and six perfluoro-n-alkanes in a jet.249 They showed that there are systematic microscopic solvent shifts of the S2 origin bands depending on the number of carbon atoms in n-alkanes in the 1:1 complexes, although the shift is small for perfluoro-nalkanes.249 Excited-state dynamics have been investigated for arylthioketones in solution and in the vapor phase including in a jet.251−257 Motyka and Topp measured the S0 → S2 excitation spectra by monitoring the S 2 fluorescence and the T1 phosphorescence for jet-cooled xanthione.252 It was demonstrated that the phosphorescence/fluorescence intensity ratio is affected both by predissociational cooling of T1 and by nonradiative deactivation of S2 and that there is modespecificity of the nonradiative process in S2.252 The S2 fluorescence lifetime of thiocoumarin was reported to be 450 ± 50 fs in n-hexane and 130 ± 50 fs in acetonitrile, while the S1 lifetime was reported to be ∼10 ps.255 Burdzinski et al. investigated orientational anisotropy decay of benzopyranthione in S2 in hydrocarbons and showed that the dependence of the rotational time constant on the solvent viscosity is approximately linear.256 The S2 fluorescence lifetime of jetcooled xanthione was reported to be 240−370 ps following excitation into different vibrational levels in S2.257 Maciejewski et al. determined the S3 → S2, S3 → S1, S3 → S0, and S2 → S1 internal conversion yields [ΦIC(S3 → S2), ΦIC(S3 N

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Figure 13. Typical metalloporphyrins that exhibit S2 fluorescence. M indicates an appropriate metal ion.

→ S1), ΦIC(S3 → S0), and ΦIC(S2 → S1), respectively], using a method based on the steady-state measurements of absorption, T1 phosphorescence, and S2 or S1 fluorescence along with selective excitation to the S2 and S3 states for 4H-1-benzopyran4-thione, xanthione-d0, and xanthione-d6 in perfluoroalkane.258 They obtained ΦIC(S3 → S2) = 0.9, 0.72, and 0.74; ΦIC(S3 → S1) = 0, 0.04, and 0.11; ΦIC(S3 → S0) = 0.1, 0.24, and 0.15; and ΦIC(S2 → S1) = 0.96, 0.85, and 0.88, respectively, for xanthione-d6, xanthione-d0, and 4H-1-benzopyran-4-thione. These results indicate that the deactivation of the S3 state is not strictly sequential and that the S3 → S1 and S3 → S0 internal conversions occur with relatively high yields. Other than the aforementioned molecules, several thiocarbonyl compounds such as 4-thiouridine have also been reported to show S 2 fluorescence. 259 In addition, S 2 fluorescence resulting from triplet−triplet annihilation was observed for 4H-1-benzopyran-4-thione in perfluoro-1,3dimethylcyclohexane.260

Figure 14. Absorption (solid-line curves) and fluorescence (dottedline curve) spectra of ZnTPP (Zn tetraphenylporphyrin) in ethanol.268

weakness of the emission.266 Absorption and fluorescence spectra of a typical metalloporphyrin (ZnTPP) are shown in Figure 14. The reported quantum yields and lifetimes of the S2 fluorescence of metalloporphyrins range from 10−3 to 10−5 and from 0.1 to 5 ps, respectively.266−288 The S2 and S1 fluorescence quantum yields of metalloporphyrins depend on the porphyrin ring structure, the metal ion, and the solvent used. Kotlo et al. reported that addition of a compound containing a heavy atom (CH3I) to the solvent decreases S1 fluorescence but does not decrease S2 fluorescence for Zn-, Mg-, Cd-TBP, Zn-tetraphenylporphine, Zn-tetrapropylporphine, nor Zn-monoazatetrabenzoporphine.279 Martarano et al. measured emission spectra of Y3+, Lu3+, and Th4+ complexes of tetraphenylporphyrin (TPP) and showed that the S2 fluorescence yields are essentially the same for the three complexes, although the S1 fluorescence yield exhibits a clear heavy-atom dependence.271 These observations indicate that the main nonradiative process from S2 is the internal conversion to S1, whereas that from S1 is intersystem crossing, possibly to T1. The emission properties of metalloporphyrins containing lanthanide ions were investigated in detail in conjunction with the heavy-atom effects. Tsvirko et al. investigated the emission properties of complexes of tetra-p-tolylporphyrins with lanthanide ions (Sm3+, Eu3+, Gd3+, Tb3+, Yb3+, and Lu3+) in ethanol.272 The quantum yields of the S2 fluorescence of these complexes were found to be almost in the same order except for the complexes containing Yb and Eu.272 The low S2 fluorescence quantum yields for the complexes with Yb and Eu ions were interpreted in terms of the charge-transfer states

4.5. Metalloporphyrins

Metalloporphyrins are biologically important systems possessing a π-electron cyclic-conjugation system with a metal ion inside the porphyrin ligand. These complexes exhibit characteristic absorption bands, called Q and B (Soret) bands, in the visible (500−700 nm) and near-ultraviolet (400−500 nm) regions, respectively, which are considered to correspond to the S0 → S1(π, π*) and S0 → S2(π, π*) transitions. Several metalloporphyrins and their derivatives are known to exhibit both S2 and S1 fluorescence in addition to T1 phosphorescence. The values for ΔE(S2 − S1) of metalloporphyrins are normally about 8 000 cm−1, with the oscillator strength of the S2 state being much larger than that of S1. Thus, the excited-state features of metalloporphyrins resemble those of azulene. Typical metalloporphyrins that exhibit S2 fluorescence are shown in Figure 13. Bajema et al. first observed the S2 fluorescence of zinc tetrabenzoporphyrin (ZnTBP) in octane and in argon matrices in 1971 along with the S1 fluorescence.261 Later, S2 fluorescence was observed for a variety of metalloporphyrins and their derivatives, as will be described later. The photophysics of porphyrins was reviewed in 1986 by Gouterman et al.262 Although much of the evidence for S2 fluorescence has been observed for metalloporphyrins, extremely weak S2 fluorescence emissions have also been reported for a free-base tetraphenylporphyrin (H2TPP), which does not possess a metal ion.263−265 The S2 fluorescence was not detected from freebase porphyrins in early studies, probably due to the extreme O

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Figure 15. Some of the lesser-known molecules that are reported to show fluorescence from higher excited states.

lying between the S1 and S2 states.272 Later, Tsvirko et al. also investigated the emission properties of lanthanide ion (Sm3+, Eu3+, Gd3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, and Lu3+) complexes of tetrabenzoporphyrin (TBP) in ethanol. 280 The S 2 fluorescence decay rate and quantum yield were found to vary from 3 × 1011 to >2 × 1013 s−1 and from 1.45 × 10−3 to near zero, respectively, depending on the lanthanide ion.280 There seems to be no significant correlation between the S2 fluorescence properties and the identity of the lanthanide ion, whereas there is a linear relationship between the T1 level and the deactivation rate constant of the T1 state.280 The applicability of the energy-gap law to the nonradiative decay of the S2 state of metalloporphyrins has been discussed and examined by a number of researchers.281−287 In the case of metalloporphyrins, however, there are a number of parameters which influence the ΔE(S2 − S1) values, such as the metal ion species and the structure of porphyrin ligand as well as the solvent used. Kokubun and co-workers reported a linear relationship between the ΔE(S2 − S1) value and the logarithmic value of the nonradiative decay rate of S 1 for AlCl tetraphenylporphyrin (AlClTPP), ZnTPP, and CdTPP.281 Steer and co-workers showed that there is a linear correlation between ΔE(S2 − S1) and the solvent polarizability for ZnTPP and MgTPP.282−286 They have shown that the energy-gap law concerning the internal conversion from S2 applies for these metalloporphyrins in solvents with different polarizabilities.282−286 The main nonradiative path of the S2 state is considered to be the S2 → S1 internal conversion with almost unit efficiency for most metalloporphyrins.284 Fujitsuka et al. showed for Zn porphyrin polypeptides with different peptide lengths that the S2 → S1 internal conversion rate obeys the energy-gap law.287 In the case of Zn porphyrin polypeptides, the ΔE(S2 − S1) value changes depending on the degree of polymerization of the polypeptides attached to the metalloporphyrin, so that the relationship between the S2 → S1 internal conversion rate and ΔE(S2 − S1) could be investigated systematically.287 In any case, the energy-gap law for the S2 → S1 internal conversion rate seems to apply for metal-

loporphyrins under well-designed conditions where only one parameter is varied. Recently, Steer and co-workers reported S2 fluorescence of ZnTPP in an ionic liquid at room temperature, which is essentially similar to that observed in common solvents of the same polarizability.286 Gustavsson and co-workers demonstrated that the time constant (2.4 ps) of the decay of S2 fluorescence agreed with that of the rise in S1 fluorescence for ZnTPP in ethanol.267 Mataga et al. obtained similar results for ZnTPP and Zndiphenylporphyrin derivatives in solution.268 They observed also weak hot fluorescence from unrelaxed vibronic levels in S1 formed immediately after the S2 → S1 internal conversion.268 Steer and co-workers investigated the effects of perdeuteration, substituent nature and position, and macrocyclic structure and conformation on the dynamics of a variety of metalloporphyrins and showed that the rate of increase in S1 population equals the rate of S2 population decay.284 Although most of the spectroscopic measurements of metalloporphyrins were carried out in solution or rigid matrices, excitation spectra with a well-resolved vibrational structure in a jet were reported for MgTPP in the S0 → S2 transition region.288 S2 fluorescence resulting from triplet− triplet annihilation was also observed for MgTPP, ZnTPP, CdTBP, and SnCl2TPP in solution,285,289 and for ZnTPP in a polymer matrix.290 Further, the S2 fluorescence observed by means of stepwise two-photon excitation through the S1 state was reported for ZnTPP.269 S2 fluorescence has been observed for a number of metalloporphyrins and their derivatives other than those mentioned herein. Hirakawa et al. investigated quenching of S2 fluorescence for Zn 5-(1′-pyrenyl)-10,15,20-triphenylporphyrin derivatives.291 Steer and co-workers investigated quenching of S1 and S2 fluorescence by halide ions (Cl−, Br−, and I−) for Zn tetrakis(4-sulfonatophenyl)porphyrin.292 S2 and S1 fluorescence were observed also for Al and Ga corroles,293 metalloporphyrin analogues, and metalloporphyrins and their derivatives such as SbTPP,295 a J-aggregate of 5,10,15,20tetraphenyl-21H,23H-porphinetetrasulfonic acid,278 Ga hydroxP

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The excited-state ordering of this molecule is S2(π, π*) > S1(n, π*) > T2(n, π*) > T1(π, π*). In a rigid matrix at 77 K, only the T1(π, π*) phosphorescence is observed, but at room temperature both the S2(π, π*) and S1(n, π*) fluorescence are observed in degassed CCl4 solution. Since the ΔE(S1 − S2) value of 1,4-anthraquinone is only 1 100 cm−1, the occurrence of the S2 fluorescence is considered to be due to thermal activation from the S1 state. S2 fluorescence was observed for isoquinoline vapor, the ΔE(S2 − S1) value of which is extremely small (∼1100 cm−1).311 Because of the small S2(π, π*) − S1(n, π*) energy gap of isoquinoline, the level densities of S1 near the S2 origin are expected to be rather small.311 Thus, internal conversion from the low-lying vibrational levels of S2 to S1 does not occur in the absence of collisional deactivation of the vibrationally excited S 1 , which accounts for the observation of S 2 fluorescence from low vibrational levels of S2.311 In addition, 1,2,3,4-tetrahydroisoquinoline is also reported to show S2 fluorescence in addition to S1 fluorescence and T1 phosphorescence in solution.312 Other molecules showing fluorescence from higher excited states are listed below. Weak S2 fluorescence was reported for azobenzene.313,314 S2 → S0 fluorescence was observed for several triphenylmethane dyes,315 for which occurrence of the S2 fluorescence was interpreted in terms of the large ΔE(S2 − S1) values (14 000 cm−1). Fluorescence from the S2(π, π*) state is reported for o-hydroxybenzaldehyde vapor, the quantum yield of which is ∼10−4.316 The occurrence of S2 fluorescence of o-hydroxybenzaldehyde was interpreted in terms of the large ΔE(S2 − S1) value (14 300 cm−1).316 S2 and S1 fluorescence emissions were observed for 1,1,1-trifluoro-3-bromoacetone (CF3COCH2Br) vapor with an S2/S1 fluorescence quantum yield ratio of 0.1.317 The ΔE(S2 − S1) value of this molecule is ∼10 000 cm−1.317 Some thiophene compounds (e.g., 1-cyano-2[5-[2-(1,2,2.4-tetramethyl-1,2,3,4-tetrahydroquinolin-6-yl)]thiophen-2-yl]vinylphosphonic acid diethyl ester) were reported to show S2 fluorescence, the occurrence of which can be explained in terms of the large ΔE(S2 − S1) value.318 The S2 fluorescence of [18]annulene and the S2 and S1 fluorescence of a monofluorinated annulene derivative were observed in 3methylpentane at 77 K.319 The S2 and S1 states of [18]annulene are located at 21 400 and 12 500 cm−1 with the S2/S1 oscillator strength ratio of ca. 200. [18]Annulene may be more accurately included in the polyene family of molecules, but it is tentatively included in the category of other fluorescent molecules. Fullerenes (C70) have been reported to show the fluorescence from the closely located S1(1T1g), S2(1T2g), and S3(1Gg) states in a Ne matrix at low temperature.320,321 Fluorescence from the S1(n, π*) and S2(π, π*) states was observed for 3,6-diphenyl-stetrazine along with weak phosphorescence from T1(n, π*).322 The ΔE(S2 − S1) value of this molecule is ∼8 300 cm−1. The occurrence of the S2 fluorescence of diphenyl-s-tetrazine was interpreted in terms of slow S2 → S1 internal conversion due to the large ΔE(S2 − S1) value.322 Naphtho[1,8-cd][1,2,6]thiadiazine and its derivatives, the ΔE(S2 − S1) values of which are 16 000 cm−1, are reported to show S2 fluorescence.323 A ketocyanine dye, 2,5-bis[(2,3-dihydroindolyl)propylene]cyclopentanone, is reported to show dual fluorescence from the S2 and S1 states in various solvents.324 The observed S2 fluorescence of this dye is reported to be weak in solution at room temperature but can be intensified in rigid matrixes at 77 K. The appearance of the S2 fluorescence of this dye was interpreted in terms of the large ΔE(S2 − S1) value, which

yl tetra-para-tolylporphyrin,294 Er hydroxyl TPP,274 AlCl-, GaCl-, and InCl-TPP,266 Zn 5-(1′-pyrenyl)-10,15,20-triphenylporphyrin derivatives,291 SbTPP derivatives,295 Zn tetrakis(4sulfonatophenyl)porphyrin,292 and Cu and Zn 5,10,15,20tetrakis[4-(1-octyloxy)phenyl]porphyrins.296 Photoinduced intramolecular electron transfer processes have been investigated by monitoring the S2 fluorescence of metalloporphyrins for a number of directly linked metalloporphyrin dyads or derivatives.295,297−302 Intermolecular electron transfer processes from ZnTPP to Cl2CH2 also were studied by monitoring the S2 fluorescence of metalloporphyrins.277 There exist a wealth of spectroscopic data for a variety of metalloporphyrins. The general consensus, so far recognized in considering the photophysical processes from the S2 state, is that S2 fluorescence is prompt fluorescence and that the main deactivation process of the S2 state is S2 → S1 internal conversion, which presumably obeys the energy-gap law. 4.6. Other Fluorescent Molecules

Other than the molecules described above, there are also a number of perhaps lesser-known molecules from which fluorescence from higher excited states has been observed. Chemical structures of some of these molecules are provided in Figure 15. Wakasa et al. observed the S2 (π, π*) and S1 (n, π*) fluorescence from silyl and germyl ketones (PhnMe3−nSiCOPh and PhnMe3−nGeCOPh with n = 0, 1, or 2) at room temperature in solution.303 The ΦF(S1) and ΦF(S2) values of these ketones are reported to be approximately 10−3 and 10−4, respectively.303 Since the ΔE(S1 − S2) values of silyl and germyl ketones can be 11 000 cm−1, the excited-state feature is similar to that of azulenes, except that the S1 state is 1(n, π*) in nature. Fluorescence of diphenylacetylene (DPA) and some of the derivatives of DPA is recognized as an emission from the S2 state.304 DPA belongs to the D2h point group, but the symmetry of the low-lying excited state does not seem to be well understood.304 The location and ordering of the low-lying singlet excited states of DPA have been revealed by measuring one-photon excitation and two-photon resonant ionization spectra in a jet as well as one- and two-photon excitation spectra in low-temperature matrices.305,306 The state ordering of the low-lying excited states of DPA and its derivatives changes depending on the solvent as well as the substituents on the phenyl rings. It was suggested that in the condensed phase such as in EPA the fluorescing state of DPA is optically allowed S1(11B1u).307 In the case of 4-methoxy-DPA, hydroxy-DPA, chloro-DPA, N,N-dimethylanilino-DPA, and aminophenylDPA, the S2 and S1 states are assigned as the fluorescent 1B1u and nonfluorescent 1Ag or 1B2u, respectively, whereas the S1 states of cyano-DPA and CH3COO-DPA are considered to be the fluorescent 1B1u states.308,309 In the vapor phase, the allowed S2(11B1u) state is located slightly higher in energy than the forbidden S1 state.309 In room-temperature solution, the emission of DPA consists solely of the S2 fluorescence.304,309 The ΔE(S2 − S1) values were evaluated for some of the derivatives of DPA through the temperature dependence of the S2 fluorescence decay time.309 The photophysical and photochemical processes of DPA derivatives were reviewed by Hirata.304 1,4-Anthraquinone is reported to show weak fluorescence from the S2(π, π*) state in addition to the S1(n, π*) fluorescence in degassed CCl4 solution at room temperature.310 Q

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varies from 3 100 to 4 000 cm−1 depending on the solvent used as well as the sparse level density of the active modes for the S2 → S1 internal conversion. Biphenylene was also reported to show S1 and weak S2 fluorescence in solution.325,326 In addition, S2 and S3 fluorescence occurring through the triplet−triplet annihilation (T1 + T1 → Sn (n = 2 and 3) + S0) was reported for 9,10-anthraquinone and xanthone in solution.327 Finally, T2 → T1 fluorescence was observed for 9-bromo- and 9,10-dibromoanthracene in heptane at room temperature.328 Although there are a number of lesser-known molecules that show anomalous emission as mentioned above, we are not yet convinced that all of the reported data are nontrivial. Some of the reported data do not include even the excitation spectrum of the observed emission, required for the confirmation of the results.313,315,317,322,324,326 Nevertheless, the published data themselves may warrant attention as the basis of future work.

Table 2. Several Molecules Showing the Phosphorescence from Higher Excited Triplet States along with the Emitting Statesa molecules

emitting states

p-benzoquinone 1,4-naphthoquinone p-methoxybenzaldehyde p-cyanobenzaldehyde chlorobenzene p-dichlorobenzene p-dibromobenzene p-bromotoluene p-chlorotoluene fullerene (C70)

S1(n, π*), T2(n, π*), T1(n, π*) S1(n, π*), T2(n, π*), T1(n, π*) S1(n, π*), T2(n, π*), T1(π, π*) S1(n, π*), T2(n, π*), T1(π, π*) S1(π, π*), 3(π, π*), 3(π, σ*) S1(π, π*), 3(π, π*), 3(π, σ*) 3 (π, π*), 3(π, σ*) 3 (π, π*), 3(π, σ*) 3 (π, π*), 3(π, σ*) S1(π, π*), T2(π, π*), T1(π, π*)

references 352, 354, 331, 332, 356, 356, 356 361 360 364

353 355 332 333 358 358

a

Normally, the two or three types of such emissions cannot be observed simultaneously. For example, only the T1 phosphorescence can be observed at 77 K for p-cyanobenzaldehyde, but the S1 fluorescence and T2 phosphorescence appear gradually with increasing temperature.

5. PHOSPHORESCENCE FROM HIGHER EXCITED STATES In contrast to molecules showing fluorescence from higher excited states, only a small number of examples of molecules that show phosphorescence from higher triplet states are known. This is in part due to the difficulties inherent in detecting phosphorescence emission and in the separation of T1 and T2 phosphorescence emissions. Further, the number of phosphorescent molecules is in general smaller than that of fluorescent molecules. Normally, phosphorescence has been observed in matrices at low temperature. To observe the phosphorescence emission in fluid solution or in the vapor phase at temperatures near room temperature, the removal of molecular oxygen that is a triplet in nature (3Σg−) from the sample is necessary. During the 1980s, experimental data on the phosphorescence from higher excited states were accumulated. At present over 17 molecules are reported to exhibit T2 phosphorescence, of which ∼10 molecules seem to be accompanied by reasonable data to justify the occurrence of T2 phosphorescence. Several of the molecules showing T2 phosphorescence are shown in Figure 16. Typical molecules

showing phosphorescence from higher excited states are listed in Table 2 along with the emitting states. To the best of our knowledge, only the emission from the T2 state has been reported as phosphorescence from higher excited states, although that from the S3 or S4 state has been reported for fluorescence emission.116,121,197 Furthermore, except for halogenated benzenes or toluenes, most of the observed T2 phosphorescence is considered to originate from the thermal population from the T1 state. In the case of halogenated benzenes, the observed dual phosphorescence was interpreted as consisting of slow and fast decay components corresponding to two different transitions. 5.1. Aromatic Carbonyl Compounds

The T1 states of some benzaldehydes possessing an electrondonating group in the para position, such as p-methoxybenzaldehyde, are attributed to be 3(π, π*) in nature.329,330 In such molecules, the excited state ordering is S1(n, π*) > T2(n, π*) > T1(π, π*), along with the comparatively small T2 − T1 energy separation, ΔE(T2 − T1). Some of the derivatives of benzaldehyde with such an excited-state ordering have been reported to exhibit phosphorescence from the T2 state. These are p-methoxy- and p-cyanohydroxybenzaldehyde.331−333 The reason for the occurrence of the T2 phosphorescence is that the T2 and T1 states are located close to each other, with the oscillator strength of the former state being much higher than that of the latter. Thus, at temperatures where the Boltzmann distribution in the T1 state is sufficient to populate the upper excited state, the T2 phosphorescence and even S1 delayed fluorescence are observable through the thermal population from the T1 state. On the other hand, in the case of benzaldehyde derivatives with excited-state ordering of S1(n, π*) > T2(π, π*) > T1(n, π*), we can observe only the T1 phosphorescence and S1 delayed fluorescence. In Figure 17, emission spectra of p-cyanobenzaldehyde in pdichlorobenzene host at different temperatures are displayed. The emission at −196 °C (77 K) consists mostly of the phosphorescence from the T1(π, π*) state with the origin band seen at ∼23 700 cm−1 (422 nm). The emission at temperatures near room temperature is the phosphorescence from the T2(n, π*) state with the origin seen at ∼24 150 cm−1 (414 nm). The T2 phosphorescence exhibits a prominent progression in the CO stretching vibration with an interval of ∼1 650 cm−1.

Figure 16. Some of the molecules that are reported to exhibit T2 phosphorescence. R

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π*) phosphorescence origin agrees with that of the S0 → T2 absorption origin. (iii) The temperature-dependent spectral change was observed even at temperatures near room temperature, although normally the site effect is not observable at higher temperatures. (iv) The T1 and T2 emission origins are separated by ∼450 cm−1, which is too large to be the value for the site splitting. In the vapor phase at temperatures near room temperature, p-methoxy- and p-cyanobenzaldehyde are reported to show the T2(n, π*) phosphorescence and the S1(n, π*) fluorescence.332 Increasing the temperature of these vapor samples results in an increase of the T2 phosphorescence quantum yield. Therefore, the T1 phosphorescence is masked by stronger T2 phosphorescence at higher temperatures due to the increased thermal population of T2 from T1. This observation is markedly different from that for benzaldehyde vapor whose triplet-state ordering is T2(π, π*) > T1(n, π*). In the case of benzaldehyde vapor, the T1(n, π*) phosphorescence yield decreases with increasing temperature. Invisible energy levels of the T1(π, π*) state of p-methoxy and p-cyanobenzaldehyde vapors were estimated through the analyses of the temperature dependence of the T2(n, π*) phosphorescence and S1(n, π*) delayed fluorescence intensities.332 These energy levels were found to be in reasonable agreement with those obtained in rigid matrices at 77 K.332 Anomalous emission, which can be assigned to T1(n, π*) and T2(π, π*) dual phosphorescence, was reported also for pchlorobenzaldehyde in methylcyclohexane at liquid He temperature334 and for p-chlorobenzaldehyde and p-bromobenzaldehyde in p-dibromobenzene or in p-dichlorobenzene at 105 to 172 K.335 Hayashi and Nagakura assigned the observed T2 phosphorescence of p-chlorobenzaldehyde and p-bromobenzaldehyde as thermally activated emission.335 Further, dual phosphorescence from 3(n, π*) and 3(π, π*) states was reported for 2,4,5-trimethylbenzaldehyde and 2,4-, 2,5-, and 3,4-dimethylbenzaldehydes in durene at low temperature.336,337 However, the observed emission of 2,4,5-trimethylbenzaldehyde was considered to arise from the site effects or rotational isomers.338 Xanthone was reported to show dual phosphorescence from the closely located T2(n, π*) and T1(π, π*) states.339−341 Thermally activated T2 phosphorescence in polyethylene films was also reported for xanthone.342 However, it is likely that most of the observed dual phosphorescence can be attributed to intermolecular interactions or site effects.341,343 The emission of 1-indanone and its derivatives was once thought to be an example of dual phosphorescence from the T2(n, π*) and T1(π, π*) states. Phosphorescence spectra of these molecules are known to consist of two spectral components with short and long lifetimes, with the former exhibiting progressions of the CO stretching vibration. However, it was reported later that the observed emission can be attributed to the results of the intermolecular interactions or photochemical byproduct (vide infra). As the history of dual phosphorescence of 1-indanones is somewhat complicated, it is described here in detail. Yang and Murov first reported dual phosphorescence showing different lifetimes in rigid matrices at 77 K for 1-indanone and assigned the slow and fast components to phosphorescence from the 3(π, π*)-like and 3 (n, π*) states, respectively.344 Long and Lim later confirmed this observation for 1-indanone and showed that the phosphorescence arises from two close-lying triplet states in nonpolar solvents by measuring the phosphorescence spectra at

Figure 17. Emission and excitation spectra of p-cyanobenzaldehyde in a p-dichlorobenzene matrix at different temperatures.333 The excitation spectrum was measured at 20 °C. The emission spectra are normalized to a common magnitude.

Closer inspection of the spectra reveals that at elevated temperatures there is also a weak band at ∼25 700 cm−1 (389 nm), which is assigned to the S1(n, π*) fluorescence. Successive occurrence of the T1(π, π*) and T2(n, π*) phosphorescence and S1(n, π*) fluorescence with increasing temperature is seen clearly in Figure 17. In this case, the quantum yield ratio of the T2 to T1 phosphorescence, ΦT2/ΦT1, is expressed approximately by ΦT2/ΦT1 = kT2 exp[−ΔE(T2 − T1)/kBT]/kT1, where kB is the Boltzmann constant and kT1 and kT2 are, respectively, the radiative rate constants of the states T1 and T2. The values for ΔE(T2 − T1) obtained from the temperature dependence of the T1 and T2 phosphorescence intensities agree well with the spectroscopically estimated energy separations. Although the observed emission originates from two or three different excited states, the observed decay profiles cannot be multiexponential, because these excited states are in thermodynamic equilibrium: the observed emission decay rate, kobs (=1/τobs), at a particular temperature T is given by333 kobs = [kP1 + kNP1 + (kP2 + kNP2) × exp( −ΔE(T2 − T) 1 /kBT )]/[1 + exp(−ΔE(T2 − T)/ 1 kBT )]

(1)

where kP1 and kNP1 are, respectively, the radiative and nonradiative rate constants of T1 and kP2 and kNP2 are those of T2. The value of τobs is 0.9 s at −196 °C, but it is shortened to 6.3 × 10−2 s at −71 °C. By fitting the phosphorescence lifetime data to eq 1, the obtained ΔE(T2 − T1), kT1, and kT2 values were shown to be in reasonable agreement with the spectroscopically estimated ΔE(T2 − T1) and the radiative rate constants for the 3(π, π*) and 3(n, π*) states, respectively, for cyanobenzaldehyde.333 In general, it is not easy to distinguish the dual emission from two different excited states from that caused by a site effect for the samples in a solid matrix. In the case of p-cyano- and pmethoxybenzaldehydes, however, the observed phosphorescence is considered to originate from the two different excited states. This is based on the following observations: (i) The shapes of the two types of phosphorescence emission differ from each other, because the T1 and T2 states are of (π, π*) and (n, π*) types, respectively. (ii) The location of the T2(n, S

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different temperatures ranging from 20 to 38 K.345 Amrein et al. analyzed the emission spectra of 1-indanone and six annelated 1-indanones and showed that the two spectral components can be selectively excited, which excludes a thermal equilibrium between the two triplet states.346 Kanda et al. observed three spectral components in the phosphorescence of 1-indanone at 77 K but only one component for 2,2-dimethyl-1-indanone.347 On the basis of the solvent effect of the relative emission intensities, they assigned the short-lived component to the phosphorescence from 1-indanone and the long-lived one to that of enolate anions.347 However, Chu and Kearns ruled out emission from enolate ions and enols as well as that from thermally populated upper triplet states by measuring the phosphorescence and phosphorescence excitation spectra at 77 K.348 They interpreted the dual phosphorescence of 1-indanone and 1-tetralone in terms of photochemically produced byproduct. By means of selective photobleaching of the phosphorescence emission, Wild and co-workers showed that the dual phosphorescence of 1-indanone and its two derivatives in a glassy ethanol matrix at 77 K originates from two different species: a short-lived 3(n, π*)-like phosphorescence was attributed to the emission from free molecules, and a longlived 3(π, π*)-like phosphorescence was attributed to that from indanone hydrogen-bonded to ethanol.349 More recently, Nakayama et al. investigated the phosphorescence of 1indanone in protic solvents containing water at 77 K.350 They ascribed the short-lived emitting species to 1-indanone hydrogen-bonded to trifluoroethanol, and the long-lived species to 1-indanone hydrogen-bonded with varying numbers of water molecules.350 In summary, appearance of the dual phosphorescence of 1-indanones seems to be related to the intermolecular interactions or photochemical byproduct. Dual phosphorescence was reported also for propiophenone and butyrophenone in a rigid matrix at 77 K, but the occurrence of the emission was interpreted in terms of the conformational difference caused by cage effects.351

Figure 18. Emission spectrum of 1,4-naphthoquinone vapor.354

allowed character, whereas the origin band of the T 1 phosphorescence is missing as observed in the phosphorescence of 9,10-anthraquinone.7 The ΔE(T2 − T1) value of 1,4naphthoquinone vapor obtained from the temperature dependence of the T2/T1 phosphorescence intensity ratio agrees with that estimated spectroscopically (180 cm−1).354 Semiempirical MO calculations along with group theory considerations indicated that T1 and T2 are assigned to 3B1 and 3A2, respectively. Thus, p-benzoquinone and 1,4-naphthoquinone can be regarded as unique molecules in the sense that these show three kinds of emission from the S1(n, π*), T2(n, π*), and

5.2. Quinones

Because of the presence of two CO groups, quinone molecules possess two nonbonding orbitals, n+ and n−, almost localized on the two oxygen atoms. Thus, there are always two types of 3(n, π*) states in quinones, 3(n+, π*) and 3(n−, π*). The two low-lying triplet (n, π*) states of p-benzoquinone vapor are energetically very close to each other, and the observed phosphorescence emission in the vapor phase is considered to originate from the T2(3Au: n, π*) state.7 It was reported also that the two 3(n, π*) states of p-benzoquinone in a neon host are essentially degenerate, with a splitting of only 64 cm−1, and that dual phosphorescence from the T2(3Au) and T1(3B1 g) states is observed at low temperatures.352 Further, in carefully degassed CCl4 solution and in p-dichlorobenzene matrix at temperatures near room temperature, p-benzoquinone was reported to exhibit T1(n, π*) and T2(n, π*) phosphorescence in addition to the weak delayed S1(n, π*) fluorescence.353 The ΔE(T2 − T1) value in these hosts is ∼200 cm−1, and the appearance of the T2 phosphorescence was ascribed to the thermal population of T2 from T1.334,353 In the case of 1,4-naphthoquinone, phosphorescence from the T1(n, π*) and T2(n, π*) states was observed clearly in the vapor phase.354 The phosphorescence of 1,4-naphthoquinone vapor exhibits a doublet structure in the CO stretching vibration bands corresponding to emission from the two (n, π*) triplet states (Figure 18). The T2 phosphorescence shows

Figure 19. Emission spectra of p-benzoquinone in a p-dichlorobenzene matrix at different temperatures.353a Data taken from ref 353a.

T1(n, π*) states depending on the situation (Figures 18 and 19). In contrast to 1,4-naphthoquinone, its 2-methyl derivative (vitamin K1) exhibits only T1(n, π*) phosphorescence and S1(n, π*) delayed fluorescence.355 The emitting states of benzaldehyde derivatives and quinones are illustrated in Figure 20 along with the energy levels. 5.3. Halogenated Aromatic Compounds

Halogenated benzenes and toluenes such as p-dichlorobenzene, p-chlorotoluene, and p-bromotoluene are reported to show dual phosphorescence from the 3(π, π*) and 3(π, σ*) states, T

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Figure 20. Schemes showing the emission processes of benzaldehyde derivatives and quinones. F1, P1, and P2 represent S1 fluorescence and T1 and T2 phosphorescence, respectively.

Figure 21. Schemes showing the three intramolecular mechanisms for occurrence of fluorescence emission. F1 and F2 represent S1 and S2 fluorescence, respectively.

356−361

although it is not clear which is the T1 or T2 state. Dual phosphorescence of these molecules was observed in rigid matrices at low temperatures. In these systems, the observed dual phosphorescence was interpreted as being composed of slow and fast decay components corresponding to the 3(π, π*) → S0 and 3(π, σ*) → S0 transitions, respectively. Because the observed spectra are broad and because the two spectra are nearly coincident, each phosphorescence spectrum could be separated through time-resolved measurements. The excitation spectra of the time-resolved phosphorescence are reported to agree with the corresponding absorption spectra. 356,357 Quantum chemical calculations indicate that the 3(π, π*) and 3 (π, σ*) states are located close to each other and that the C− Cl equilibrium bond length in 3(π, σ*) is longer than that in 3 (π, π*).362 Qualitative features of the calculated results are found to be consistent with those suggested on the basis of the phosphorescence properties.362,363

under collision-free conditions (in this case the higher and lower singlet states are mixing); (C) directly from the higher singlet state without involvement of the fluorescence component via reverse internal conversion from the lower singlet state, i.e., prompt fluorescence. Mechanisms (A) and (B) resemble each other, but in case (B) the fluorescence originates from unrelaxed vibronic levels of the excited state and it disappears in condensed phases or under high-pressure conditions where the collisional deactivation occurs effectively. In case (A) the relative quantum yield of the S2 fluorescence tends to increase with decreasing ΔE(S1 − S2) value, whereas in case (C) the relative quantum yield of the S2 fluorescence tends to decrease with decreasing ΔE(S1 − S2) due to the energy-gap law for the S2 → S1 internal conversion. Further, in case (B) the S2 fluorescence quantum yield in the vapor phase normally tends to increase with decreasing pressure, whereas in case (C) it is almost invariant against pressure. The S2 fluorescence of diphenylhexatriene and diphenyloctatetraene as well as pyrene in solution corresponds to case (A). The ΔE(S1 − S2) values of these molecules are normally 1.5. S2 fluorescence of most of aromatic acenes of medium to large size such as pyrene and benzanthracene in the vapor phase at low pressure corresponds to case (B). For these molecules, the ΔE(S1 − S2) value is normally 10. The mechanism for the appearance of T2 phosphorescence was discussed also by Chu and Goodman.371,372 In the case of halogenated benzenes or toluenes, the occurrence of dual phosphorescence was interpreted using a relaxation scheme somewhat similar to case (B). The idea originated from the observation that the phosphorescence decay curves are biexponential. The observed dual phosphorescence was interpreted as consisting of slow and fast decay components corresponding to the 3(π, π*) → S0 and 3(π, σ*) → S0 transitions, respectively. Strictly speaking, however, the mechanism of the occurrence of the dual phosphorescence does not apply to any of the cases (A), (B), or (C) for these molecules. Thus, it appears necessary to investigate the emission properties in more detail. Even if a particular molecule satisfied the conditions that the emission from the upper state can be observable, as mentioned above, it cannot always be said that the molecule actually shows the emission from higher excited states. Whether or not the emission from higher excited states is observable depends also on other factors such as the stability of the molecule with respect to photon irradiation and relative rates of the radiative and nonradiative processes. For example, even if a molecule possesses a large ΔE(S1 − S2) value, it may be difficult to detect the S2 fluorescence when the S2 → S1 internal conversion rate is

much faster than the radiative rate of S2 by a factor of >105. In this sense, the emission from higher excited states can be considered as anomalous or rare and provides an opportunity to reveal the dynamic behavior of the molecules in the excited states.

7. SUMMARY Considerable progress has been made during these three or four decades in the accumulation of the anomalous emission spectral data of organic molecules, including the emission from higher excited states. In particular, experiments in a supersonicjet expansion provided information on the intrinsic photophysical properties of cooled and isolated molecules in which the Boltzmann distribution is almost negligible. Organic molecules showing emission from higher excited states form unique systems to which a number of modern techniques should be applied to obtain more detailed information on their photophysical properties. These data will provide useful information for understanding the dynamical behavior of isolated molecules in their excited states. There seems to be at least three mechanisms for occurrence of fluorescence from higher excited states: (A) the fluorescence occurs through the thermal repopulation of the lower excited state, e.g., S1; (B) it occurs via the reverse internal conversion from the lower singlet state under collision-free conditions; and (C) it occurs directly from the higher singlet state without involvement of the fluorescence component via the reverse internal conversion from the lower singlet state, i.e., prompt fluorescence. In case (C), the internal conversion rate from the higher singlet state to the lower state is considered to be slow enough to compete with the fluorescence from the higher state. In terms of phosphorescence from higher triplet states, there is mainly one mechanism corresponding to case (A) for the occurrence of the emission, except for the case of halogenated benzenes, for which dual phosphorescence from the 3(π, π*) and 3(π, σ*) states was observed. That is, in most cases the phosphorescence from the higher triplet excited state occurs as the result of the thermal activation of the lower triplet state. Further, at present only the T2 phosphorescence is known as an observed emission from the higher triplet excited state. APPENDIX A Under collision-free conditions, the relaxation processes of pyrene vapor excited into the S3 or S4 state can be interpreted by the kinetic scheme shown in Figure 22. Since the molecule is free from collisions during the excited-state lifetime, its energy is conserved throughout the internal conversion processes. Thus, the energy in the molecule that has reached high vibrational levels in the S1 state, S1*, may return to isoenergetic levels in the S2 state, S2*, through the reverse S1* → S2* internal conversion with the rate constant expressed by k21. On the basis of the kinetic model shown in Figure 22, the fluorescence quantum yields of the S1 and S2 fluorescence, ΦF(S1) and ΦF(S2), are expressed in the forms107 ΦF(S1) = (k 21/β)[kF1/(kF1 + kFQ1 + k12)]

(A1a)

ΦF(S2 ) = kF2/β

(A1b)

where β = kF2 + kFQ2 + k 21[(kF1 + kFQ1)/(kF1 + kFQ1 + k12)]

The ΦF(S2) value can be divided into two parts: V

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the π → π* transitions are concerned, therefore, one may consider only au and bg orbitals due to the symmetry of the π or π* orbitals. Hence, the excited (π, π*) states of C2h polyenes are either Ag or Bu. Both of the electron promotions au → au* and bg → bg* can produce the excited Ag states, whereas both au → bg* and bg → au* can produce the Bu state. Because the ground state is of Ag symmetry and because (x, y, z) = (bu, bu, au), the transition probability between the ground and an excited singlet (π, π*) state gains when the excited state is 1Bu ( ≠ 0) but does not gain when it is Ag ( = 0). Thus, one can recognize that the 1Ag state is forbidden while 1Bu is allowed in the electronic transition of symmetrically substituted all-s-trans polyenes, where the first singlet Ag state (the ground state) is normally described by 11Ag and the second is described by 21Ag. An energy-level scheme illustrating emitting states of typical all-s-trans C2h polyenes is shown in Figure 23.

Figure 22. Relaxation model of pyrene vapor at low pressure excited into the S3 state: k21 and k12 indicate the rate constants for the S2* → S1* internal conversion and the reverse S1* → S2* internal conversion, respectively. kFQ1 is the rate constant for the S1* → S0 internal conversion. kF1 and kF2 are the radiative rate constants of the S1 and S2 states, respectively. kFQ2 is the rate constant for the S2* → S0 internal conversion.

ΦF(S2 ) = ϕfast(S2 ) + ϕslow (S2 )

(A2)

with ϕfast(S2) = kF2/(kF1 + kFQ1 + k12) and ϕslow (S2 ) = (k12/β)[kF1/(kF1 + kFQ1 + k12)] × [kF2/(kF2 + kFQ2 + k 21)]

ϕfast(S2) corresponds to the yield of the S2 fluorescence directly emitted from S2*, whereas ϕslow(S2) corresponds to that of the S2 fluorescence, which is emitted from S2* after the molecules return to S2* from S1*. It follows from eqs A1a and A1b that

Figure 23. Energy-level scheme and emitting states of typical all-strans C2h polyenes.

AUTHOR INFORMATION Corresponding Author

ΦF(S2)/ΦF(S1) = (kF2/kF1)[(kF1 + kFQ1)/k12 + k12/k 21]

E-mail: [email protected].

(A3)

Notes

Here, it is not unreasonable to assume that kF1 + kFQ1 ≪ k12. Therefore, we have eq A4. ΦF(S2 )/ΦF(S1) ≈ kF2/kF1 × k12/k 21

The authors declare no competing financial interest.

(A4)

Biography

Applying Fermi’s golden rule, the values for k21 and k12 can be expressed in the forms k12 = (2π /ℏ)|V |2 ρ(S2 *)

(A5a)

k 21 = (2π /ℏ)|V |2 ρ(S1*)

(A5b)

where V is the coupling matrix element between the vibronic states S2* and S1* and ρ(S1*) and ρ(S2*) are the densities of the vibrational state in S1* and S2*, respectively. The ratio of the rate constants, k12/k21, is then given by k12/k 21 = ρ(S2 *)/ρ(S1*)

(A6)

Thus, we obtain an approximate expression A7. ΦF(S2 )/ΦF(S1) ≈ kF2/kF1 × ρ(S2 *)/ρ(S1*)

(A7) Takao Itoh is currently professor at Graduate School of Integrated Arts and Sciences of Hiroshima University. He was born in Tokyo, Japan, in 1951 and received B.Sci., M.Sci., and D.Sci. degrees all from Hokkaido University. From 1980 to 1992, he was working for research divisions of chemical companies, except for 1985−1986, during which he was a research associate at the University of California, Riverside. In

APPENDIX B Most of the symmetrically substituted all-s-trans polyenes belong to the C2h point group. It can be recognized from the character table of the C2h point group that the symmetry of the molecular orbitals of C2h polyenes can be ag, au, bg, or bu. When W

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1993, he became associate professor of science at Kanto Junior College, along with lecturer at Kanto Gakuen University. In 1999, he was appointed to professor of chemistry at Miyazaki Medical College, and then in 2004 he moved to Hiroshima University as professor of physical chemistry. He is an author or coauthor of over 125 refereed papers on physical chemistry, chemical physics, and material science. In addition to his interests in physical chemistry, Takao Itoh is an entomologist and a member of the Entomological Society of Japan and the Lepidopterists’ Society (U.S.A.), in addition to Chemical Society of Japan, Japan Society of Molecular Science, Photochemistry Association of Japan, and American Chemical Society. He has published also 20 papers on Lepidoptera.

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dx.doi.org/10.1021/cr200166m | Chem. Rev. XXXX, XXX, XXX−XXX