Photophysical and Redox Properties of Perylene Bis-and Tris

Article pubs.acs.org/JPCA

Photophysical and Redox Properties of Perylene Bis- and TrisDicarboximide Fluorophores with Triplet State Formation: Transient Absorption and Singlet Oxygen Sensitization Lucia Flamigni,*,† Alberto Zanelli,† Heinz Langhals,‡ and Bernd Böck‡ †

Istituto per la Sintesi Organica e Fotoreattivita’ (ISOF), CNR, Via P. Gobetti 101, 40129 Bologna, Italy Department of Chemistry, LMU University of Munich, Butenandtstr. 13, D-81377 Munich, Germany

‡

S Supporting Information *

ABSTRACT: A detailed photophysical characterization of a couple of new perylene imide derivatives, a carboxylic trisimide PIx, and an asymmetrically substituted carboxylic bisimide PIa is presented. PIx and PIa have the lowest singlet excited state just below 2.6 eV. The dyes are remarkably fluorescent (ϕf = 0.37 ± 0.03 for PIa and ϕf = 0.58 ± 0.04 for PIx in toluene), but they also display an efficient intersystem crossing. This leads to typical excited triplet photophysics/photochemistry, with intense triplet state absorption spectra and efficient singlet oxygen (1Δg) photosensitization (ϕΔ = 0.68 ± 0.02 for PIa and 0.44 ± 0.02 for PIx in toluene). On the basis of the measured ϕΔ, a ϕisc of 0.65 ± 0.02 for PIa and 0.43 ± 0.02 for PIx in toluene is derived. PIx reduces at −0.58 eV vs SCE, almost similarly to the corresponding symmetrically substituted perylene bisimide PI0, but unlike the latter, it has the first oxidation potential above +1.9 V. PIa is more electron rich and displays a more difficult first reduction at −0.95 V with a more facile oxidation at +1.75 V, similar to that of the parent PI0. The absorption spectra of the excited singlet and triplet states and that of electrochemically generated monoanions are reported.

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INTRODUCTION Aromatic carboxylic bismides, from the simple pyromellitimide to naphthalene bisimide and perylene bisimide up to the most complex periarylene imides, are attracting increasing attention.1 Dye chemists, material chemists, and biochemists are interested in these dyes for the superb colors, the electron transport properties, and the intense luminescence, which finds application in photonic devices as well as in imaging techniques. Furthermore, the excellent stability and the wide tunability of these dyes make them very attractive. These dyes have been and are widely used, as such or in molecular assemblies, for light energy collection, and conversion purposes.2−4 Under this respect, the high molar absorption coefficient, the strong signatures of the singlet excited state (both in absorption and in emission spectroscopy), the good ability to act as electron acceptors, and the intense and characteristic spectroscopic signatures of the radical anion have favored their exploitation, allowing for detailed mechanistic studies following light absorption process.2,3 In this article, we examine the two new asymmetrically substituted perylene imide derivatives, PIx and PIa (Figure 1), in order to establish their photophysical and electrochemical properties. This information will be valuable to establish their potential as photo- or electro-active building blocks in the construction of new functional photoactive arrays. Both compounds are less symmetric than the parent PI0 structure, © 2012 American Chemical Society

they have a five member imide ring at the 1,2 position: PIx has three imide functionalities, whereas PIa has two angularly arranged carboximide functionalities.

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EXPERIMENTAL METHODS Synthesis. The synthesis of PIx5 and PIa6 have been reported elsewhere. Electrochemistry. Electrochemical experiments have been performed at room temperature, after Ar purging, with an AMEL 5000 electrochemical system in DCM (C. Erba RPE, distilled over phosphoric anhydride and stored under Ar pressure) and 0.1 M tetrabuthylammonium perchlorate (TBAP, Fluka, puriss. crystallized from methanol and vacuum-dried). Cyclic voltammetries have been performed in a homemade three compartment cell with Pt semisphere electrode (diameter 2 mm), Pt wire counter electrode, and aqueous KCl saturated calomel electrode (SCE = 0.47 V vs ferrocene/ferricinium).7 Spectroelectrochemistry experiments have been carried out in a quartz cell (ALS, Japan), 10 mm width in the upper part but with an optical path of 1 mm in the lower one. The spectra were recorded across a Pt (8 × 5 mm) grid that is the working electrode. The counter electrode was Pt wire, and the reference Received: October 21, 2011 Revised: January 9, 2012 Published: February 2, 2012 1503

dx.doi.org/10.1021/jp210132w | J. Phys. Chem. A 2012, 116, 1503−1509

The Journal of Physical Chemistry A

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Figure 1. Nomenclature and structural formulas of the asymmetric PIs and the parent PI0.

electrode was Ag/Ag1+ 10−3 M, TBAP 0.1 M in DCM . This reference electrode was checked with SCE before and after each experiment. The spectra were recorded at 240 nm/min by using a Perkin-Elmer Lambda 9 spectrophotometer. Photophysics. The solvents used were spectroscopic grade toluene (C. Erba), spectroscopic grade dichloromethane (C. Erba), and HPLC grade benzonitrile (SIGMA-Aldrich). Absorption spectra were recorded with a Perkin-Elmer Lambda 9 spectrophotometer, and the emission spectra, corrected for the photomultiplier response if not otherwise stated, were detected by a Spex Fluorolog II spectrofluorimeter equipped with a Hamamatsu R3896 photomultiplier. Luminescence quantum yields of the sample, ϕs, were evaluated against a standard with known emission quantum yield ϕr by comparing areas under the corrected luminescence spectra by using the equation ϕs/ϕr = Arns2(area)s/Asnr2(area)r, where A is the absorbance, n is the refractive index of the solvent employed, and s and r stand for sample and reference, respectively. PI0 in TL (ϕfl = 0.92)8 or PI0 in DCM (ϕfl = 0.99)9 or quinine sulfate in air-equilibrated 1 N H2SO4 (ϕfl = 0.546)10 were used as standards. The near IR emission (range 800−1500 nm) was probed on TL solid glasses at 77 K with an FLS920 spectrofluorimeter (Edinburgh) equipped with an Hamamatsu R5509-72 supercooled photomultiplier tube at 193 K and a TM300 emission monochromator with a NIR grating blazed at 1000 nm. An Edinburgh Xe900 450 W xenon arc lamp was used as the exciting light source. The latter spectrofluorimeter was also used to measure the singlet oxygen luminescence (ϕΔ) of PIa and PIx in TL against 5,10,15,20-tetraphenylporphyrin (TPP) in toluene as a reference (ϕΔ = 0.70).11 Fluorescence lifetimes were detected by a Time Correlated Single Photon Counting apparatus (IBH) with excitation at 465 nm. Transient absorbance in the picosecond/nanosecond range made use of a pump and probe system based on a Nd:YAG laser (Continuum PY62/10, 35 ps pulse). The third harmonic (355 nm) with an energy of ca. 3 mJ/pulse was used to excite air equilibrated samples whose absorbance at the excitation wavelength was ca. 0.5. Transient absorbance to detect triplet properties (microsecond range) was performed on air-free solutions if not otherwise specified. To this aim, solutions contained in homemade, 10 mm optical path cuvettes were bubbled with argon for 10−20 min and sealed. The apparatus was based on a Nd:YAG laser (JK Lasers) delivering pulses of

18 ns. The third harmonic (355 nm) was used for excitation. Absorbance of the solutions at the exciting wavelength was ca. 1, and the energy used was of 3 mJ/pulse for the determination of the spectra and ca. 0.8 mJ/pulse for lifetime determination, in order to prevent undesired second order triplet−triplet annihilation reactions. Triplet excited state molar absorption coefficients were measured against a benzophenone (BP) in a benzene actinometer, using ϕisc= 1 and εT = 7220 M−1 cm−1 at 530 nm.12 For more details on the apparatuses, see previous reports.13 Estimated errors are 10% on lifetimes, molar extinction coefficients, and quantum yields; the working temperature, if not otherwise specified, was 295 ± 2 K.

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RESULTS AND DISCUSSION Absorption and Luminescence. The absorption spectra of the asymmetrically substituted perylene carboximides examined, PIx and PIa, are shown in Figure 2 for toluene

Figure 2. Absorption spectra of the asymmetric PIs and the parent PI0 in TL.

(TL) solutions and compared to that of the parent PI0. Both asymmetrically arranged PIs display absorption features in the UV region, rather strong in the case of PIa (ε ca. 50 000) and weaker in PIx (ε ca. 25 000). These are absent in PI0 and therefore are a consequence of the five-membered ring functionality. In the visible region, the typical absorption pattern of symmetric perylene carboxylic bisimides is present 1504



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only in PIx, but with a reduced molar absorption coefficient with respect to PI0, whereas it is replaced in PIa by a weaker, less symmetric and broader absorption feature. This latter feature has to be ascribed to the increased degree of asymmetry of the structure. The absorption spectra in solvents of increasing polarity, dichloromethane (DCM) and benzonitrile (BN), are reported in Figure S1 (Supporting Information). There is a very minor variation of the absorption spectra, a slight bathochromic shift in passing from DCM to TL and from the latter to BN for both PIs. The fluorescence spectrum for the two carboximides in TL is very similar and displays a slightly higher energy maximum in PIx, 475 nm, vs 481 nm for PIa, Figure 3. A similar trend is

Table 1. Luminescence Data for the Examined Compounds in Toluene, Dichloromethane, and Benzonitrile. Dielectric Constants (ε) and Refractive Index (n) are Indicated 295 K λmax (nm)a PI0 PIa

534, 574, 622 481, 512

PIx

475, 507, 545

PI0

529, 571, 629 489, 520 (sh) 474, 505, 544

PIa PIx

PI0 PIa PIx

536, 578, 627 496, 522(sh) 478, 510, 548

Φfl

77 K τ (ns)f

krad × 107 (s−1)

λmax (nm)a

TL (ε = 2.4, n = 1.497) 4.0 23.0 543, 587, 638 0.37c 8.4 4.4 483, 516, 555 (sh) 7.3 7.9 479, 515, 0.57d 0.59e 554 DCM (ε = 8.9, n = 1.424) 0.99b 4.0 24.7 0.92b

0.38c

6.7

5.7

0.40d0.36e

7.2

5.3

τ (ns)f

E (eV)g

4.0

2.28

10

2.56

7.0

2.58

BN (ε = 25.9, n = 1.528) 0.98b 4.0 24.5 0.40c

8.3

4.8

0.55e

7.4

7.4

a

Data from uncorrected spectra. bLuminescence quantum yield after excitation at 490 nm in toluene, 489 nm in dichloromethane, and 492 in BCN; PI0 in DCM as standard (ϕfl = 0.99). cLuminescence quantum yield after excitation at 474 nm in toluene, 456 nm in dichloromethane, and 440 in BCN. The same results are obtained for excitation in the UV band: 349 nm in TL, 348 nm in DCM, and 351 nm in BCN; PI0 as standard. dLuminescence quantum yield after excitation at 450 nm in toluene and 448 nm in dichloromethane; PI0 as standard. eExcitation at 351 nm vs quinine sulfate as standard. f Excitation at 465 nm. gFrom luminescence maxima at 77 K in TL glass.

Figure 3. Uncorrected fluorescence spectra of PIs at room temperature and at 77 K in toluene.

observed at 77 K in TL glass, where the vibronic structure of the spectra is more resolved. Excitation spectra read on the maxima of the emission overlapped perfectly with the absorption spectra, Figure S2 in Supporting Information, indicating that there is only one emitting state that is formed with the same efficiency, irrespective of the excitation wavelength. The emission in different solvents, Figure S3 in Supporting Information, displays different behavior for the two PIs. In PIx, there is a slight bathochromic shift similar to that of ground state absorption (Figure S1 in Supporting Information), the wavelength maximum follows the order DCM < TL < BN (see Table 1). The bathochromic shift in the absorption and emission bands of PIx follows the polarizability and not the polarity (see Table 1). This is an indication that there is no change in dipole moment from the ground to the excited state. At variance with these results, in the case of PIa, the emission maximum shifts to lower energies by increasing the solvent polarity (Table 1), i.e., the wavelength maximum in TL is lower than that in DCM, which in turn is lower than that in BN. Furthermore, it can be noticed that the spectral shape in the more polar solvents (DCM and BN) is broader, Figure S3 in Supporting Information. This points to formation in PIa of a state with some charge transfer character; however, the fluorescence quantum yield is almost the same for all solvents within experimental errors, ϕfl = 0.38 ± 0.02. On the contrary, for PIx, a clear decrease in luminescence quantum yield is noticed in DCM, ϕfl = 0.38 ± 0.02, compared to the value measured in TL and BN, ϕfl = 0.57 ± 0.02. This is not ascribable to the DCM polarity since BN is much more polar, and we are unable to provide explanations for this, unless the well-known photoinduced acid generation in DCM is

responsible for the anomalous behavior. Excitation of the two asymmetric PIs in the ultraviolet and visible bands gave identical emission quantum yields. The luminescence lifetimes of PIx, Table 1, are unaffected by the solvent polarity, τ = 7.3 ± 0.1 ns; PIa displays in DCM a slightly shorter lifetime, τ = 6.7, than that detected in the other two solvents, τ = 8.3 ± 0.1. Overall, the calculated radiative rate constants krad (krad = ϕfl/τ) is of the order of 5 × 107 s−1 for PIa and approximately 6−7 × 107 s−1 for PIx, therefore lower than that of the parent PI0 (krad = 2.3 × 108) by a factor of ca. 4, but the emission of these derivatives is still remarkably high to conceive their application as luminophores with a different spectral range with respect to the parent PI0. In view of the high triplet yield of these PIs (see below), we explored the luminescence properties of the dyes in a glass at 77 K, in order to detect a possible phosphorescent state. Perylene carboxylic bisimide similar to PI0 are known to have a triplet with a rather low excited state level, 1.19 eV.14 This value was determined by sensitization experiments since their intrinsic intersystem crossing efficiency is virtually zero. The luminescence spectrum of the new PIs is dominated by the fluorescence also at 77 K; only in the NIR region very weak bands could be detected, which were absent in the parent PI0. The NIR luminescence spectra in TL glassy solutions are reported in Figure 4 with the signal detected from reference 1505



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The spectra of DCM solutions of PIx and PIa after 600 s at a potential about 0.25 V more negative than the first reduction potential of each compound are reported in Figure 6. Under

Figure 4. NIR uncorrected luminescence from PIs and reference PI0 in TL matrix at 77 K.

PI0. Detectable bands with maxima around 900 and 990 nm for PIa and even weaker bands in the region 920−990 nm for PIx could be observed. We tentatively ascribe these bands to phosphorescence. Electrochemistry and Spectroelectrochemistry. Figure 5 shows cathodic voltammograms of PIx and PIa. Cyclic

Figure 6. Spectra of a neutral molecule (line) and a mixture of neutral molecule and radical-anion (dash) of PIx and PIa at 2 × 10−4 M, in DCM 0.1 M TBAP. The spectra were taken after applying, for 600 s, −0.85 V vs SCE to PIx solution and −1.2 V vs SCE to PIa.

these conditions, the working electrode generates on the light path a detectable amount of radical-anion in equilibrium with the oxidation reactions at the counter electrode. The radical anion of PIx shows three absorption bands with maxima at 650, 720, and 880 nm. From the spectra, a conversion of ca. 80− 90% can be calculated, and consequently molar absorption coefficients of the order of 25 000, 15 000, and 17 000 M−1 cm−1 can be derived for the peaks at 650, 720, and 880 nm, respectively. The results are not so different from those reported for the anion of PI0, which displays three similar but bathochromically shifted maxima at 710, 800, and 960 nm with molar absorption coefficients noticeable higher (ca. 90 000 on the 710 nm band).8,19 The radical anion of PIa shows only one absorption band with a maximum at 575 nm. From the spectra, assuming a very low absorption of the anion in the wavelength region of the neutral state band, a conversion ratio of PIa of the order of 10−20% can be estimated, and a molar absorption coefficient of the order of 25 000 M−1 cm−1 can be calculated for the band at 575 nm of PIa radical anion. Transient Absorption Spectroscopy. Absorption spectroscopy in the pico- and nanosecond range were performed in TL solutions in order to explore the properties of the singlet and, possibly, triplet excited states. Figure 7 reports the singlet excited state absorbance for the two asymmetrically substituted carboximides and for PI0 in TL solutions with matched optical density at the excitation wavelength (355 nm). The absorption features of 1PI0, with the stimulated emission bands around 575 and 620 nm and absorbance at 685 nm and around 850 nm, are much stronger than those of the other compounds. PIx exhibits

Figure 5. Cyclic voltammetries at 0.1 V s−1 of PIx (gray) and PIa (black) in DCM 0.1 M TBAP; concentration is 1.3 × 10−4 M.

voltammetries of PIx produces three reduction waves, one per carboximide group, whose standard potentials result −0.58 V, −0.77 V, and −1.00 V vs SCE. The first and second wave, reversible if the potential scan is inverted at −0.9 V, are 40 and 60 mV less negative than the corresponding ones for PI0.15 Consequently, the third wave, the one at the more negative potential, can be attributed to the contribution of the fivemembered carboxy imide ring. No current peak has been detected in the oxidation range up to 2 V vs SCE, thus the oxidation potential of PIx can be assumed higher than 1.9 V, as expected because of the addition of the third imide group.16 Cyclic voltammetry of PIa produces two reversible waves only, with standard potential at −0.96 and −1.30 V vs SCE. The more negative potential of the first wave indicates a less conjugated π-system with respect to both PI0 and PIx; that is, the five-membered carboximide ring contributes to the π-orbital less strongly than the symmetric six-membered carboximide group. Furthermore PIa has an oxidation wave with standard potential at 1.75 V vs SCE, relatively close to that reported for symmetric perylene carboxylic bisimides.17 The reduction peak currents versus the square root of the scan rate display a linear relationship, except for the third wave of PIx, indicating a diffusion driven process.18 1506



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Figure 7. Transient absorption spectra at the end of a 35 ps pulse (355 nm) after excitation of optically matched solutions of asymmetric PIs and PI0 in TL. The spectra are due to 1PIs.

Figure 8. Transient absorption spectra at the end of a 18 ns pulse (355 nm) after excitation of optically matched, oxygen-free solutions of asymmetric PIs in TL. No signal was detected from PI0 solutions. The spectra are due to 3PIs. The absorbance of the optically matched actinometer BP in benzene solution is also shown as a line, see text.

a band around 685 nm but noticeably weaker than the one of PI0, and PIa has a broad and very weak absorption band around 570 nm. The stimulated emission features of the asymmetric PIs are weaker and at lower wavelength, out of the accessible spectral window. The spectra decay moderately on the time window of the experiment (3 ns) in agreement with a lifetime of 7−8 ns, as measured from luminescence experiments. The nanosecond laser flash photolysis experiment allows to detect longer lived transients (τ ≥ 20 ns), as is the case of triplet excited states. PI0 does not display any transient absorbance, in agreement with an almost zero yield of intersystem crossing. Perylene carboxylic bisimide triplets have only been produced and detected after sensitization, either by inter- or intramolecular reactions.14,20−22 3PI has also been observed as a product of charge separated state recombination in PI containing multicomponent arrays undergoing electron transfer or via formation of a highly polar transition state in a PI dimer.23 The data so far reported agree in assigning to 3PIs an intense and positive ΔA spectrum with band(s) in the region 450−550 nm, overlapped with the strong bleaching of the low energy ground state absorption band and, in some cases, less intense positive broad absorption extending up to the NIR region. Going back to the present asymmetric PIs, they display in TL oxygen free solutions excited at 355 nm intense triplet absorption spectra, indicating a high yield of intersystem crossing. In Figure 8 are reported 3PIa and 3PIb spectra at matched optical densities at the exciting wavelength of 355 nm. 3PIa exhibits a maximum at 510 nm and a bleach at 435 nm,3PIx shows a maximum at 530 nm and a bleach at 470 nm. In air-free TL solutions, PIa has a lifetime of 90 μs and PIx has a lifetime of 140 μs. According to their nature, both triplets react efficiently with O2. In air equilibrated solutions, the lifetime for PIa is 248 ns and that of PIx is 325 ns. By taking into account a concentration of oxygen in TL air equilibrated solutions of 1.8 × 10−3 M,24 reaction rates with oxygen kox of the order of 2 × 109 s−1 can be calculated for both PIs. This value is similar to those of well-known triplet photosensitizers as porphyrins25 and corroles.26 In order to derive some quantitavive information on the production of 3PIs, benzophenone in benzene was used as the actinometer. The measured ΔA for the benzophenone triplet 3 BP in matched conditions is reported in Figure 8 as a continuous line. From the experiment the product of intersystem crossing (ϕisc) and molar absorption coefficient (εT) on the wavelength of the maximum can be derived. ϕisc ×

εT for the actinometer at 530 nm is 7220 M−1 cm−1,12 and in our samples, ϕsc × εT appears lower by only 20% for PIx and 40% for PIa. Assuming as ϕisc = 0.43 in the case of PIx and ϕisc = 0.65 in the case of PIa, (see following section), molar absorption coefficient ε of the order of 7100 M−1 cm−1 for 3PIa at 510 nm and of the order of 14 200 M−1 cm−1 for 3PIx at 530 nm can be calculated. Reactivity with Oxygen. In view of the high reaction rate with oxygen of the present 3PIs, we explored this reactivity and the related products. In principle, the mechanism of quenching of triplet excited states by ground state oxygen O2, could be due either to energy transfer or to electron transfer, yielding, respectively, singlet oxygen 1Δg (E0−0 = 0.98 eV) or superoxide ion O2−. In the present case, however, electron transfer reaction with production of O2− does not seem possible in view of the redox potential of the species. Whereas the reduction potential of O2 is of the order of −0.8 V, the oxidation potentials of the present PIs is ≥1.75 V, making an electron transfer reaction from an excited state with a stored energy of less than 1.4 eV as the present one almost impossible from a thermodynamic viewpoint. Therefore, reaction of 3PI with oxygen can only lead to photosensitization of 1 Δ g . Accordingly, the typical luminescence of 1Δg with a maximum at 1268 nm was detected upon excitation at 419 nm of air equilibrated, TL solutions of PIs. Figure 9 displays the results of the NIR luminescence experiment compared with a standard of 5,10,15,20-tetraphenylporphyrin (TPP) in TL (ϕΔ = 0.70).11 The measured yield of 1Δg, ϕΔ, is 0.68 for PIa and 0.45 for PIx. This, together with the measured fluorescence yields ϕfl (see Table 1), allows to derive a yield of triplet, ϕisc, of 0.65 ± 0.02 for PIa and 0.43 ± 0.02 for PIx in TL.

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CONCLUSIONS PIs are, in general, characterized by a very high fluorescence quantum yield and a ca. zero yield of intersystem crossing. In some cases, quenching of the fluorescence occurs upon intramolecular electron transfer, see, for example, ref 6, but in no case a sizable triplet yield was detected before. Peculiar to the present PIs are the asymmetry in substitution and the presence of maleimide as a bay group. The effect of this substituent and of the symmetry disruption might affect indirectly the triplet yield by decreasing the radiative rate constant. We can in fact detect experimentally a decrease of 1 1507



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS Financial support from Italian CNR (MACOL PM.P04.010), from the CIPSM cluster in Munich, and a scholarship of the Bavarian State are gratefully acknowledged.

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Figure 9. Uncorrected singlet oxygen luminescence from optically matched toluene solutions of PIs and standard TPP; excitation at 419 nm.

order of magnitude in the radiative rate constant krad between the symmetric PI0 and the new PIs (Table 1). The decreased radiative rate can allow competition by the intersystem crossing path in the new PIs. However, we cannot exclude also a direct effect of the above-mentioned peculiarities in the structure of the new PIs on the intersystem crossing rate. The interpretation is undoubtedly more complex, and several parameters should be considered. We are presently engaged in the study of several PIs in order to rationalize the observed phenomena. We have studied the spectroscopic, photophysical, and electrochemical properties of the two new asymmetrically substituted perylene polycarboximide derivatives. The main parameters of deactivation of excited state formed by light absorption have been derived, and the spectroscopic features of the excited and radical states have been determined. The examined perylene imide derivatives have proven to be excellent and stable luminophors in a variety of solvents; the deactivation of their excited state is characterized by a reasonably high radiative rate constant but also by a high intersystem crossing rate constant with a ϕisc= 1 − ϕfl. In these dyes, the properties of strong luminophores coexists with a typical triplet photoreactivity characterized by strong T−T absorption bands and singlet oxygen photosensitization up to levels comparable to those of typical photosensitizers. Because of their ability to act as fluorophores and as oxygen photosensitizers, these dyes might have great potential in fluorescence imaging associated to photosensitized reactions, either in biological or in material applications. They can furthermore be easily used as simple, effective, and very stable laboratory sources of singlet oxygen upon excitation with the 451 nm line of an indium lamp or the mercury lamp line at 436 nm. In addition, due to their photo- and electro-reactivity, they are, in analogy to the symmetric PI0 parent molecule, interesting building blocks for molecular arrays with application in light energy collection and conversion.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Additional absorption spectra in various solvents. Excitation spectra in toluene. Additional fluorescence spectra in various solvents. This material is available free of charge via the Internet at http://pubs.acs.org. 1508



Article

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Photophysical and Redox Properties of Perylene Bis-and Tris

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