J. Phys. Chem. 1996, 100, 2001-2011
2001
Comprehensive Model of the Photophysics of N-Phenylnaphthalimides: The Role of Solvent and Rotational Relaxation Attila Demeter, Tibor Be´ rces,* and La´ szlo´ Biczo´ k Central Research Institute for Chemistry, Hungarian Academy of Sciences, H-1025 Budapest, Pusztaszeri u. 59-67, Hungary
Ve´ ronique Wintgens, Pierre Valat, and Jean Kossanyi Laboratoire des Mate´ riaux Mole´ culaires, CNRS, 2-8 rue H. Dunant, 94320 Thiais, France ReceiVed: April 21, 1995; In Final Form: August 23, 1995X
Absorption and fluorescence spectra, fluorescence decay times, and quantum yields of fluorescence and triplet state formation have been determined for N-phenyl and substituted N-phenyl derivatives of 1,2-, 2,3-, and 1,8-naphthalimides, using stationary irradiation and laser flash excitation methods. The effects of substituents on the N-phenyl group on solvent polarity and viscosity have been studied. A short-wavelength (SW) fluorescence, similar to the luminescence emitted by the N-alkyl derivatives, and/or a considerably red shifted long-wavelength (LW) luminescence are observed, and the ratio of the SW and LW fluorescence components is found to depend on substitution and on solvent properties. A striking characteristic of the Nphenylnaphthalimides (in contrast to the N-alkyl derivatives) is the very efficient internal conversion which results in short fluorescence decay times and in low fluorescence and triplet yields. On the basis of the experimental results, it is suggested that solvent and geometrical relaxation of the Franck-Condon state yields two emitting excited states, the SW and LW states, which emit the short-wavelength and long-wavelength fluorescence, respectively. The geometry of the SW state is similar to that of the ground state, while twisting of the phenyl group toward a coplanar geometry is assumed to be required in the formation of the LW state. The extended conjugation comprising the phenyl and naphthalimide moieties, attributed to the coplanar geometry, together with the charge transfer character endows the LW excited state with an extra stability. Solvent cage and geometrical (twisting) relaxation induces efficient internal conversion by virtue of pseudoJahn-Teller coupling of the two low-lying excited states (“proximity effect”) as well as by the decrease of the energy gap between the LW excited state and ground state (“energy gap law”).
I. Introduction publication1
In a previous we reported on the spectroscopic and photophysical properties of 1,2-, 2,3-, and 1,8-naphthalimides and of their N-methyl derivatives. It was shown that these compounds emit fluorescence around 400 nm and are characterized by fluorescence decay times and quantum yields which decrease considerably and by triplet yields which increase significantly, as a consequence of coupling between the first excited singlet and an upper triplet state, in the order of 1,2-, 2,3-, and 1,8-naphthalimides. Some observations made with N-phenyl-1,2-naphthalimides2 and with N-phenyl-2,3-naphthalimide3 indicated that the spectroscopic and photophysical properties of the N-phenylnaphthalimides are fundamentally different from those of the N-CH3 derivatives. The tendencies mentioned above for the N-CH3 compounds do not prevail in the case of the N-phenylnaphthalimides. Moreover, (i) under certain conditions the N-phenyl derivatives show red-shifted, so-called “long wavelength” (LW) fluorescence apart from the “short-wavelength” (SW) emission; (ii) the excited state lifetimes of the N-phenyl derivatives are significantly shorter than those of the corresponding N-CH3 compounds. Many nitrogen heterocycles and aromatic carbonyl compounds possess low-lying n* and π* singlet excited states, which may significantly influence the photophysics if these are close neighboring states.4,5 Very efficient internal conversion from X
Abstract published in AdVance ACS Abstracts, January 1, 1996.
0022-3654/96/20100-2001$12.00/0
the lowest excited state to the ground state has been suggested to be the consequence of vibronic interaction between the closelying n* and π* states.6,7 This vibronic interaction may result in potential energy distortion and displacement along the vibronically active out of plane bending mode, which is called the pseudo-Jahn-Teller effect,6,8 or is now generally referred to as the proximity effect.5 The force constant of the vibronically active mode increases in the upper state and decreases in the lower state, thus indicating a red shift of the emission. It has been suggested9 that the dual fluorescence of cyanosubstituted anilines is the result of solvent-induced pseudoJahn-Teller coupling of the two lowest excited singlet states. The present paper reports on the results of a systematic study of the absorption, fluorescence and other photophysical properties of the three N-phenylnaphthalimide isomers, O
O
N
N
O
O N-phenyl-2,3-naphthalimide (N-Ph-2,3-NI)
N-phenyl-1,2-naphthalimide (N-Ph-1,2-NI) O N
O N-phenyl-1,8-naphthalimide (N-Ph-1,8-NI)
© 1996 American Chemical Society
2002 J. Phys. Chem., Vol. 100, No. 6, 1996
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and of their methyl-, methoxy-, and trifluoromethyl-substituted N-phenyl derivatives. The purpose of this work is to develop a comprehensive model of fluorescence and photophysical properties based on the solvent cage relaxation and phenyl rotation induced processes including pseudo-Jahn-Teller coupling. II. Experimental Section A. Materials. The 1,2- and 1,8-naphthalimides were prepared from the naphthalic anhydrides (Chemsyn Science Laboratory and Aldrich, respectively), while the 2,3-naphthalimides were obtained from 2,3-naphthalenedicarboxylic acid (Aldrich) with the appropriate anilines (Aldrich) in a way similar to that previously described1,3 for N-phenylnaphthalimides. The crude compounds were purified by column and thin-layer chromatography (Merck PLC silica gel) and finally by recrystallization from an n-hexane/benzene solvent mixture. High purity grade solvents were used, as described previously.1 B. Experimental Techniques. UV-vis spectra were obtained using either an HP 8452a spectrometer or a Varian-Cary Model 219 instrument. Fluorescence spectra were recorded by using three different apparatus: (i) a homemade photon-counting spectrofluorimeter equipped with a Princeton Applied Research 1140 A/B detection system and using 366, 333, and 311 nm excitation, respectively; (ii) a Perkin-Elmer Model MPF 44B apparatus equipped with a DCSU-2 spectral correction unit; and (iii) a SLM-Aminco Model 8000C spectrofluorimeter. Fluorescence quantum yields were measured by comparison with quinine sulfate (in 0.5 mol dm3 aqueous sulfuric acid solution) for which φf ) 0.55 was taken.10 Nanosecond fluorescence decay measurements were made with a time-correlated single-photon-counting technique. For sub-nanosecond lifetime determinations, a frequency-tripled pulsed YAG laser (B. M. Industries) of 30 ps fwhm and a TSN 506 streak-camera with about 10 ps time resolution were used. In this way, lifetimes of 50 ps or longer could be determined. Two methods were used to obtain triplet quantum yields: (i) one was an energy transfer method with 9,10-dibromoanthracene as energy acceptor and a 308 nm excitation wavelength from a Lambda Physic EMG 101 excimer laser;2 and (ii) the other method was a laser flash photolysis experiment carried out with a frequency-doubled pulsed ruby laser (347.5 nm, 20 ns fwhm, J. K. Laser Co.), using 5,6,8,9-tetrahedrodibenzo[c,h]xanthylium perchlorate as energy acceptor. N-Me-1,8-NI was used as a reference compound with a triplet yield of 0.95 and 1.00 in acetonitrile and hexane, respectively.1 The transient absorption measurements were made at the absorption maximum of the triplet excited state, (i.e. at 505, 440, and 480 nm for N-Ph-1,2-NI, N-Ph-2,3-NI, and N-Ph-1,8NI, respectively, in acetonitrile). Absorption and fluorescence measurements at different temperatures were carried out using a liquid-nitrogen-cooled dewar (Oxford Instruments). III. Results A. Absorption and Fluorescence Spectra. Absorption Spectra. N-phenyl derivatives of 1,2-, 2,3-, and 1,8-naphthalimides show absorption in n-hexane in the 23 000-50 000 cm-1 wavenumber range. The long-wavelength S1 band exhibits vibrational fine structure (Figure 1), while the two strong shortwavelength ones (not shown in the figure) are broad and structureless. These three bands are characteristic for the naphthalimide structure; the corresponding bands at similar wavelengths were observed also for N-Me compounds.1 All
Figure 1. Absorption and fluorescence spectra of N-Ph-1,2-NI (A1 and F1), N-Ph-2,3- NI (A2 and F2), and N-Ph-1,8-NI (A3 and F3) in n-hexane. Designations: experimental absorption and emission spectra (9 and b, respectively), calculated spectrum (s), model spectrum for the absorption of the naphthalimide moiety (0), band corresponding to the S0 f S2 transition (- - -), contribution from short-wavelength bands (‚‚‚).
three bands show a 100-200 cm-1 red shift when hexane is replaced by acetonitrile. However, the bands are relatively insensitive to methyl substitution on the N-phenyl ring: ortho methyl substitution causes no or only a small change in the location and in the oscillator strength of the bands. A relatively weak band has been observed2 in the spectrum of N-Ph-1,2-NI, with a maximum located at 34 030 cm-1 (294 nm) in hexane and with a 870 cm-1 blue shift in acetonitrile. Such a large shift was taken as an indication that this band corresponds to a transition to an excited state with net charge transfer character. There is some indication in the spectrum of N-Ph-2,3-NI in n-hexane that a similar transition occurs for N-Ph-2,3-NI in nonpolar solvents. Unfortunately, overlap by the close-lying and intense short-wavelength bands did not permit us to detect this band in more polar solvents. For similar reasons, the band could not be directly identified in the spectrum of N-Ph-1,8-NI in any of the solvents used. However indirect evidence has been found which support the assumption that an S2 state with charge transfer character may play a role in the photophysics and spectroscopy of all three types of N-phenylnaphthalimides. Namely, for all three N-phenylnaphthalimide isomers, absorption bands with maxima around 31 000 cm-1 are required in addition to those corresponding to the naphthalimide moiety in order to describe the spectra. These bands seem to correspond to the transitions to the charge transfer type S2 state. The resolution of the experimental absorption spectra, made in order to extract the S2 bands, is shown in Figure 1. The spectra of the N-(2,6-dimethylphenyl)naphthalimides were used to represent the contributions of the naphthalimide moieties to the absorption. Namely, these compounds had spectra similar to those of the N-alkyl derivatives, which made them good models of the S1 bands of the N-phenyl compounds. The determination of the S2 absorption band was carried out by obtaining the best fit to the measured spectrum with the spectrum of the model compound of the S1 band, the Gaussian curve representing the S2 band in question, and in some cases one or more Gaussians accounting for the contribution from higher energy bands. As can be seen from Figure 1, the band associated with the S0 f S2 transition is more significant in the case of the 1,2and 2,3-naphthalimides but small for the 1,8-compounds. This
Photophysics of N-Phenylnaphthalimides
Figure 2. Absorption and fluorescence spectra of N-(2-Me-Ph)-2,3NI (A1 and F1), N-(p-CF3-Ph)-2,3-NI (A2 and F2), and N-(p-CH3OPh)-2,3-NI (A3 and F3) in n-hexane. Designations: model spectrum for the fluorescence of the naphthalimide moiety (O); for other designations see Figure 1.
band is blue shifted to 30 000 ( 600 cm-1 for (N-Ph-1,2-NI), 30 900 ( 600 cm-1 for (N-Ph-2,3-NI), and 32 500 ( 600 cm-1 for (N-Ph-1,8-NI). Substitution in the N-phenyl group influences much more the S0 f S2 than the S0 f S1 transition. Thus, methyl substitution in the ortho position of the phenyl ring of N-Ph-1,2-NI2 and N-Ph-2,3-NI causes a 800-1200 cm-1 blue shift of the charge transfer band (Figure 2) and at the same time decreases considerably its oscillator strength, as compared to the lowest energy band. Moreover, the 2,6-dimethyl substitution in N-Ph2,3-NI results in the disappearance of the S2 band. It was expected from the charge transfer character of the S2 state that electron-donating or electron-withdrawing substituents on the N-phenyl ring may have a strong effect on the absorption and emission properties of the N-phenylnaphthalimides. Therefore, the para-methoxy (p-CH3O) and para-trifluoromethyl (pCF3) derivatives were studied (Figure 2). p-CF3 substitution has an insignificant effect on the oscillator strength and caused a 1000-2000 cm-1 blue shift of the S2 band. However, as a result of p-CH3O substitution, a significant increase of the oscillator strength associated with the S0 f S2 transition could be observed for all three naphthalimide isomers. At the same time, the p-MeO substitution caused a ∼1000 cm-1 red shift. The identification of the first and second excited singlet states gains strong support from the Hu¨ckel calculations carried out for all three N-phenylnaphthalimide isomers. The calculated electron distributions for various molecular orbitals12 are given for N-Ph-2,3-NI in Figure 3 and electron distributions were published2 previously for N-Ph-1,2-NI. The results obtained for all three naphthalimide isomers are qualitatively similar and indicate that electron transfer in the S0 f S2 transition occurs from the aniline moiety to the π* orbitals of the carbonyl groups (compare the electron distributions in the LUMO and HOMO-1 orbitals). This reverses the direction of the dipole moment on excitation to the S2 state. On the other hand, electron transfer in the S0 f S1 transition occurs from the naphthalene moiety to the π* orbitals of the carbonyl groups (compare LUMO and HOMO orbitals), in which the electrons of the aniline moiety do not participate. The results of the Hu¨ckel calculations supply also information on the symmetry properties of the excited states. N-Ph-2,3-NI and N-Ph-1,8-NI belong to the C2 point group (or to C2v for the planar molecule). It follows from the orbital symmetries
J. Phys. Chem., Vol. 100, No. 6, 1996 2003
Figure 3. Hu¨ckel molecular orbitals12 for N-Ph-2,3-NI. Full and open circles correspond to positive and negative charges, respectively. The orbital symmetries are give in parentheses.
that the lowest excited singlet S1 state is a 1A state (the ground state is also a 1A state), while the S2 is a 1B state (or a 1B1 if coplanar). Fluorescence Spectra. The fluorescence spectra of the N-phenylnaphthalimides show multifold features, as depicted in Figure 1. The most important characteristic of the luminescence is that a short-wavelength (SW) fluorescence, similar to the luminescence emitted by the N-alkyl derivatives,1 and/or a considerably red shifted long-wavelength (LW) component can be observed in their spectra. The unsubstituted N-Ph-1,8-NI emits only SW fluorescence, while N-Ph-1,2-NI shows only LW luminescence, but dual (SW and LW) fluorescence is characteristic for N-Ph-2,3-NI. The relative importance of the SW and LW luminescence depends significantly on the molecular structure, on both the solvent polarity and viscosity, and on the temperature. The fluorescence properties, together with some other photophysical data, are summarized for the unsubstituted and p-CH3O- and p-CF3-substituted N-phenyl naphthalimides in Table 1, while such results are given for the o-Me-substituted compounds in Table 2. Thus, the location of the SW fluorescence maximum is practically independent of the substitution (and is similar to the one found for the N-Me compounds), while a significant blue shift occurs with ortho substitution in the LW fluorescence (when observable). On the other hand ortho methyl substitution in N-Ph-1,2-NI and N-Ph-2,3-NI increases the importance of the SW fluorescence, i.e. decreases the contribution of the LW component to the total fluorescence which is given in Tables 1 and 2 in parentheses by the r ) φfLW/(φfSW + φfLW) ratio. This results in dual fluorescence of the methyl derivatives of N-Ph1,2-NI and N-Ph-2,3-NI. The effect of p-CH3O and p-CF3 substitution on the ratio of LW and SW fluorescence is particularly significant. It can be concluded from the data given in Table 1 that the contribution of the LW component to the fluorescence (i.e. their r ratio) increases in general with p-CH3O substitution and decreases with p-CF3 substitution (see Figure 2). Therefore, the N-(pMeO-Ph) compounds show practically no SW emission; however, LW emission is observed in hexane for the N-(p-MeOPh)-1,8-NI. On the other hand, the SW fluorescence (which is small for the unsubstituted N-Ph-2,3-NI) becomes comparable to the LW emission in the case of N-(p-CF3-Ph)-2,3-NI. The location of the LW fluorescence maximum is sensitive to the substitution in the para position of the N-phenyl ring by electron-donating or electron withdrawing groups. This can be demonstrated in the case of N-Ph-2,3-NI: In nonpolar solvent,
2004 J. Phys. Chem., Vol. 100, No. 6, 1996
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TABLE 1: Fluorescence and Other Photophysical Properties of N-Methyl-, N-Phenyl-, N-(p-Methoxyphenyl)-, and N-(p-(Trifluoromethyl)phenyl)naphthalimides
p-CH3O substitution causes strong red shift while p-CF3 substitution results in blue shift. It can be seen from Table 1 that the solvent effect on the SW fluorescence of the N-phenylnaphthalimides is similar to the effect observed in the case of the N-methyl compounds, i.e. a small red shift with increasing polarity. On the other hand, the increase of solvent polarity (from n-hexane to acetonitrile) causes a stronger red shift of the LW fluorescence: about 1670 and 890 cm-1 in the case of N-Ph-1,2-NI and N-Ph-2,3-NI, respectively. This effect on the fluorescence of N-Ph-2,3-NI is presented in Table 3. The change in dipole moment between the ground state and the LW excited state, ∆µLW, is deduced from the dependence of the ∆νjmax difference between the maxima of the absorption and LW fluorescence bands on Lippert’s ∆f function.13 Thus, using 4.3, 4.2, and 4.3 Å for the radius of the Onsager cavity of the 1,2-, 2,3-, and 1,8naphthalimides, respectively, leads to the ∆µLW dipole moment
differences given in Table 4. The ∆µSW values appearing in Table 4 are estimations obtained by assuming that the ∆µSW dipole moments of the N-phenylnaphthalimides are similar to the ∆µ values of the appropriate N-methyl compounds (and the Onsager cavity radius for the N-phenylnaphthalimides are larger by 0.5 Å). The solvent effect on the fluorescence maxima of the ortho methyl substituted naphthalimides is similar to that already described for the unsubstituted compounds. However, the ratio of the SW and LW fluorescence intensities is particularly sensitive to the solvent polarity. For the ortho methyl substituted naphthalimides, the contribution of the long-wavelength fluorescence component (i.e. r) decreases markedly with increasing solvent polarity. This solvent effect can be very well demonstrated if the fluorescence band is resolved into a SW and a LW component. The luminescence from the N-(2,6-di-Me-Ph)NI was used to model the fluorescence associated with the naph-
Photophysics of N-Phenylnaphthalimides
J. Phys. Chem., Vol. 100, No. 6, 1996 2005
TABLE 2: Fluorescence and Other Photophysical Properties of Various Ortho Methyl Substituted N-Phenylnaphthalimides in n-Hexane
TABLE 3: Fluorescence and Other Photophysical Properties of N-Ph-2,3-NI in Different Solvents solvent
(∆f)
λLW f /nm
τLW/ns
φLW f
(r)
φISC
φIC
LW φLW f /τ
φISC/τLW
n-hexane benzene diethyl ether ethyl acetate acetonitrile
(0.00) (0.03) (0.164) (0.201) (0.301)
475 478 483 487 496
1.00 2.58 1.96 1.76 1.30
0.007 0.012 0.010 0.008 0.006
(0.98) (0.99) (0.99) (0.99) (0.97)
0.44 0.49 0.43 0.34 0.22
0.55 0.50 0.56 0.65 0.77
0.0067 0.0047 0.0051 0.0046 0.0047
0.44 0.19 0.22 0.19 0.17
thalimide moiety (i.e. the SW fluorescence), and a Gaussian curve was chosen to represent the LW component of the fluorescence. Resolution was carried out by obtaining the best fit to the experimental fluorescence spectrum in the 17 000-26 000 cm-1 wavelength range. The results of such an analysis were given for N-(2-Me-Ph)-1,2-NI in a previous publication2 and are shown for N-(2-Me-Ph)-2,3-NI in four solvents in Figure 4. The fluorescence of the p-MeO-substituted compounds
showed strong red shift (ca. 70-100 nm) with increasing solvent polarity, while the fluorescence of the p-CF3 compounds was relatively insensitive to the change of the media and showed only on 11-22 nm red shift when hexane was replaced by acetonitrile. A systematic study14 of the solvent effect on the luminescence of these compounds gave ∆µLW dipole moments as high as 8 and 9 D for N-(p-MeO-Ph)-2,3-NI and N-(p-MeOPh)-1,8-NI, respectively. The ∆µLW values of the N-(p-CF3Ph) compounds were found to be small.
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TABLE 4: Differences of the Excited State and Ground State Dipole Moments for SW and LW, as Well as Estimated (Solvent + Structural) Relaxation Energies (in kcal mol-1) for the Formation of the LW State structure O
∆µSW/D
∆µLW/D
solvent
∆Erel 1
∆Εrel 2
Εrel 1
Εrel 2
rel Εrel 2 - Ε1
5.0
5.5
hexane acetonitrile
-10.7 -14.7
-20.1 -29.1
62.7 58.7
65.9 56.9
3.2 -1.8
2.5
3.4
hexane acetonitrile
-11.4 -12.8
-18.9 -22.1
69.5 68.1
69.1 65.9
-0.4 -2.2
hexane acetonitrile
-3.1 -4.0
-6.5 -8.5
81.2 80.3
86.5 84.5
5.3 4.2
N O O N O O
3.0
N O
Figure 4. Fluorescence spectra of N-(2-Me-Ph)-2,3-NI in hexane (A), diethyl ether (B), ethyl acetate (C), and acetonitrile (D). Designations: experimental (b ) and calculated (s) fluorescence spectra, model spectrum for the fluorescence of the naphthalimide moiety (O), LW fluorescence component (- - -).
B. Photophysical Properties. The photophysical properties of the unsubstituted N-phenyl- and the N-methylnaphthalimides are given in Table 1. These data show very important differences in the photophysics of the N-alkyl and N-phenyl compounds: while internal conversion accounts for only a few percent of the excited singlet state deactivation processes of the N-methyl derivatives, it is by far the most important primary photophysical step of the N-phenylnaphthalimide singlet excited state. The efficient internal conversion (IC) occurs from the SW state; however, significant IC appears to originate also from the LW state. The fast IC processes result in short lifetimes and small fluorescence and intersystem crossing quantum yields for most unsubstituted N-phenylnaphthalimides. Effect of Ortho Substitution. Ortho methyl substitution on the phenyl ring of N-Ph-1,2-NI increases the LW lifetime and changes to a similar extent the fluorescence and intersystem crossing quantum yields at the expense of internal conversion (see Table 2). In the case of the unsubstituted N-Ph-1,2-NI, fluorescence and triplet formation appear to occur exclusively from the LW state. A small SW contribution can be observed in the case of the 2-Me-Ph derivative, while SW and LW
fluorescence components are comparable for the 2,6-di-Me-Ph compound. Methyl substitution in the phenyl group of N-Ph-2,3-NI changes the ratio of SW and LW fluorescence in a similar way as described above for N-Ph-1,2-NI: substitution gradually decreases the contribution of the LW state. There is a change from the practically LW-state-controlled photophysics in the case of the unsubstituted compound to the SW-state-controlled photophysics in the case of N-(2,6-di-Me-Ph)-2,3-NI. The lifetime, fluorescence yield, and triplet yield decrease due to the first ortho methyl substitution and then increase back again to reach more or less the original values as a result of the second methyl substitution. Ortho methyl substitution of N-Ph-1,8-NI influences the processes occurring from the SW state. The SW fluorescence decay time of both unsubstituted and alkyl-substituted compounds was shorter than the time resolution of our instrumentation (i.e. 50 ps). A triplet yield as high as φISC ) 0.80 for a compound characterized by a singlet lifetime shorter than 50 ps indicates very efficient internal conversion and the occurrence of a specific effect. The photophysical properties of the N-(2,5-di-Me-Ph)-NI’s have values which are intermediate between those of the unsubstituted and 2-Me-substituted N-Ph-NI’s. Effect of Para Substitution. The fluorescence quantum yield is low for all p-CH3O- and p-CF3-substituted compounds: the photophysics is determined by nonradiative processes, i.e. intersystem crossing (ISC) and internal conversion (see Table 1). Specific effects determine the efficiency of the ISC and IC processes and influence thereby their relative importance: low ISC and high IC yields are characteristic for N-(p-MeO-Ph)2,3-NI, while comparable ISC and IC yields are found for N-(pMeO-Ph)-1,8-NI. A further important trend is observed for the LW state processes of N-Ph-2,3-NI’s: the ISC yield increases while the IC yield decreases in the order N-(p-MeO-Ph)-2,3NI, N-Ph-2,3-NI, N-(p-CF3-Ph)-2,3-NI. SolVent Polarity Effect. The solvent effect on the photophysical properties of naphthalimides has been studied in some detail previously,2,3 and new experimental results obtained for N-Ph-2,3-NI are presented in Table 3. The LW lifetime increases with decreasing solvent polarity, and the same trend is obeyed by the quantum yields of fluorescence and triplet formation. This solvent effect is obviously caused by the decrease of the kIC rate constant of the dominating internal conversion process with decreasing solvent polarity. The results obtained in nonpolar hexane do not fit in the above described trend: the LW lifetime is shorter and the internal conversion
Photophysics of N-Phenylnaphthalimides
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TABLE 5: Viscosity (Temperature) Effect on the Photophysical Properties of N-Phenylnaphthalimides in Triacetin O
O
O
N N
N
O O
λf/nm
T/K 303 283 273 263 253 243 233 223
SW LW SW LW SW LW SW LW SW LW SW LW SW LW SW LW
607
τ/ns
