J . Phys. Chem. 1989, 93, 7985-7988
7985
ARTICLES Observation of Dual Fluorescence for Fluoranthene in the Vapor Phase Ki-Min Bark and R. Ken For&* Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881 (Received: February 9, 1989; In Final Form: May 30, 1989)
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Dual fluorescence for fluoranthene in the gas phase is reported with 337.1-nm excitation. The strong fluorescence emission at wavelengths longer than 400 nm is the F, band (SI So). The weaker signal at wavelengths shorter than 400 nm is So). The fluoranthene lifetimes of the F1 and F, bands are 32 & 1 and 22 f 1 ns, respectively. The the F2 band (S, changes in the spectra as a function of signal acquisition delay time and excitation wavelength support this dual fluoresence. The lifetime with 368-nm excitation, which pumps only the SI band, is 41 f 1 ns. This unusual fluorescence property in opposition to Kasha's rule is postulated to be due to slow internal conversion from S2 to SI. The coupling between the SI and S2 states appears to be small owing to bond length changes for different electronic states of fluoranthene.
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Introduction It is well-known that fluorescence emission arises from the lowest excited singlet state in complex molecules because the radiationless transitions from higher excited singlet states to the first excited singlet state occur with very high efficiency. Kasha's rule has proved a remarkably accurate generalization to date with only few exceptions. The first authentic exception to Kasha's rule was discovered by Beer and Longuet-Higgins,lJ who showed that the fluorescence of azulene could only be interpreted as an S2 Sotransition. This anomalous fluorescence is also observed from many derivatives of a ~ u l e n e . ~ -Rentzepis6*' ~ has observed the weak SI So fluorescence intensity of azulene at 13 200 cm-I. Azulene thus exhibits two fluorescence emissions originating from excited singlet states, S2 So fluorescence with a relatively high fluorescence quantum yield and SI So fluorescence with an extremely low fluorescence quantum yield. Azulene is an unusual molecule since the energy of S1(ILb)= 14200 cm-I is approximately half that of S2(IL,) = 28300 cm-I. The primary reason for the high quantum yield of S2 So fluorescence is the large S2-SIenergy gap and the consequent small Franck-Condon factor for the internal conversion process. Another exception to Kasha's rule is the S2 So fluorescence observed from vapor-phase pyrene and from a solution of 3:4 pyrene-benzopyrene.* The S,-SIenergy gaps in these compounds are only 1300 cm-I. Fluoranthene has been suggested as a possible compound which may exhibit dual fluorescence from SI and S2 states in the condensed phase at 77 K.9v'0 The Fl band (SI So) originates at So) 402 nm and extends to 571 nm, while a weaker F2 (S, emission is observed from 353 to 393 nm. The Fl fluorescence originates from the SI state which is "hidden" in the room-temperature absorption spectrum by the adjacent strong absorption to the S2 state. The weak So S, absorption may be resolved at low temperature. An unusual feature of the dual emissions reported for fluoranthene is the proximity of the two emissions. The energy difference between F2 and Fl is only 3450 cm-' compared with an S2-SI energy gap of 14 200 cm-' in azulene.' If F2 originates from the molecular state S2which is only 3450 cm-l above SI,it would be expected that S2-Sl internal conversion, unless symmetry forbidden, would quench F2. Birksl reported that F2 emission may originate either (a) from photodegradation products or (b) from fluoranthene molecules in a different solvent environment from those yielding F,. The emission of F, exhibits
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'Author to whom correspondence should be addressed.
0022-3654/89/2093-7985$01.50/0
vibrational structure, but Fl also includes a broad structureless band at lower energies, suggestive of possible agglomeration. Therefore, it is possible that Fl and F2 originate from different solid solution or microcrystalline phases. Also, it has been reported that the F2 band of fluoranthene in frozen matrix is due to impurities."J2 In this article we present evidence of dual fluorescence for vapor-phase fluoranthene. In the condensed phase, this kind of distinctive property is not observed at room t e m p e r a t ~ r e . ' ~
Experimental Section The fluorescence lifetime and emission spectra of fluoranthene vapor in a collision-freestate have been obtained by using a system described previo~sly.'~The excitation sources used were a pulsed nitrogen laser having output at 337.1 nm (29 665 cm-I) with a nominal pulse width of 12 ns (fwhm) and a nitrogen laser pumped dye laser. By use of BPBD-365 (in a 7:3 mixture of toluene and ethanol) and an output beam of 368 nm, a pulse width of 10 ns (fwhm) was obtained. A Schott WG 360 or GG 375 cutoff filter was placed in front of the monochromator to remove the scattering light from the excitation source. From previous work in this l a b ~ r a t o r yit' ~has been demonstrated that this system is linear over more than 4 orders of magnitude in fluorescence intensity and shows no response time sensitivity change with wavelength over the range 350-600 nm. At all times the light emission level was kept low by the use of neutral-density filters so that saturation (1) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: New York, 1970; p 166. (2) Beer, M.; Longuet-Higgins, H. C. J . Chem. Phys. 1955, 23, 1390. (3) Ruzevich, 2. S. Opt. Spectros. 1963, 15, 191. (4) Binsch, G.; Heilfronmer, E.; Jankow, R.; Schmidt, D. Chem. Phys. Lett. 1967, 1, 135. (5) Dhingra, R. C.; Poole, J. A. Chem. Phys. Lett. 1968, 2, 108. ( 6 ) Rentzepis, P. M. Chem. Phys. Lett. 1968, 2, 117; (b) Photochem. Photobiol. 1968, 8, 579. (7) Rentzepis, P. M. 155th National Meeting of the American Chemical Society, San Francisco, 1968; Abstract 91. (8) Geldof, P. A,; Rettschnick, R. P. H.; Hoytink, G. J. Chem. Phys. Lett. 1969, 4, 59. (9) Birks, J. B. Organic Molecular Photophysics; Wiley: New York, 1973; p 169. (10) Philen, D. L.; Hedges, R. M. Chem. Phys. Leu. 1976, 43(2), 358. ( 1 1) Nauman, R. V.; Holloway, H. E.; Wharton, J. H. Chem. Phys. Let?. 1985, 122(5), 523. (12) Hofstraat, J. W.; Hoornweg, G. Ph.; Gooijer, C.; Velthorst, N. H. Spectrochim. Acta 1985, 41A(6), 801. (13) Bark, K. M.; Forct, R. K. To be submitted for publication. (14) Jandris, L. J.; Forct, R. K. Anal. Chim. Acta 1983, 251, 19-27.
0 1989 American Chemical Society
7986 The Journal of Physical Chemistry, Vol. 93, No. 24, 1989
Bark and Forci I
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A
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2oj;;;
T,
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400 450 WAVELENGTH (NM)
500
Figure 2. Wavelength dependence of the lifetime of vapor-phase fluoranthene with 337.1-nm excitation: (O), 412 K; ( O ) , 425 K; ( 0 ) ,432 K; (A), 438 K. The F, band lifetime is 32 f 1 ns, and the F, band lifetime is 22 f 1 ns.
t
600
r
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500 _ _ _ WAVELENGT~,“Q,
Figure I . Fluorescence emission spectra of vapor-phase fluoranthene as a function of delay time and excitation wavelength over the temperature range of 412-438 K. (A) Excitation wavelength, 337.1 nm: (-), no delay in data acquisition time; (- - -), 50 ns delayed in data acquisition time. (B) No delay in data acquisition time: (-), 337.1-nm excitation; ( - --), 368-nm excitation.
of the photomultiplier tube (PMT) was avoided. To prevent the condensation of fluoranthene on the windows of the fluorescence cell, it was modified as a dual-chamber structure. The sample chamber was positioned slightly out of the heating furnace. The temperature of the cell main body including the windows was higher than the sample chamber. The fluorescence cell was connected to a vacuum pump so that a constant back-pressure of 0.005 Torr could be maintained within the cell. The temperature of the cell was regulated by a tube furnace, and the temperature was measured to f 2 OC with an iron-constantan thermocouple. Background traces obtained by using a cold cell with all appropriate filters in place showed no evidence of any fluoroescence. The equilibrium vapor pressure of fluoranthene was calculated by using the average enthalpy of sublimation (AH) reported by Sonnefield and Zoller.ls It was assumed that AH was constant over the temperature range involved in this study. The fluorescence signal was dispersed by an ISA Model H-20 monochromator with 2-nm slits and detected by an RCA-1P28 PMT. The signal from the PMT was processed by a PAR Model 162/164/165 boxcar integrator with a 5-11s window on the Model 165 module and displayed on an X-Y recorder or strip chart recorder. Data were passed to an IBM-XT compatible computer for analysis. Reagent-grade fluoranthene (Aldrich) was used without further purification. This compound was analyzed for impurities by reverse-phase high-pressure liquid chromatography (HPLC) on a CIScolumn under conditions previously reported in the literature and was found to be 99.9% pure.I6 The fluoranthene was found to have two impurities, anthracene and phenanthrene, composing no more than 0.1% of the total sample weight. To examine the influence of impurities, the fluoranthene was purified by recrystallization from benzene (Fisher, crystallizable) four times. This purified fluoranthene was then analyzed by HPLC. The (15) Sonnefield, W. J.; Zoller, W. H. Anal. Chem. 1983,55(2), 275-80. (16) Krstulovic, A. M.; Rosie, D. M.; Brown, P. R A n d . Chem. 1976, 48, 1283.
amount of impurities was reduced to less than 0.03%. A sideby-side comparison of decay times and spectral profiles for purified and unpurified fluoranthene showed no differences. In this work we used the deconvolution technique known as least-squares iterative reconvolution to obtain the fluorescence lifetime.”
Results The fluorescence emission spectrum of vapor-phase fluoranthene excited by the nitrogen laser 337.1-nm line is shown in Figure 1A. No changes were observed over a temperature range from 100 to 200 “C. The observed lifetime at 460 nm which is the peak of the fluorescence intensity is also constant within experimental error over this temperature range. At 165 OC, the collision probability between fluoranthene molecules is calculated to be less than 0.2 during the lifetime of the excited state. Below this temperature the collision probability is negligible. Under these conditions, the system can be regarded as in a collision-free state. All of the results reported in this work were obtained under these conditions. The fluoranthene S1 .+ So 0-0 transition is 396.5 nm in a supersonic jet.I8 This 0-0 transition was reported as 402.2 nm in s o l ~ t i o n , 404.4 ~ ~ J ~nm in frozen matrix,20 and 409.4 nm from calculation^.^^ Therefore, we assume that the 0-0 transition of the vapor phase at elevated temperature is around 400 nm. In Figure 1, we can observe the emission over the S2 .-, So emission region (350-400-nm region). It does not seem likely that the electronic state of the molecule is shifted greatly by changing of sample phase from solution to vapor. In solution or solid crystals at room temperature, no fluoranthene fluorescence emission is observed in this region. The fluorescence emission spectra of purified and unpurified fluoranthene did not exhibit any differences. Therefore, it is reasonable to assume that the shortwavelength emission (shorter than 400 nm) originates from a different electronic state. To verify this assumption, we measured the lifetime as a function of emission wavelength. The results of these measurements are shown in Figure 2. These data clearly indicate the dual fluorescence, the F, band (SI So) and the F2 band (S2 So). The lifetimes of the F, and F2 bands are 32 f 1 and 22 f 1 ns, respectively. All of the measured lifetimes are constant within experimental error, and the slopes of least-squares fits for these data points are essentially zero in each band area. The lifetime of the F1 band is slightly shorter than the previously
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(17) OConner, D. V.; Ware, W. R.; Andre, J. C. J . Phys. Chem. 1979, 83, 1333. (18) Chan, I. Y . ;Dantus, M. J. Chem. Phys. 1985,82(11), 4771. (19) Siihnel, J.; Kempka, V.; Gustav, K. J . Prakt. Chem. 1980, 322(4), 649. (20) Kolc, J.; Thulstrup, E. W.; Michl, J. J. Am. Chem. Soc. 1974, 96(23), 7188.
Dual Fluorescence for Fluoranthene in the Vapor Phase
The Journal of Physical Chemistry, Vol. 93, No. 24, I989 7987
TABLE I: Analysis of the Fz Fluorescence Band Showing Biexponential Decay as a Function of Emission Wavelength"
h,. nm 370 380
T ~ .ns 21 i 1 22 i 2 22 i 1
390 400
22 f 1
T?. ns 32 f 2 31 i 1
lYl
a,
0.52 f 0.09 0.58 i 0.14
0.48 f 0.09 0.42 f 0.14
33 i 2 34 i 2
0.29 & 0.05 0.21 i 0.04
0.71 & 0.05 0.79 i 0.07
T, and T2 are the lifetimes of the short- and long-lived components of the decay curve, and a, and a2 are the emission proportions of the short- and long-lived components of the decay curve, respectively. The uncertainties quoted are one standard deviation.
(A)
ALPHR- I TAU- 27 NS CHISU- 4,516986
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600
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Figure 4. Fluorescence emission spectra of vapor-phase fluoranthene as
a function of delay time and excitation wavelength over the temperature range of 412-438 K. (A) (-) Excitation wavelength is 337.1 nm with 50-11sdelay of signal acquisition time. (--) Excitation wavelength is 368 nm with zero delay of signal acquisition time. (B) Excitation wavelength is 368 nm. Signal acquisition times are (-) zero delay, (---) 20-11s delay, and (--) 30-11sdelay.
B
25 TIME (NS)
75
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Figure 3. Representative fluorescence decay curves at 390 nm for a single-exponentialfit (A) and a double-exponentialfit (B): experimental data points ( 0 ) ; calculated data points (-). The residuals (RES) are the differences between the experimental and calculated data points.
reported value of 38 f 2 ns from this laboratory.21 The present system has an improved detector with more effective EM1 shielding that greatly decreases the noise. This noise modifies the data analysis and affects the observed lifetimes. Therefore, the lifetime reported in this paper is more reliable. The short lifetime and low intensity of F2are understandable because the internal conversion (IC) from S2 to S, will quench to some extent the fluorescence emission from the S2state. The signal in the F2band region is composed of two decay components, fast ( 7 , ) and slow decay portions (Q),owing to the scattered light from the strong F, emission and some overlap of the F, and F2bands. The fraction of the total intensity C Y , is due to the F2 fluorescence, and the fraction CY, is attributed to incomplete resolution of F1emission. The results of the F2 band analysis are shown in Table I. The 7 2 values are essentially the same as the liftime of the F, band. We attribute this to the high intensity of the F, band relative to the F2 band and the low resolution of the single monochromator. As the emission wavelength approaches the F, band, C Y , decreases and a2increases, consistent with scattered light from the high ~~
(21) Jandris, L.J.; Ford, R. K.;Yang, 266.
S.C. Appl. Specrrosc. 1985,39(2),
relative intensity of the F, band. Representative fluorescence decay curves at 390 nm are shown in Figure 3. Figure 3A is the best fit between a single-exponential and the experimental decay curve. Figure 3B is the best fit between a double-exponential and the experimental decay curve. The double-exponential curve was derived by using the parameters shown in Table I. The observed decay curve does not match the single-exponential fit but does match well with the double-exponential fit. These observations clearly support the double-exponential decay of fluorescence at this wavelength arising from the mixing of two different emissions, F, and F,. Fluorescence emission spectra were obtained at different delay times as seen in Figure 1A. The intensity of the F2band decreases rapidly compared with that of F, as the signal acquisition time is delayed. The change in the spectrum is minor because of the weak intensity of the F2band, but these changes are larger than the experimental error. Fluorescence emission spectra with 368-nm excitation are shown in Figure 4B. There is no significant change with different delay times. A small amount of signal intensity is observed at wavelengths shorter than 400 nm. We attribute this emission to the low resolution of the single monochromator of our system. In Figure lB, we can compare the emission spectra at two different excitation wavelengths with zero delay time. The spectrum excited at 368 nm has smaller intensity in the F, band region relative to the 337.1-nm excited spectrum. In Figure 4A, the two spectra, 337.1 nm excited with 50-11sdelay and 368 nm excited with zero delay, have similar intensity in the F, band region. With 368-nm excitation, which corresponds to SI manifold excitation only, the fluorescence emission spectra at different signal acquisition times are the same within experimental error. The lifetime of vapor-phase fluoranthene with 368-nm excitation is 41 f 1 ns. This long lifetime compared to the 337.1-nm excitation is not surprising because usually the fluorescence lifetime decreases as a function of the excess vibrational excitation energy above the 0-0 transition. This can be regarded as the increased probability of radiationless conversion from higher vibrational
7988 The Journal of Physical Chemistry, Vol. 93, No. 24, 1989
Bark and For&
levels. Also, the emission of F2 will partly contribute to the short the nuclear momentum and vibrational overlap integrals. The lifetime from 337.1-nm excitation of the F, band. vibrational modes can be classified into two categories:28 the The model of Heller and co-workers22 can be applied to the promoting modes and the accepting modes. Approximate selection variation of the nonradiative rate with vibrational energy, provided rules for the Franck-Condon factors are as follows: First, mixing rapid vibrational energy redistribution is not occurring. In this will occur primarily between vibronic states that differ by the model, totally symmetric C-C modes are optically populated while smallest possible number of excited vibronic modes which ensure totally symmetric C-H modes act as accepting modes to take up energy conservation. The higher frequency vibrations are favored the electronic energy gap. The increasing radiationless rate with as acceptor modes because it is unfavorable to distribute too many vibrational quanta into a low-frequency acceptor mode. Second, excess energy, within this model, is due to the increasing ability the most favorable change in the promoting mode involves one of the C-C modes to act as accepting modes as the number of quanta in the C-C modes in the initial state is i n ~ r e a s e d . , ~ . ~ ~ vibrational quantum. For the case of internal conversion, the selection rules for the electronic matrix elements are the same Discussion as those for vibronic coupling. The matrix element will be nonUsually the internal conversion between excited singlet states vanishing only for vibrations which corresponds to the same occurs on the picosecond time scale. But the fact that the F2 band representation of the molecular point group. For nondegenerate was observed indicates this internal conversion rate is so slow that electronic states, there is only one symmetry type of a molecular the two decay processes, S2 fluorescence and internal conversion vibration which scrambles these states. from S2 to SI,can compete with each other. To explain these The approximate selection rules for the nuclear and the elecphenomena, we must consider the geometry changes for different tronic matrix elements indicate that whether or not statistical electronic singlet states. Bond angles exhibit only negligible mixing occurs in a large molecule is determined by the size of the changes due to excitation, but remarkable bond length changes normalized average energy parameter p X 3, where p is the mean do o c c ~ r In . ~the~SI ~ state ~ the two bonds connecting the benzene density of vibronic states and 2 is an average coupling term. In and naphthalene moieties are shortened by about 5 pm, but in vapor-phase fluoranthene the emission spectrum is very broad and the Sz state these bonds are shortened by about 3 pm compared totally structureless. Therefore, we can postulate that the So state to the Sostate. While these bonds are shortened, some other bonds has high vibronic state density. Since it is unlikely that the density are lengthened by electronic excitation. The greatest lengthening of vibronic states changes dramatically by excitation, we may of a C-C bond is about 5 pm for So SI excitation. These rather assume the S, and S, states have enough density of levels for the large changes of bond length between So and SI relative to that radiationless transition. The value of p X v is likely to be small between So and S2may, to a large extent, explain the anomalous for the S2 SI internal conversion for the following reasons: fluorescence properties of f l ~ o r a n t h e n e . ~ ~ ~ ~ ~ ~ ~ ~ 1. The Franck-Condon factor will be small because the sigInformation from spectroscopic measurements and theoretical nificant bond length change between SI and S2states may reduce calculations, such as Pariser-Parr-Pople (PPP) type, leads to the vibrational overlap integrals. This reduces the average coupling assignment of electronic symmetry B2 for SI and A, for S2 for strength v. fluoranthene.'* Therefore, in the excited states SI and Sz, sig2. A large part of the vibrations do not correspond to the same nificant changes in bond length minimize the rate of internal representation of the molecular point group because the electronic conversion. A quantitative study of the intramolecular electronic symmetries of SI and S2are different (B2 for SI and A, for S2). relaxation time in large molecules requires the evaluation of the For nondegenerate electronic states, there is only one symmetry vibronic matrix elements that couple the zero-order Born-Optype of molecular vibration that scrambles these states. Also, this penheimer (BO) states corresponding to different electronic condition will hardly be satisfied owing to the different electronic configuration^.^' To do this, molecular geometry and the ansymmetries of SI and S2. This leads to only a small fraction of harmonicities of the vibrations in the excited electronic states of the vibronic states that can couple S2 with SI. polyatomic molecules must be considered. The nature of the Therefore, the average energy parameter p X 8 is small owing intramolecular coupling terms arising from the breakdown of the to the small value of 8, and we conclude that the IC rate of S,-SI BO approximation has been studied intensively. is unusually slow. A recently developed theoretical model, based The coupling matrix elements that determine the rate of the on the reaction path concept and the calculated potential energy . radiationless transition can be displayed as sums of products of surface characteristics, was used to calculate the rate of internal electronic terms and Franck-Condon integrals. The Franckconversion as a function of excess vibrational energy of SI Condon factors contain two contributions: matrix elements of benzene.29 It is likely that fluoranthene has too many degrees of freedom for this type of theoretical model to be useful for calculating the radiationless transition rate. (22) Heller, D. F.; Freed, K. F.; Gelbart, W. M. J. Chem. Phys. 1972,56,
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2309. (23) Beddard, G. S.; Fleming, G . R.; Gizzeman, 0.L. J.; Porter, G. Proc. R . Sac. London, A 1974, 340, 519. (24) Spears, K. G.; Rice, S.A. J . Chem. Phys. 1971, 55, 5561. (25) Berlman, I. B.; Wirth, H. 0.;Steingraber, 0.J. J . Am. Chem. Sac.
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Acknowledgment. We thank Dr. Sze Cheng Yang for his helpful comments and suggestions. Registry No. Fiuoranthene, 206-44-0.
1968, 90, 566. ( 2 6 ) Gusten,
H.;Heinrich, G. J. Photochem. 1982, 18, 9. (27) Pitts, J. N.; Hammond, G. S.;Noyes, W. A. Adu. Photochem. 1969, 7, 221.
(28) Lin, S. H.; Bersohn, R. J . Chem. Phys. 1968, 48, 2732. (29) Kato, S. J. Chem. Phys. 1988, 88(5), 3045.
