J . Phys. Chem. 1991, 95, 2971-2975
2971
Effect of Pressure on the Natural Radiatlve Lifetimes of Anthracene Derivatives in Solution Satoshi Hirayama,* Hiroya Yasuda, Laboratory of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo- ku, Kyoto 606, Japan
Masami Okamoto, Faculty of Engineering and Design, Kyoto Institute of Technology, Matsugasaki, Sakyo- ku, Kyoto 606, Japan
and Fujio Tanaka College of Integrated Arts and Sciences, University of Osaka Prefecture, Mom- umemachi, Sakai, Osaka 591, Japan (Received: July 18, 1990; In Final Form: October 19, 1990)
The fluorescence lifetimes of five anthracene derivatives (9-cyanoanthracene (CNA), 9,lO-dicyanoanthracene (DCNA), 9, IO-dimethylanthracene (DMEA), 9,lO-dimethoxyanthracene(DMEOA), and 9,lO-diphenylanthracene (DPHA)), whose fluorescence lifetimes can be regarded as radiative lifetimes, were examined as a function of refractive index, n, by varying the applied hydrostatic pressure from 0.1 to 700 MPa. The fluorescence lifetime for each compound decreases monotonically with increasing pressure. In particular, the fluorescence lifetimes of CNA, DMEA, and DPHA exhibited almost the same trend with increasing pressure and the reciprocals of their radiative lifetimes are proportional, to a good approximation, to their square of the refractive index. The factor J(2eo - e)%(~)/e de, which may also influence the magnitude of the radiative lifetime, was found to remain almost constant with increasing pressure. An important consequence of the n2 dependence of the radiative lifetime is discussed in terms of the effect of pressure on the intersystem crossing process, where 9methylanthracene (MEA) is used as an example.
Introduction The spontaneous radiative process is one of the most fundamental deactivation processes for the electronically excited states of atoms and molecules. The reciprocal of the rate constant for this process is called the natural radiative lifetime ( T ~ ) .Since the absorption and spontaneous radiative processes are paired, the radiative lifetimes of atoms and simple molecules can be predicted from the properties associated with absorption through the use of quantum mechanics.' However, even for diatomic molecules, the evaluation of the radiative lifetimes is often not straightforward when unstable electronically excited states are involved.* For large organic molecules, the situation is more complicated, even if the transition under consideration is allowed and the molecule does not undergo a significant structural change in the excited state.' Of particular interest is the effect of environment on the radiative lifetime. However, although the study of molecules located in heterogeneous systems, such as interfaces or membranes, are attracting more and more attention: a complete understanding of the effect of environment has not been r e a ~ h e d . ~ The radiative lifetime is considered to be influenced by the refractive index of the medium for various reason^^,^ and, although several functional forms of the refractive index dependence have been derived from theory, none of these have been confirmed experimentally. A comparative work5 in solution and gas phase has shown that the radiative lifetimes in solution and the gas phase can be related by eq 1, where n is the refractive index of a medium. n2r,(soln) = .r,(gas)
= constant (for a given molecule) (1) Also, our recent work,' in which the relationship between the radiative lifetimes in supersonic jets and solution was investigated, (1) Hertzberg, G. Molecular Spectra and Srrucrure; D. van Nostrand: Toronto, 1950; Vol. 1. (2) Telle, H. H. A m Phys. Polon. 1986, A70, 79. (3) Strickler, S.; Berg, R. G. J . Chem. Phys. 1962, 37, 814. (4) Yamasaki, H.; Hirayama, S.;Okamoto, M. J . Photochem. Phorobiol., A: Chem. 1990, 51, 127. ( 5 ) Hirayama, S.; Phillips, D. J . Phorochem. 1980, 12, 139. (6) Shibuya, T. J . Chem. Phys. 1983,78,5175; Chem. Phys. Leu. 1983, 103. .- -,46. .-. (7) Hirayama, S.: Iuchi, Y.: Tanaka, F.: Shobatake, K. Chem. Phys. 1990, 144, 401.
provides further confirmation of the validity of eq I. Furthermore, it was demonstrated in this latter work that radiative lifetime for an allowed transition is independent of vibronic level for vibronic levels within several vibrational quantum numbers above the electronic origin. To our knowledge, however, the only systematic studies of verifying the validity of eq 1 in the condensed phase (Le., the constancy of eq 1 for a given molecule), are those of Lampert et a1.* and O l m ~ t e d . ~This is mainly because the change in the radiative lifetime expected in the condensed phase is small. In fact, the change in the radiative lifetime of 9,10-bis(phenylethyny1)anthracene observed by Lampert et al.,8 which was achieved by varying temperature, is rather small (less than 7%) and, hence, the accuracy of fluorescence lifetime measurements must be very high in order to establish the functional dependence of radiative lifetime on refractive index. Pressure is a better method for varying the refractive index of a solvent compared with temperature, because thermal energy assisted intramolecular processes, if any, will also affect the fluorescence lifetime by changing the temperature. A wide range of solvents with different refractive indices may be employed,'O but it is often hard to distinguish a refractive index dependent part in the fluorescence lifetime from a solvent-specific one. In this work, therefore, the effect of pressure on the fluorescence lifetimes of several anthracene derivatives has been examined with the aim of testing thoroughly the n2 dependence of eq 1 in the condensed phase.
Experimental Section Fluorescence lifetimes were measured by the time-correlated single photon counting technique and the decay curves obtained were analyzed by the iterative nonlinear least-squares method. Experimental details for an excitation light source and data accumulation were the same as reported in a previous paper." The anthracene derivatives were either synthesized according to the literature or purchased from Aldrich Chem. Co. Ltd. Each compound was purified by TLC. Methylcyclohexane (MCH) (8) Lampert, R. A.; Meech, S. R.;Metcalfe, J.; Phillips, D.; Schaap. A. P. Chem. Phys. Lerr. 1984, 94, 137. (9) Olmsted, 111, J. Chem. Phys. Lerr. 1976, 38. 287. (10) Maciejewshi, A.; Steer, R. P. J . Photochem. 1986, 35, 59. (1 1) Hirayama, S.: Shimono, Y. J . Chem. Soc., Faraday Trans. 2 1983, 80. 94 1.
0022-365419 112095-297 1%02.50/0 , 0 1991 American Chemical Society I
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2912 The Journal of Physical Chemistry, Vol. 95, No. 8, 1991
Hirayama et al.
High Pressure Optical Vessel
\
.
Monochromator MC-1 ON
Flash Lamp PRA 510
Water (25OC) from Thermoregulator L
I
____
L
L
_____
J
Lens
Photomultiplier Tube
Tc-SPC
Figure 1. High-pressure optical vessel used for fluorescence decay measurements. TABLE I: Effect of Rearum on the Fluorescence Lifetimes of the Antbncene Mvrtives Whow Fluoreeecaec Qwntum Yields Are Close to Unity’ 7,Insa prw.1 PIB MPa DCNA DMEA DMEOA CNA DPHA cm-’ n(25 “C)
0.1 50 100
150 200 250 300 350 400 500 600 700
11.41 11.12 10.92 10.81 10.68 10.57 10.49 10.44 10.37 10.27 10.16 10.12
13.38 13.00 12.77 12.59 12.43 12.26 12.19 12.07 11.97 1 1.79 11.67 1 1.56
14.63 14.43 14.17 14.06 14.03 13.90 13.92 13.72 13.74 13.63 13.56 13.50
13.18 12.77 12.55 12.36 12.21 12.10 11.96 11.84 11.73 11.58 11.46 11.37
7.46 7.28 7.14 7.03 6.96 6.87 6.82 6.76 6.70 6.62 6.56 6.47
0.7650 0.7984 0.8232 0.8433 0.8602 0.8750 0.8881 0.9001 0.9109 0.9302 0.9450 0.9580
1.42058 1.4417 1.4576 1.4706 1.4816 1.4914 1.5000 1.5080 1.5153 1 S283 1.5384 1.5473
‘The errors associated with the fluorescence lifetimes are less than 1.5% in most cases.
(fluorescence grade, Dojin Laboratories) was used as received. The high-pressure vessel used for the fluorescence lifetime measurements is shown in Figure l . The applied pressure was calibrated against a manganin gauge. The temperature of the solutions in the cylindrical quartz sample cell (6 mm inner diameter) was kept at 25 f 0.1 “C. The sample solutions were deoxygenated by bubbling with solvent-saturated nitrogen gas for 20 min. The concentration of each sample solution used for the fluorescence decay measurements was kept low ( M)in order to avoid the effects of reabsorptionI2 or self-quenching on the fluorescence lifetime. The absorption and fluorescence spectra under high pressures were recorded by using a Shimadzu UV-260 spectrophotometer and a home-built fluorospectrophotometer, respectively.
Results and Discussion The n2 Dependence of rr. The radiative lifetime, T ~ can , be calculated by eq 2, when the values for the fluorescence quantum T f = T,@f (2) yield, ar,and the fluorescence lifetime, rr, are available. When arvaries with pressure, both rf and @ f must be measured as a (12) Sakai, Y.;Minami, T.; Kawahigashi, M.; Inoue, T.; Hirayama, S.J . Lumin. 1989, 42, 31 7.
0.0
10.0
20.0
30.0
4.0
50.0
, T I E / tUN0
Figure 2. Fluorescence decay curves measured for DPHA in deoxygenated MCH at 25 OC: (a) 0.1 MPa and (b) 700 MPa. The solid lines through the dots are the best-fit single-exponential decay curves obtained by the nonlinear least-squares iterative reconvolution.”*’4
function of pressure in order to evaluate rl. Since the measurement of @f is influenced greatly by experimental conditions, the experimental errors associated with this measurement are much greater than those associated with the measurement of r p For this reason it is desirable to test the validity of eq 1 for compounds whose fluorescence quantum yields are very close to unity over the pressure range examined. It is assumed that the anthracene derivatives studied in the present work meet this requirement.” (13) The fluorescence lifetimes measured in ethanol at 77 K, at which all the radiationless transition channels for these compounds are closed, are very close to those measured at room temperature at 0.1 MPa,’ supporting the present assumption.
The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 2973
Natural Radiative Lifetimes of Anthracenes
1
;2001 300
O
O
Oi
0 0
4
0 a m
0
8
0
0
6
a 0
I 0.01
1 0.02
I 0.03
I 0.04
I 0.05
J 0.06
Af (n2)
Figure 4. Linear correlations of the spectral red shifts observed for D C N A ( 0 ) ,DPHA ( 0 ) ,DMEA (A), and DMEOA (0) and the Onsager polarity functionfln2) given by eq 4.
(DMEOA), DPHA, and DMEA are plotted against the so-called Onsager polarity function (eq 4),20 where the calculated values
s
I 0
200
400
An2)=
600
Pressure ( m a) Figure 3. Effect of pressure on the fluorescence lifetime for DMEOA
(a)), D C N A (0). DPHA
( O ) , DMEA (A), and C N A ( 0 ) . The ratio
of rl(P) to rl(O.1) is plotted against pressure. The broken line represents the ratio of n2(0.1)to n2(P).
In Figure 2 the fluorescence decay curves of 9,10-diphenylanthracene (DPHA) measured in deoxygenated MCH at 0.1 and 700 MPa are compared. The best-fit single-exponential-decay curves yield lifetimes of 7.46 and 6.47 ns, respectively, indicating that Tr (=7,,in the present case) decreases as the pressure is increased. The results obtained for the other anthracene compounds are summarized in Table I. In each case, the fluorescence lifetime decreases monotonically with increasing pressure. This behavior makes a sharp contrast to a significant increase with pressure of Tr observed for 9-methylanthracene (MEA) or 9,lOdichloroanthracene (DCLA) for which fluorescence quantum yield is less than unity since the intersystem crossing process is competitive with the radiative p r o c e s ~ . ’ ~ In Figure 3 the ratio of q ( P ) to ~ d 0 . 1MPa) is plotted against pressure for each of the five anthracene compounds examined. The plots for 9, IO-dimethylanthracene (DMEA), 9-cyanoanthracene (CNA), and DPHA are very similar and are reasonably close to the broken line which represents the square of the ratio of the refractive index at the pressure, P, to the value a t 0.1 MPa and at 25 O C (=1.42058).16 Since the values for the refractive index of MCH at higher pressures are not available, they were calculated by using the Lorentz-Lorenz e q ~ a t i o n . ’ ~ J ~ (3)
The density, p, of MCH at each pressure is known as is given in Table I.l9 The constant, K,was calculated at 0.1 MPa at which both n and p are known. The values for n calculated thus are also given in Table 1. These values must be close to the true values for n. To show this, in Figure 4 the spectral red shifts observed for 9.1 0-dicyanoanthracene (DCNA), 9,lO-dimethoxyanthracene (14) OConnor, D.V.; Phillips, D. Time-Correlated Single Photon Counring, Academic Press: London, 1984. (IS) Yasuda, H.; Scully, A. D.; Hirayama, S.;Okamoto, M.; Tanaka, F. J . Am. Chem. Soc. 1990, I 12, 6847. ( 16) Organic Solvents. In Techniques of Organic Chemistry; Weissberger, A., Ed.; Interscience: New York, 1955; Vol. VII. (17) Poulter, T. C.; Ritchey, C.; Benz, C. A. Phys. Rev. 1932, 41, 366. (18) Vcdam, K.;Limsuwan, P. J . Chem. Phys. 1978,69, 4772. (19) Jonas, J.; Hasha, D.; Huang, S.G. J . Chem. Phys. 1979, 71, 3996.
(nZ- 1 ) (2n2 + 1 )
(4)
for the refractive index (eq 3) are used. A good linear relation, which is expected to hold for the general spectral red shift arising from the dispersion interaction,20indicates that the calculated values for n are reasonably close to the true ones (see also ref 18). The trend observed in the Tf(P)/TLo.l) vs P plots provides evidence that the effect of pressure on the fluorescence lifetimes for these three compounds (and probably DCNA as well) is the result of a change in the refractive index of the solvent and is not due to some inherent property of the solutes themselves. Thus the fluorescence lifetimes at any pressure, P, can be related, to a good approximation, to the lifetime measured a t 0.1 MPa by the use of eq 5 with the exception of DMEOA. It should be noted
n2(P) 7,-(P) = n2(P)r,(P) = n2(P=0.1) r,(P=O.1) ( 5 ) that eq 5 holds because, in the present case, Tf N 7,. Apart from the explicit dependence on refractive index, the radiative lifetime is also proportionally dependent on the factor given by eq 6.3321 The terms in eq 6 have the same definitions 1 / ~ , 0: $(200
- F ) ~ E ( B ) / F dB
(6)
as those given in ref 21. The values for the wavenumber of the “mirror symmetry” point, v0, and the decadic molar extinction coefficient at B, e(s), are dependent on pressure and, hence, they were determined experimentally at several pressures. Figure 5 compares the absorption and emission spectra of DMEA in MCH measured a t several pressures from 0.1 to 400 MPa. The value for po was calculated by taking the arithmetic mean of the wavelength of the longest wavelength absorption maximum and that of the shortest wavelength emission maximum a t each pressure. Figure 6 shows the variation with pressure of the values of the integral in eq 6 relative to the value at 0.1 MPa, for DCNA, DMEOA, DPHA, and DMEA. Despite the significant spectral red shifts in the absorption spectra, the relative values of the integral in eq 6 display practically no change or only a slight decrease with increasing pressure, i.e., only a slight increase in 22 7,.
The insensitivity of the factor given by eq 6 to molecular environment also appears to be the case in going from solution to (20) Suppan, P. J . Photochem. Photobiol., A: Chem. 1990, SO, 293. (2 1) Birks, J. B.Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970; Chapter 2. (22) The slight deviation from unity of the relative values of the factor given by eq 6 (Figure 6) for the anthracene derivatives studied in this work cannot account for the increasing divergence of the experimentally determined data (solid lines) from the trend predicted by eq 5 (dashed line) shown in Figure 3. At higher pressures the factor given by eq 6 tends to unity rather than showing any systematic divergence from unity.
2974 The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 1
280
I
330
1
I
1
430
480
I
380 WAVELENGTH/NM
1
I
I
I
0
a0
430
480
WAVELENGTH/HI I
1
520
570
I
I
Hirayama et al.
1
1
I
I
r
0 370
420
470 WAVELENGTH/NM
Figure 5. Effect of pressure on (a) the absorption and (b) fluorescence spectra of DMEA in MCH at 25 "C. From left to right 0.1, 100, 200, 300. and 400 MPa in each figure.
3 WAVUENGWNM
Figure 7. Effect of pressure on (a) the absorption and (b) fluorescence spectra of DMEOA in MCH at 25 "C. From left to right 0.1, 100,200, and 300 MPa in each figure.
quently, it is important that the radiative lifetime be corrected for the refractive index according to the n2 dependence when the radiative lifetime is compared in various media. The $ dependence of r, given in eq 1 can be used to estimate the refractive index of microscopically heterogeneous systems whose refractive indices are either unknown or difficult to measure.23 The reason for the significant deviation from eq 5 exhibited by DMEOA is not clear a t present. The factor given by eq 6 hardly varies with pressure for DMEOA and, hence, it does not modify the radiative lifetime to any appreciable extent. The molecular shape of DMEOA is not much different from those of the other anthracene derivatives examined in this paper. The cavity around a DMEOA molecule, therefore, must be similar in shape to those of the other anthracene derivatives. Consequently, it is not expected that a different type of dependence of the radiative lifetime on the refractive index6 can be derived for DMEOA. Furthermore, judging from the value for the fluorescence lifetime of DMEOA observed in a supersonic jet (28.1 n ~ )there , ~ is~ no doubt that the fluorescence lifetime of DMEOA measured in MCH at 0.1 MPa (1 5.6 ns) can be regarded as its radiative lifetime (see also ref 13). Thus, radiationless transitions from S , of DMEOA are already slow at 0.1 MPa and are most unlikely to affect the fluorescence lifetime when the pressure is increased. Nevertheless, it should be pointed out that the presence of two stereoisomer^^^ for DMEOA may make the electronic relaxation process for this molecule more complicated than for the other anthracene derivatives studied in this work. Consistent with this, the fluorescence spectra of DMEOA shown in Figure
' 0
100
200
300
400
Pressure ( m a ) Figure 6. Variation with pressure of the factor given by eq 6 for DCNA ( O ) , DMEOA ((D), DPHA (0),and DMEA (A) in MCH at 25 "C. The values at the higher pressures are normalized to the value at 0.1 MPa
in each case.
gas phase since the n2 dependence of 7rgiven in eq 1 holds reasonably well as has already been It has now been shown in the present work that this equation also holds to a good approximation in the condensed phase (see Figure 3).8*9 Conse-
(23) Gibson, E. P.; Rest, A. J. Chem. Phys. Left. 1980, 73, 294. (24) Hirayama, S.;Tanaka, F.; Shobatake, K. J . Phys. Chem. 1990, 94, 1317.
2975
J . Phys. Chem. 1991,95. 2975-2982
in the fluorescence lifetime is ascribed to the retardation of the rate of intersystem crossing to the nearby triplet m a n i f ~ l d , ' ~ * ~ ~ i.e., a decrease in k,,(P) with pressure. In order to calculate the true pressure dependence of k,,(P) the change in 7, with pressure must be taken into account by using the correction given by eq 5, since, as was discussed in the previous section, T , is not constant but depends on pressure as a result of changes in the refractive index. Equation 7 can then be rewritten as follows:
19
knr(P) = 1 / ~ f i P-) 1/Tr(P)
18
= 1/~1(P)- n2(P)/n2(P=0.1)T,(P=O.I)
J
(8)
The values for k,,(P) which were calculated without correcting for the dependence of T,(P)on pressure is denoted k::(P), whereas the values corrected for changes in the refractive index are denoted k z ( P ) . These values are plotted in Figure 8. The two curves deviate significantly from each other at higher pressures and, consequently, the apparent activation volumes, AV',,, calculated by using eq 9, Le., the slopes of the two curves given in Figure
E
4
17
RT 16
0
200
400
600
Pressure (MPa) Fipre 8. Pressure dependence of the nonradiative rate constant of MEA in MCH at 25 O C (a) after and (b) before correcting for the change in the refractive index with pressure by using eq 8. The solid lines represent the best-fit quadratic polynomials.26
7b are much broader than those of DMEA, though both compounds exhibit similarly structured sharp absorption spectra as is seen from a comparison of Figure 5a and Figure 7a. An Important Consequence ofthe n2 Dependence of 7,. Since the fluorescence lifetime is more often affected by nonradiative processes whose overall rate constant is given by k,,, 7fiP) is more generally written as follows for many other anthracene derivatives:
7dP) = 1 / [ 1 / ~ r+ knr(P)) (7) In many cases the fluorescence lifetime increases as the pressure is increased. For example, as has been reported previou~ly,~~ the fluorescence lifetime of 9-methylanthracene (MEA) is 5.0 ns at 0.1 MPa and it increases gradually with increasing pressure and appears to level off at a value of 10.2 ns at 700 MPa. This increase
a In k,,/aP = -AVnr
(9)
8, also differ significantly. The corrected and uncorrected activation volumes calculated at 300 MPa are 9.1 f 0.5 and 6.4 f 0.7 cm3 mol-', respectively. This correction for changes in refractive index given by eq 8 would also be necessary when activation energies for radiationless processes are calculated from Tf measured a t different temperatures, since the refractive index of a medium also changes with temperature.20 In conclusion, it has been shown clearly in this work that the natural radiative lifetime is influenced by changes in the refractive index of the medium in the form of n2,but hardly by the factor given by eq 6 . Since the n2 dependence given in eq 1 can be used to predict satisfactorily the radiative lifetime, this relation should be of great use in many cases where experimental values for fluorescence quantum yields are difficult to obtain.
Acknowledgment. This work is partly supported by Grantsin-Aid 01470010 and 02245102 to S.H. and 6254033 to M.O. Registry No. DCNA, 1217-45-4; DMEA, 781-43-1; DMEDA, 2395-97-3; DNA, 1210-12-4; DPHA, 1499-10-1; MEA, 779-02-2. ( 2 5 ) Tanaka, F.; Okamoto, M.; Yamashita, S.; Teranishi, H. Chem. Phys. Lett. 1986, 123, 295. (261 Isaacs. N. S . Liauid Phase Hiah Pressure Chemistry. _ . Wilev: . ChiChester, U.K., 1981; p 183.
-
Collision-Induced Intersystem Crossing in NH(a'A)
-
NH(X3Z-)
J. Stephen A d a d and Louise Pasternack* Chemistry Division, Code 61 10, Naval Research Laboratory, Washington, D.C. 20375-5000 (Received: August 24, 1990; I n Final Form: November 9, 1990) Laser-induced fluorescence (LIF) of individual rovibronic states of gas-phase NH(a'A) and NH(X31T) is used to study intersystem crossing (isc) induced by collisions with Nz, CO, Xe, and Oz. Hydrazoic acid, HN3, is photolyzed at 266 nm to produce NH(a'A). The time-dependent behavior of NH(a'A) in its ground and excited vibrational levels is probed by LIF. Appearance rates for NH(X3Z-) in several vibrational levels are also measured. The vibrational level distribution of NH(X%) is measured and compared to prior distribution calculations in order to predict the position of the potential energy curve crossing for the collision-induced isc. The results of these measurements on the mechanism of NH(a'A) decay are discussed in light of recent theoretical calculations and by comparison to isc occurring in the isoelectronic O(ID).
Introduction The study of electronically excited species is of considerable interest to both the experimentalist and theoretician. Various electronic relaxation processes have been extensively studied Over *NRL/NRC Postdoctoral Research Associate.
the past few years, particularly the electronic relaxation of small molecules in which spin-orbit coupling is h " a n t . When these molecules are in the collision-free regime, decay may occur via a radiative transition; however when the molecular system is perturbed by collisions, nonradiative processes may occur. Our interest is in the study of collision-induced intersystem crossing
0022-3654/91/2095-2975%02.50/0 0 1991 American Chemical Society
