Radiative and radiationless rate constants of aromatic hydrocarbons. Chrysenes

JOURNALOF MOLECULAR SPECTROSCOPY 32, 368-374 (1969)

Radiative

and Radiationless

Aromatic

Rate Constants

Hydrocarbons.

of

Chrysenes

J. S. BRINEN, AI. I<. ORLOFF, J. B. GALLIVAN, R. F. STARING,AND B. G. ROBERTS Central Iiesearch Division,

Anzerican Cyanalnid

Connecticut

Company, Stamford,

06904

Quantitative luminescence and ESR triplet measurements have been performed on chrysene, chrysene-dl, , and 6-fluorochrysene and the data used to evaluate the relevant rate constants involved in populating and depopulating the lowest excited singlet and triplet states in these molecules. The resulbs show that dellteration significantly affects only nonradiative losses from the lowest triplet, while monofluoro sukitution affects the radiative fluorescence rate, the radiative and nonradiative rates from the lowest triplet but does not appreciably alter the rate of intersystem crossing. Quantitative

Auorescerxe

and

rescence and phosphorescence

phosphorescence

lifetime

yields

data to evaluate

can be used with fluothe rate constants in-

volved in the population and depopulation

of the lowest triplet state of aromatic

hydrocarbons

the above measurements are used to

(1). In this communication,

obtain these rate constants for chrysene and chrysene-& measurements may be combined with quantitative test the validity

. Quantitative

ESR

luminescence measurements to

of the equations and assumptions used. In lieu of some relevant

data as to changes in these rates with regard to substitution

on the chrysene

nucleus, the ESR results may be used to predict some of the missing parameters. To this end, the effects of a fluorine substituent on intersystem crossing, radiative triplet and singlet decays and nonradiative EXPERIMENTAL

triplet decay are determined. METHODS

Vacuum sublimed and zone refined chrysene was obtained commercially

from

James Hinton and was used without further purification. Chrysene-& was obtained from ,IIerck Sharpe and Dohme, Ltd and was further purified by chromatography. The Wluorochrysene was obtained from Aldrich Chemical Company Ultraviolet

and was chromatographed

and recrystallized

absorption, low temperature

ments indicated

that the compounds

by Dr. I<. hlatsuda.

luminescence and triplet ESR measure-

were of high purity.

The Z-methyltetra-

hydrofuran, purified by conventional means (2), was used as the solvent both room temperature and low temperature measurements. 368

for

RATE CONSTANTS FOR CHRYSENES

369

Room temperature fluorescence yield measurements (&.) were performed using a calibrated (S’) Bausch and Lomb monochromator in conjunction with an AH-3 mercury arc. Solutions of 5 X 10d4 moles/liter of the chrysenes in g-methyltetrahydrofuran were degassed on a vacuum line through freeze-pump-thaw cycles until a pressure of less than 1 p was achieved at 77°K. These were then warmed to room temperature for fluorescence yield measurements. The sensitivity of & measurements to oxygen quenching was monitored by comparison with measurements in air and argon atmospheres. Relative yields (at 77°K) of phosphorescence to fluorescence (&/4,) were obtained using a modified PerkinElmer model 12C spectrometer (4) with excitation from a 1000 W xenon arc with proper filtering. A computer program to correct for photomultiplier response (5) and the nonlinear wavenumber drive of the spectrometer was used to obtain the (&/&) ratios. These ratios were relatively unchanged by removal of oxygen. The equipment and technique used for the ESR measurements of triplet growth curves and concentrations has been previously described (6, 7). In all the experiments performed, 3130 A radiation was the primary source of excitation, and the assumption is made that, for chrysene, chrysene-& and 6-fluorochrysene, the overall rate of internal conversion from S, to & , proceeds with the same efficiency. Triplet lifetimes (TV) were determined both from ESR and phosphorescence measurements. The lifetimes obtained by these two methods differed by less than 10%. The fluorescence lifetimes (TV) used in the calculations were values taken from the literature (8, 9). Room and low temperature ultraviolet absorption spectra were obtained using a Gary 14 spectrophotometer and a low temperature (10) absorption cell.

The long wavelength absorption band system (I&) of chrysene is significantI;* intensified with fluoro-substitution in the 6 position, with the intensity of the O-O band increased by a factor of -3. The second absorption band system (‘L, ) is only slightly changed in intensity. Excitation into the second band system using 3180 A radiation results in nearly equal absorption of light by all three molecules and removes the usual difficulties associated with fluorescence measurements when the exciting light is in the same region as the fluorescence. This, together with the assumption the S? w+ S1 internal conversion is the same in each case, permits a valid comparison of achievable steady state triplet concentrations. The results of the quantitative measurements are shown in Table I. The effective quenching by O2 of the fluorescence properties of the chrysenes in room temperature solutions is readily seen by comparing the first three columns in Table I. The value of c#+for chrysene in column 3 is in agreement with recentI> reported values (11, 12). The low temperature measurements are relativel)

370

BRINEN

ET AL.

TABLE

I

QUANTITATIVE LUMIN&SCT:NCI’:AND ESR Molecule

Chrysene Chrysene-dl? 6-Fluorochrysene

bFa

$JFb

‘$F’

7,” (set)

.05 .06 .09

.ll .ll

.19 .22 .28

50 X 10-9 50 X 10-9

.175

a Room temperat,ure measurements

in air; accuracy

DATA 7Te (set)

.29 1.13 .18

2.7 f 13.5 f 2.2 f

0.1 0.5 0.1

for all +F measurements

:-

T’

TCISHIIF

1.2 3.1 (1.0) is estimated

as *7.5%. b Room temperature measurements; solutions bubbled with argon taken directly from cylinder. o Room temperature measurements on solutions degassed on vacuum line. d See Ref (9); measurements at 77°K. e Measurements at 77°K. f The ratio of triplet molecules achieved at steady state relative to 6-fluorochrysene; the maximum concentration of triplet,s observed in these experiments (for C!,,D,,) varied between 3.03.5 X lO+ moles/liter. (depended upon thickness of filters).

insensitive

to the removal of oxygen presumably because diffusion processes are negligible. The increased fluorescence yield for 6-fluorochrysene is compatible with the increased intensity of the ‘LB transition and in turn for the lower (&/c#+) value. Deuteration significantly enhances 7T and (&/&) as expected. The relative triplet state concentrations achieved at steady state’ are shown in the last column of Table I. Rise curves were measured by following the rate of growth of triplet molecules as a function of excitation time until steady state conditions were achieved. Using the ratio of the steady state concentrations determined from the ESR measurements, the curves shown in Fig. 1 were obtained. The point to be noted is that the initial slopes for all three molecules are the same. The significance of this point will be seen later.

Kinetic Calculations The quantitative luminescence data may be used to calculate the radiative and radiationless rate constants involved in the population and depopulation of the lowest excited singlet and triplet states for the molecules under investigation. Vigure 2 serves to define the terms used in the kinetic equations. 1The ESR signal (Arr~= 2) for G-fluorochrysene was composed of two peaks with a separation of 24 G. This splitting is similar to t,hose observed in 1-fluoronaphthalene and 4,4’bifluorobiphenyl and is due to the fluorine isot’ropic hyperfine interaction (13). In addition, the low and highfieldAnz = 1% axis peaks (14) in6-flllorochrysene are alsosplit into twoeqllal peaks, with a splitting of ~45 G. This splitting probably arises from the anisotropic portion of the electron-flllorine hyperfine structure tensor.

RATE

CONSTANTS

FOR

CHRYSENES

371

80 -

a0 -

40 -

20 -

1.U t

.5.Y

(sec.)

FIG. 1. Growth of triplet molecules in chrysene, chrysene-dls and 6-fluorochrysene. At steady state, the ratio of triplet molecules formed, relative to 6-fluorochrysene is 1.2 for chrysene and 3.1 for chrysene-dl,

From the basic definitions for & and &,

the following expressions may be obtained.

u-here:

BRINEN ET AL.

372 s2 t

FIG. 2. Schematic energy level diagram used to define the rates involved in populating the lowest excited singlet and triplet states. The radiative rate of flnorescence is given by kF , the rate of intersystem crossing by ki, , the radiative and nonradiative rates from the lowest triplet to the ground state by k, and kNR , respectively.

&A_

L (

71

kF=L

--

4F

>

NR =

!&

1

=_f_

4F 72 k, >

(

-

kp,

0)

-

78

kix,

(3)

7s

Izl--F

kp 0 4’F TT

-

TT

I+-!?73

k, =% 1x

1

;k

’

(4)

The only assumptiorl made in the derivation of the above equations is that the rate for X1-A SO process is slow with respect to kF and Izi, . While this may not always be valid (6), the identical values for 7, and & for chrysene and perdeutero-chrysene and the same initial slopes in the triplet rise curve certainly support this position here. If it is further assumed that & for chrysene is independent of temperature,’ then it may be used with the (&/4,) ratio measured at 77°K to obtain c#+ . The rate constants obtained using Eqs. (24) are tabulated in Table II. The first point of interest is that deuteration slows k,, by a factor of 7 and leaves k, unchanged, accounting for the increased phosphorescence lifetime. Fluorine substitution increases both k, and kNR by about the same relative 2Partial slIpport of this is that r1 measured at 77°K is 50 X 10mgset (9) while r at room temperature is nearly the same, 45 X lO+ set (8).

RATE

CONSTANTS TABLE

FOR

373’

CHRYSENES

II

RADIATIVE AND RADIATIONLESS RATE CONSTANTS

(X 10-7 SeO)

(SC’) kp Chrysene Chrysene-dlu 6-Fluorochrysene

.025

.024 .032

km

kF

kix

.345 ,050 .423

.38 .44 .62

1.62 1.56 1.6

4ix

r9

(talc) kixlks

.81 .78 .72

45

4.26 3.55 2.58

amounts and thus accounts for the small change in 7T observed relative to chrysene. Once Iz, is calculated, Eq. (3) may be used to obtain kF (if 7s is known) or Eq. (4) may be used to calculate (kiJ&). Using available literature data for 7#, (9) k, for C&,Hlz and CI~DI~ is found to be .38 X lo7 see? and .44 X 10’ respectively. This difference in k, for the light and heavy molecules arises se?, from the small difference in the measured values of 4F (cf. Table I); therefore, it is believed that the disparity is not significant. From these values and Eq. 4, values of ki, were obtained. Within experimental error ki, was unchanged by deuteration and was found to be 1.6 X lo7 set-‘. Using these values, +ix was calculated to be -0.8 for both C18H12and ClsDlz. This is in good agreement with recently published values for C&H12 (Id). Although individual kF and ki, values reported here differ from those of Ref. (Id), the ratios (ki,/k,) are within 15%. Since r, for 6-fluorochrysene is not known only the ratio (ki,/k,) may be calculated. By use of the ESR rise curves it may be seen that ki, is the same (within experimental error) as that for C18H12.3 Using this result, I& is calculated to be 0.63 X lo7 see-‘. The significant increase in kF for 6-fluorochrysene with respect to chrysene is in line with the increase in the intensity of the & +- I% absorption band system. Using these values, +ix is calculated to be 0.73 and 7s is predicted to be 45 X lo-’ sec. A heavy atom effect on ki, for fluorine substitution is not observed, although the effect is seen in k, and kxR . 3 This involves use of the rate equation for triplet formation and is given by dT~/rll = ki,S1 - (kp + kNR)Tl . In the limit as the triplet concentration approaches zero, the initial slope of the growth curve is given by: slope = lim (dTJdl)T1 --t 0 = kixS1 . Since the exciting light absorbed for C18H12 and CMD~~ is the same, the identical initial slopes observed with the calculated values obtained using imply that (~~x)c,~H,~ = (kix)CIsDu ; in agreement the quantitative luminescence data. Similarly, since the light absorbed by CMH~F is nearly the same as for ClyH~p , it is seen that its kix is the same as for chrysene. In light of some recent work where Sp +- 81 absorption has been reported (16-17), it may be of interest to show how the above treatment may lead to an estimate of steady state singlet concentration under our experimental conditions. This may be accomplished by plotting the absolute triplet concentration versus time growth curve and obtaining the initial slope. Using ki, as 1.6 X lo7 set-1 and a steady state triplet concentration for CIyD12 as 3 X lO+ moles/liter, 81 (at t,imes where TI --) 0) is found to be ~2 X 1W2 moles/liter.

374

BRINEN

ET

SL.

CONCLUSIONS

From the quantitative experiments described above, the following observations may be deduced regarding the relevant rate constants for the chrysenes. 1. Deuteration of the chrysene nucleus has little or no effect on the radiative phosphorescence rate constant and on the rate of inter-system crossing. The nonradiative rate constant (kNR) from T1 to So is reduced by a factor of 7 upon deuteration. The radiative lifetime, 7P , is 40 sec. 2. Monofluoro substitution increases both k, and Ic,, by -25 % but has little or no effect on ki, . 3. The radiative rate of fluorescence for Cr8Hr2 is the same as that for GsDlz within experimental limits and nearly half of that for 6-fluorochrysene. 4. The quantum yield of intersystem crossing, 4ix is -0.S for Cr8Hr2and C18D12 but is reduced to 0.72 for CrsHnF. 5. Since h-i, is larger than Ic, , the observable singlet lifetime for C&HnF is predicted to be only slightly shorter than 7, for C18H12 (even though I#P~ and kF are considerably greater for 6-fluorochrysene). RECEIVED: March 13, 1969 REFERENCES 1. N. J. TURRO, “Molecular 1. 9.

4. 6. 6. 7. 8. 9. IO. 11. 12. 1s.

14. 16. 16. 17.

Photochemistry.” Benjamin, New York, 1965. J. B. GALLIVAN AND W. H. HAMILL, Trans. Faraday Sot. 61, 1960 (1965). B. G. ROBERTS AND R. C. HIRT, Appl. Spectr. 21, 250 (1967). F. HALVERSON, J. S. BRINEN, AND J. R. LETO, J. Chem. Phys. 41, 157 (1964). A. ZWEIG AND J. B. GALLIVAN, J. Am. Chem. Sot. 91, 260 (1969). J. S. BRINEN, W. G. HODGSON, AND M. K. ORLOFF, J. Mol. Spectry. 23, 112 (1967). J. S. BRINEN, J. Chem. Phys. 49,586 (1968). I. B. BERLMAN, “Handbook of Fluorescence Spect,ra of Aromatic Molecules.” Academic Press, New York, 1965. J. D. LAPOSA, E. C. LIM, AND R. E. KELLOGG, J. Chem. Phys. 42,3025 (1965). J. G. KOREN, J. S. BRINEN, AND R. C. HIRT, Appl. Opt. 3, 1431 (1964). W. R. DAWSON AND M. W. WINDSOR, J. Phys. Chem. 73, 3251 (1968). A. R. HORROCKS AND F. WILKINSON, Proc. Roy. Sot. (London), Ser. A 306,257 (1968). P. H. H. FISCHER AND K. H. HAUSSER, Chem. Phys. Letters 1,665 (1968). E. WASSERMAN, L. C. SNYDER, AND W. A. YAGER, J. Chem. Phys. 41, 1763 (1964). J. R. NOVAK AND M. W. WINDSOR, J. C&m. Phys. 47, 3075 (1967). Y. NAKATE, N. YAMXMOTO, AND H. TSIJROMURA, Chem. Phys. Letters 2, 57 (1968). R. BONNEAU, J. FAURE, AND J. JOUSSOT-DUBIIXN,Chem. Phys. Letters 2, 65 (1968).