Reprinted from THE JOURNAL OF CHEMICAL PHYSICS, Vol. 48, No. 12, 5358-5360, 15 June 1968 Printed in U. S. A.

Environmental Effects on Phosphorescence. II. "Activation Volumes" for Triplet Decay of Aromatic Hydrocarbons*

B. A. BALDWIN AND H. W. OFFEN BALD-BA 68 -0812

Department of Chemistry, University of California, Santa Barbara, Califurnia

(Received 21 February 1968) FEB 7 1969 The rate of triplet decay k has been measured to 32 kbar for 20 aromatic hydrocarbons at room tem­

perature. The measured lifetimes are generally shortened by high pressure. The slope of Ink-vs-P plots were all linear above 15 kbar. This slope is positive except for terphenyl, and can be related to "activation volumes" I!J. Vt for this kinetic process. The values -1.0<I!J. Vt <0.2 Aa/molecule are found for this class of compounds. The perdeuterated compounds have a larger negative "activation volume" than the correspond­ing perprotonated aromatics. Hence, environmental perturbations are most effective for slow processes which arise from weak intramolecular perturbations in the solute.

INTRODUCTION

The pressure dependence of rate processes can be ex­pressed in terms of the Eyring transition-state theory which leads to the concept of an activation volume Ll V t . The rate constant k of unimolecular rate processes as a function of pressure at constant temperature is

d Inkj dP= -LlV:j RT. (1)

This equation has been used in the pressure studies of chemical reactions! and molecular relaxation processes.2

The application to electronic relaxation3 in rigid media is not self-apparent. Temperature studies of phospho­rescence lifetimes in polymer matrices,4-6 organic glasses/·s and mixed crystals9•10 suggest the presence of an activated process for electronic relaxation. It is difficult to remove the possibility that temperature­dependent phenomena are caused by unknown quench­ers. This might occur by static quenching, including energy transfer, as well as dynamic (diffusional, vis­cosity-dependent) quenching which can lead to pseudo­unimolecular kinetics. Skepticism about a truly temperature-dependent mechanism for radiationless transitions is encouraged by recent theoriesll which pre­dict a negligible temperature effect at ordinary tem­peratures.

The present work is consistent with the view that

* This work is supported by the U.S. Office of Naval Research. 1 E. Whalley, Ann. Rev. Phys. Chem. 18, 205 (1967) . 2 For example, articles by G. Williams and P. W. Davidson,

Molecular Relaxatiol' Processes (Academic Press Inc., New York, 1966) .

3 G. W. Robinson,J. Chern. Phys.47, 1967 (1967). • R. E. Kellogg and R . P. Schwenker, J. Chern. Phys. 41,2860

(1964) . 6 C. Thomson, J. Chem. Phys. 41,1 (1964) . 8 J. L. Kropp and W. R. Dawson, J . Phys. Chem. 71, 4499

(1967) j P. F. Jones and S. Siegel, Pacific Conf. Chern. Spectry. Abstract 192, 1967.

7 S. J. Ladner and R. S. Becker, J. Chern. Phys.43, 3344 (1965). 8 J. E. Hodgkins and J . D. Woodyard, J. Am. Chern. Soc. 89,

710 (1967) j J. W. Hilpern, G. Porter, and L. J. Stief, Proc. Roy. Soc. (London) A277, 437 (1964) .

g S. G. Hadley, H . E. Rast, and R. A. Keller, J. Chern. Phys. 39, 705 (1963).

10 N. Hirota and C. A. Hutchison, J. Chern. Phys. 46, 1561 (1967) .

11 W. Siebrand, J. Chern. Phys. 47, 2411 (1967) .

unimolecular triplet decay in polymer matrices is an activated process. Here the temperature is held con­stant (room temperature) and the rigid matrix is subjected to high pressures (10-32 kbar). Oxygen­quenching experiments under pressure suggest!2 that the observed decrease of the triplet lifetime at high pressure is not due to dynamic quenching. It is then convenient to summarize the pressure data in terms of "activation volumes," although its physical meaning is by no means clear from the present data. It has been shown in Part I of this seriesl3 that triplet decay in aromatic hydrocarbons in ""l()-3M polymethylmetha­crylate (PMMA) is exponential. The triplet state (T1)

is depopulated by both radiative (kp) and radiationless (kp') transitions, such that the observed lifetime T= (kp+kp')-I. For aromatic hydrocarbons both rate processes are slow « 10 secl ) and the inequality kp'> kp is obeyed.

RESULTS

The measurements were made on thoroughly de­gassed samples by techniques described in 1.13 Figure 1 illustrates a plot of Ink vs P for triphenylene-dl2 in PMMA. The zero-pressure intercept corresponds to the lifetime measured at atmospheric pressure. Earlier graphsl3 of k- ! vs P also gave linear fits so that the data are too insensitive to yield the precise functional dependence of k on P, primarily because the changes in lifetime «50%) and pressure (:::;32 kbar) are small. Irregularities of unknown origin occurred in the lnk­vs-P plots for chrysene, chrysene-dI2, and terphenyl at lower pressures, but a distinct linearity is discernible above 15 kbar.

The "activation volumes," according to Eq. (1), are tabulated in Table I. It is seen that the Ll V: values are small and negative (except for terphenyl). The "ac­tivation volumes" computed from the linear plots

12 B. A. Baldwin and H. W. Offen, "Environmental Effects on Phosphorescence. III. Oxygen Quenching of Naphthalene Triplets in Compressed PolymethyImethacrylate," J. Chern. Phys. (to be published) .

13 B. A. Baldwin and H. W. Offen, J. Chern. Phys. 46, 4509 (1967) .

5358

5359 ENVIRONMENTAL EFFECTS ON PHOSPHORESCENCE. II

TABLE 1. Phosphorescence rates (seconds-1) and activation volumes (angstroms cubed/molecule) for aromatic hydrocarbons."

k(O)b -AVt • k,, (22)/k,,(0)d k,,'(32)/k,,'(0)e

Naphthalene 0.666 N aphthalene-ds 0.070 Phenanthrene 0.406 Phenanthrene-d,o 0 .089 Chrysene 0.806 Chrysene-d12 0.126 Triphenylene 0 . 117 Triphenylene-d12 0.076 Pyrene 2.81 Pyrene-d'0 0 .406 Diphenyl 0.405 Diphenyl-dlO 0.157 p-Terphenyl 0.910 p-Terphenyl-da 0 .333 Coronene 0 .154 1,2-Benzanthracene 3 .25 Fluorene 0.224 Acenaphthene 0.544 Acenapthylene 0.700 Fluoranthene 1.93

• We thank Dr. R. E. Kellogg for gifts of deuterated pyrene. chrYsene. and triphenylene.

b The average deviation is 6 %. e The uncertainty in Avt is estimated to he ±O.I AI/molecule. except

above 15 kbar are reproducible and, in contrast to l-atm lifetimes, are insensitive to sample preparation and treatment techniques as well as trace amounts of oxygen. Reversible pressure effects are evidence for the absence of photochemistry in these systems. The initial (zero-pressure) rate of triplet decay is given in the first column of Table I. With the exception of chrysene, chrysene-dl2, and terphenyl these values are in agreement with the intercept from linear pressure plots for thoroughly degassed polymer matrices. There exists a correlation between initial lifetimes r(O) and the magnitude of ~ Vt . This is illustrated in Fig. 2 by an Inkp ' (0) -vs-~ V t plot for these aromatic hydrocarbons.

It would be interesting to assign the pressure de­pendence of k to its two components. This task is for­midable because steady-state intensity measurements, required to measure kp separately, become more difficult than decay measurements at high pressure. Neverthe­less, the attempt to measure kp(P) in F3 demonstrates

FIG. 1. Pressure depend­ence of triplet decay of triphenylene-d'2 in PMMA.

I.B

,. }; I 2.2

2.6

10 30

PRESSURE (KBAR)

0.38 1.25 1.3 0.66 1.2 2.1 0.37 1.25 1.3 0.48 1.25 1.6 0.1 1.0 1.1 0.5 1.4 0.93 1.25 2.4 1.04 3.5 0.16 1.1 0.50 1.5 0.15 1.15 1.1 0.46 1.2 1.5

-0.2 1.35 0.9 0.21 1.3 1.3 0.38 1.3 1.4 0.05 1.0 0.43 1.35 1.4 0.25 1.2 0 1.0 0 1.0

that it is larger for terphenyl and chrysene . d The reproducibility is ±10 %. • The assumed values are kp(O) =0.033 sec-'. kp (32) =0.043 sec- l for

all aromatics.

that the radiative rate increases by 0%-35% in the 0-22-kbar interval for the studied aromatics. The fluctuations in the ratios kp (22 )jkp (0) for the different aromatics, tabulated in Table I, may be real but are in any case small. Since kp' (0) ~kp(O), where the equality sign is applicable to only a few deuterated aromatics,ll.14 it follows that in most aromatics the measured pressure effects arise largely from an increase in the radiationless transition probabilities. If it is assumed that kp(O) = 0.033 seclll.14.l5 and kp (32) =0.043 secl for all aro-

-3.2 • • • -2.4 • 0

0 • -1.6 0

-~ • .>< c 0 o.

-O.B 0

0 0

0 0

0 .0

0 O.B

n 0

0 .0 0.4 08

-AV· (As/molecule)

FIG. 2. Plot of the logarithm of the radiationless rate constant at 1 atm (corrected by subtracting 0.03 sec- l from the observed lifetime) against "activation volumes" A vt. Deuterated com­pounds are identified by full circles.

14 W. Siebrand, J. Chern. Phys. 44, 4055 (1966) . 16 R. E. Kellogg and R . G. Bennett, J. Chern. Phys. 41, 3042

(1964) .