Reprinted from THE JOURNAL OF CHEMICAL PHYSICS, Vol. 49, No.7, 1937-2939, 1 October 1968 Printed in U. S. A.

Environmental Effects on Phosphorescence. IV. Triplet Decay of Halonaphthalenes in Compressed PMMA * B. A. BALDWIN AND H. W. OFFEN

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

(Received 13 May 1968)

FEB 20 1969 The effects of high pressure (0-32 kbar) on the phosphorescence lifetime Tp of seven monohalogen­

substituted naphthalenes, dispersed in polymethylmethacrylate (PMMA), have been measured at room temperature. It is observed that the pressure-induced shortening of the lifetimes becomes less significant in those halonaphthalenes which possess the heavier atoms, i.e., the greater the intramolecular heavy-atom effect the less influential are environmental perturbations. "Activation volumes," ranging in magnitude between -0.25<.1. Vt <0.0 A'/molecule, were computed from linear InTp-vs-P plots. Qualitative arguments are given to support the observed magnitudes and trends in the pressure-induced volume effects on triplet decay.

INTRODUCTION

In the present paper of this series we report on the effect of high pressures on the phosphorescence lifetime Tp of several halonaphthalenes to compare environ­mental influences with the intramolecular heavy-atom effect. McClure l established that the long-lived phos­phorescence is obtained from triplets by noting that the heavier the substituted atom on a 7r-electron molecule, the greater the singlet-triplet transition probability and the shorter the lifetime. For a series of related com­pounds such as the halonaphthalenes, McClure l found agreement with the equation

T J!= constant, (1)

which follows from simple first-order perturbation calculations of the transition moment for the "spin­forbidden" transition. The parameter ~ is the spin­orbital coupling factor and represents the spin-orbit interaction in the atom. As an approximation, the ~ values computed from atomic spectroscopy are used for molecules with chemically attached heavy atoms. Then Eq. (1) implies that the triplet lifetime shortens in proportion to the square of the spin-orbit interaction energy of the substituent atom. In the absence of inter­molecular processes the lifetime (Tp-1=kp+kp') is comprised of a radiative (k,,) and radiationless decay (k,,') process. The heavy atom enhances these two rates as well as the intersystem crossing from excited singlet to triplet,2.a although the latter process will not concern us presently.

The influence of pressure on first-order rates is con­veniently summarized, as shown in Part II of this

series,4 by the equation

d InT"jdP=6.Vt jRT, (2)

where 6. V t is an "activation volume" for the triplet decay, R is the ideal gas constant, and T is room temperature for the high-pressure measurements.

EXPERIMENTAL

The substituents are fluorine, chlorine, bromine, and iodine in the 1 or 2 position on the naphthalene ring. The 1-substituted halonaphthalenes are purified by vapor-phase chromatography. The 2-substituted solid compounds are recrystallized from ethanol and sublimed twice under reduced pressure. The solute is added to purified methylmethacrylate monomer (,-,,10-3 M) and polymerized at elevated temperatures to form poly­methylmethacrylate (PMMA) rods. Difficulties with iodonaphthalene samples prepared in this way led to a modified procedure in which the solute is embedded in PMMA films by solvent evaporation. Since oxygen had a noticeable effect on the phosphorescence lifetimes, all plastic disks were thoroughly deaerated as described previously." The phosphorescence is detected in trans­mission (1800 geometry) with two sectors mounted on a common shaft and rotating out of phase. The high­pressure optical cell and other experimental details have been described.4,5

RESULTS

The phosphorescence of seven halonaphthalenes has been studied in PMMA matrices. Table I compares the triplet lifetimes in organic glasses1.2,6 with those meas-

• B. A. Baldwin and H. W. Offen, J . Chem. Phys. 48, 5358 • This work is supported by the U.S. Office of Naval Research. (1968). 1 D. S. McClure, J. Chern. Phys. 17, 905 (1949). 6 B. A. Baldw:O and H. W. Offen, J . Chern. Phys. 46, 4509 'V. L. Errnolaev and K. K. Sv~tashev, Opt. Spcctrosc. 7,399 (1967) .

(1959). 6 S. P. McGlynn, M . J. Reynolds, G. W. Daigre, and N. D . IV. L. Ermolaev. Usp. Fiz. Nauk 80, 3 (1963) [Sov. Phys.- Christodouleas, J. Phys. Chern. 66, 2499 (1962); S. P. McGlmn

Usp. 6, 333 (1963)]. R. Sunseri, and N. Christodouleas, J. Chern. Phys. 37, 1818 (1962):

2937

2938 B. A. BALDWIN AND H 'i W. OFFEN

TABLE I. Triplet lifetimes (se('onds) of halonaphthalenes at 77°K.

Ermolaev Naphthalene McGlynn and substituent McClure • et al. b Svitashev C

1-fluoro- 1.5 1.4 1.5

2-fluoro-

1-chloro- 0.30 1.23 0.29

2-chloro- 0.47

1-bromo- 0.018 0.014 0.02

2-bromo- 0 .021

2-iodo- 0.0025

a Reference 1. b Reference 6. C Reference 2.

Present work

PMMA

1.53 1.51

0 .28

0.33

0.014

0.016

0 .0030

ured in PMMA at 77°K and at atmospheric pressure. The Tp values measured in organic glasses and polymer matrices are very similar. The intensity decay is logarithmic in time over the measured decay period which is six lifetimes for the fluoronaphthalenes and approximately two lifetimes for the other halonaph­thalenes. The present value for 2-chloronaphthalene is in closer accord with Eq. (1) th~n the value measured by McClure.1

Comparison of the lifetimes at 77° and 298°K, recorded in Table II, reveals a small temperature effect. Unsubstituted aromatic hydrocarbons usually show a 50%-100% increase in Tp upon cooling to liquid-nitrogen temperatures.7 The same '-"fourfold reduction in T results in a 40%-70% increase in the phosphorescence lifetime of the heavy-atom substituted naphthalenes. Such increase is independent of the initial lifetime which differ by three orders of magnitude for this class of compounds.

TABLE II. Triplet decay of halonaphthalenes under pres£ure.

Naphthalene 7'p(O) -AVt substituent 298°K (SEC) (i3/moIEcule)

l-fluoro- 1.07 0.25±0.01

2-fluoro- 1.04 0.19±0.04

1·chloro- 0.18 0.23±0.04

2·chloro- 0.23 0.22±0 .01

I-bromo- 0.0081 0.10±0.07

2-bromo- 0.010 0.17±0.03

2-iodo- 0 .0019 0.09±0.09

7 R. E. Kellogg and R. P. Schwenker, J. Chern. Phys. 41, 2860 (1964); R. E. Kellogg and N. C. Wyeth, ibid. 45,3156 (1966).

Increasing pressures reduce the triplet lifetimes, as observed for the parent hydrocarbon.6 The lifetimes Tp

as well as In'T-p are found to depend linearly on pressure within experimental error because the variations in lifetime and in pressure are too small to determine the precise pressure dependence of transition probabilities. The slopes of InTp-vs-P plots gave the LlVt values tabulated in Table II. At least two separate runs were made to determine the uncertainties in Ll Vt (Table II) . Large errors arise from scatter in the measured lifetimes at different isobars and are attributed to photolysis and differences in the microscopic environment of the solute.7 It is seen that the environmental effects become less influential as the heavy atom increases in size and

o

"'10

o 0

-15 o 0

o

-0.3 -0.1

6V* (A'/molecull)

F1G. I. Plot of A vt, which is a measure of the pressure response, against 2 Ine which is an indication of the strength of the intra­molecular heavy-atom effect.

weight. Figure 1 shows a rough correlation between the ccmputed LlVt values and r. The ~ values for the halogens, used in plotting Fig. 1, are taken from McClure. l A similar correlation plot is obtained if InTp is plotted along the vertical axis. Naphthalene (Ll VI = -0.38)4 is also plotted in Fig. 1. The position of ring substitution appears unimportant in determining pressure effects within experimental error. The phos­phcrescence intensities are greater at high pressures . and similar to those observed for the parent naph­thalene.6 The brornine- and iodine-substituted naph­thalenes are subject to photodecomposition, especially at room temperature. The unidentified photolysis produt t phosphoresce with a lifetime of '-"0.5 sec. It was assurred that the short, exponential component can be ascribed to the halonaphthalene at all pressures. In the case of 2-iodonaphthalene, the time required for one pressure cycle (0,8, 16,32,0 kbar) presumably caused