J. Phys. Chem. 1083, 8 7 , 541-543
541
Phonon-Assisted Energy Transfer in the Mixed Molecular Crystal p -Terphenyl/Tetracene Mlchall J. Davies, Anita C. Jones, John 0. Wllllams,” and Robert W. Munn Edward Davies Chemlcal Laboratories, Unlversly College of Wales, Aberystwyth, Wales, U.K. and Department of Chemistry, Unlverslty of Manchester Instltute of Sclence and Technology, Manchester M60 100, U.K. (Recelved: November 29, 1982)
The time-independent, thermally activated transfer of singlet excitation energy from p-terphenyl (host) to tetracene (guest) over the temperature range 10-200 K is shown to be consistent with a mechanism of energy transfer assisted by phonons. certain lines merge, and others disappear. A typical decay of the p-terphenyl emission and the rise and decay of tetracene emission at 10 K is given in Figure 2. Within our experimental error (&8% for rise times and &5% for decay times), the rise time of the tetracene emission corresponding to all lines 01/02, A2, A4, and 01/02(310 cm-l) is in good agreement with the p-terphenyl decay time in the same crystal at all temperatures. Thus energy transfer from the host lattice to guest molecules occupying the various sites is unequivocally a time-independent process in agreement with our previous conclusion for the data above 70 K where emission from a pair of sites A2/A4 was m e a ~ u r e d . Furthermore, ~ over the entire temperature range 10-200 K the rate of energy transfer kTR increases with increasing temperature where km is evaluated from kTR = 1/r - l / r o ,where ro and r are the decay times of the pure and impure p-terphenyl crystals, respectively. Kruse and Small9 have shown that for tetracene in p Experimental Section terphenyl, sites A2 and A4 are observed only in emission The methods of materi$ purification and crystal growth and are present in the ground state at a concentration of for pT/T, the laser system used for excitation, and the ca. of 01/02.From selective excitation experiments detection system have been reported previ~usly,~ as have in which 01/02 and their phonon bands were excited dithe methods for treatment of the experimental r e ~ u l t s . ~ ~ ’ rectly it was found that A2 and A4 are not populated by intermolecular energy transfer from the sites O1and O2 Results and Discussion initially excited. We find that the sites 01/02, A2, and A4 Over the entire temperature range 10-300 K the receive their excitation directly from the host p-terphenyl fluorescence emission from p-terphenyl single crystals lattice since the rise time of emission from each site apdoped with tetracene in the concentration range lo-’ to proximates to the decay time of the host fluorescence. We M/M consists of the p-terphenyl emission between have treated the temperature dependence of km based on 300 and 400 nm and the tetracene emission between 450 a phonon-assisted energy transfer process between host and 600 nm. The p-terphenyl emission from pure crystals and guest. We have assumed that the probability of energy has been reported previous19 and it is no different in transfer is proportional to the thermal average number of impure crystals. Lifetimes for p-terphenyl in both pure phonons excited Np,i.e. and doped crystals are independent of wavelength at temperatures
