CbemicaS Physics zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 13 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Cl9761 121-128 o North-Holland Publishing Company PERTURBED SITES AND HOST-GUEST-HOST EXCITON CASCADE IN THE BIPHENYL ISOTOPIC MiXED CRYSTAL PH~SPHO~SC~NC~ 3 Peter S. FRIEDMAN, Paras N. PRASAD* and Raoul KOPELMAN Depmmenr of Gzemis@r. The Universi#_s of Mchigon, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF Am Arbor, ~ich~a~ 48104, US-4 Received 18 August 1974 Revised manuscript received 14 November 1975 An interesting energy cascade is observed in the phosphorescence spectra of 1% biphenyl-hto in biphe~yl-boo f2-15 K); strongly perturbed host sites, with energy levels below that of the protonated guest, quench the guest sites at higher tem- peratures (1 I-15 K). The ~de~~ificatioR of Me perturbed sites is based on vibr;ttional characteristics fboth intensity and frequency), obtained with the help of phosphorescence spectra of biphenyl-h,o and biphenyl-dlo in an argon matrix, indicating an isotope dependent vl%ronic structure. A partial vibrational analysis is presented. resulting in confirmation of the first triplet state of biphenyl as orbitally ungernde. The dynamics of the triplet excitation are discussed, including sever& possible mechanisms explaining the non-Bol~mann nature of the low-temperature steady state. The biphenyl crystalline system has been investi- gated by several workers in recent years f l--6] . This system, involving a prototype non-rigid molecule, has been shown to exhibit very compIex behavior in ffie crystalline phase, where an unusual continuous struc- tural change has been observed [ 1] at low tempera- ture (-20 K) and some evidence has been given for disorder [2-31. In this paper we present some investi- gatians of the triplet state of the biphenyl crystal, where a rather unique energy transfer has been found. In dilute isotopic mixed crystals of bipheuyl we have observed small concentrations of strongly perturbed biphenyl-~~* host sites which have energy levels lower than that of the preponderant guest biphenyl-h10 sites. Our interpretation is substantiated by the observed phosphorescence of biphenyl-h10 and biphenyl-d,o in an argon matrix. In both the argon and biphenyl environment the vibronie stm~- ture has a strong isotope dependence. $ Supported by NSF GrantGH-32578X and NIH Grant NSO8116. * Present address:Department of Chemistry, State University . of New.York at Buffalo, Buffalo, New York 14214, USA. ‘. . . . .- I . . I. ‘. The nature of the ordinary and perturbed sites is discussed in the context of excitation dynamics. In the triplet state of the isotopic mixed biphenyl crys- tals, the excitation seems to cascade to lower energy sites as the temperature is raised involving a non- Boltzmann behavior. The perturbed biphenyl-$0 sites become the predominant energy traps for the triplet excitation as the temperature is raised from 2 K to 15 K, while at 2 K only b~phenyl-~2,* is the emitting species. The present work also confirms the’assigment of -&e symmetry species of the fust triplet state, based on a vi&rational analysis of the phosphorescence OF biphenyl-fi10 and biphenyl+. 2. E~perimenral Biphenyl-h10 was obtained from James Hinton (Virginia) and was of stated purity 99.99% (zone refined). In the latter part of the work we also used biph&yl obtained from Aldrich with 99.9% quoted purity, which we further purified by 80 passes Of zone.refm@. Biphenyl-dl0 was obtained from Stohler fsotope Chemicals, Inc., and &d 99.5% deuterium atoms. Th? isotopic mixed crystals were