J. Phys. Chem. 1985,89, 5811-5815
5811
Single Excited Vibrational Level Lifetimes and Energy Dissipation Channels of Large Molecules in Inert Matrices D. Huppert,+ V. E. Bondybey, a n d P. M. Rentzepis* AT& T Bell Laboratories, Murray Hill,New Jersey 07974 (Received: September 13, 1985)
In the present manuscript, we use direct time-resolved, picosecond techniques to determine the vibrational and electronic relaxation processes occurring in large polyatomic molecules-naphthazarin and its deuterated derivatives. The molecules are trapped in low-temperature Ne and Ar matrices and excited by picosecond pulses of a tunable, synchronously pumped dye laser. The SI fluorescence of naphthazarin exhibits a sharp, well-resolved vibronic structure. Both the decay of the unrelaxed fluorescence from single excited vibronic levels and the rise time on the relaxed emission are observed and resolved in time. In several instances, the relaxed fluorescence rise times are found to be considerably longer than the measured decay times of the excited levels. This is explained in terms of intermediate “bottleneck” states. The lifetimes of these states are much longer in the deuterated species than in normal naphthazarin. It is suggested that these intermediate levels are the “tunneling states” associated with the intramolecular transfer of the phenolic protons.
Introduction Vibrational relaxation rates of molecules in the gas and condense phase have been examined extensively. For small molecules in the condense phase,lq2 the vibrational relaxation is rather slow, and one observes unrelaxed fluorescence manifested by relatively slow vibrational relaxation rates. In large molecules, in both liquid and solid media, these rates increase to the order of 1011-10’3s-I. This is commonly attributed to the high intramolecular level density and the coupling between the host matrix and guest molecule or the parity between the frequency of the lattice phonons and the vibrational frequencies of the guest species. It is obvious, therefore, that the larger the difference between lattice phonons and molecular vibration frequencies, the smaller the rate of the decay process. Essentially, this concept is within the framework of the “gap law” which proposes that increasing the vibrational frequency will result in an exponential increase in the vibrational relaxation rate. This criterion is not always followed, at least in small molecules, unless the vibration-rotation coupling is taken into consideration. In view of the above discussion, it is interesting to note that lately several observations have been made of vibrational relaxation slow enough to observe rather extensive unrelaxed fluorescence from large molecules in condensed phases3 A typical example is naphthazarin which exhibits a remarkably slow and intense fluorescence originating from upper vibrational levels with as much as 2000-cm-’ excess energy above the vibrationless level of S,.4,5 Lately, unrelaxed emission from a large number of excited vibrational levels of S2in azulene has also been observed, and the decay lifetimes of the individual vibronic levels have been measured directly.6 In the previous naphthazarin studies, the lifetimes of excited vibrational levels were calculated indirectly from the emission intensities and the lifetime of the relaxed fluorescence. Only the rise time and decay of the relaxed fluorescence were measured in these studies which, in conjunction with the unrelaxed emission spectra, allowed for the proposal that relaxation does not proceed statistically but rather via well-defined channels. In the present paper we describe a method for measuring directly data for the relaxation rates of individual excited vibrational levels and establishing the mechanism of their relaxation. With direct lifetime data the relaxation rates and mechanisms can be deduced with more certainty than by utilizing only spectroscopic and/or quantum yield data. Experimental Section (Aldrich) Naphthazarin (5,8-dihydroxy-1,4-naphthaquinone) was recrystallized from hexane and subsequently purified by ‘Present address: Department of Chemistry, University of Tel Aviv, Tel Aviv, Israel. *Address correspondence to the author at the Department of Chemistry, University of California, Irvine, CA 92717.
vacuum sublimation. As in previous the deuteration of the hydroxyl groups dl and d2 was achieved by proton exchange with D20.For fully deuterated samples, water back-exchange was prevented by conditioning the sample and entire system with D 2 0 before matrix deposition. The matrix was formed by heating the sample, contained in a quartz cell, to 50-80 “Cand deposited with a large excess of the inert gas on platinum mirror at 4 K. The sample concentrations varied from 1:2000 to 15000. The signal intensities for both spectra and kinetics were larger with the more concentrated sample; however, most of the data presented were obtained at the lower concentrations which assure a more complete isolation of the naphthazarin molecules. In particular, the emission near the 0; origin must be studied in a highly dilute sample since at higher concentrations self-absorption may distort the emission spectrum and complicate the observed relaxation behavior. The fluorescence spectra were excited by a dye laser synchronously pumped by an argon ion laser (Spectra Physics H1) operating at 246 MHz. The lasing dye medium was disodium fluorescein which emits picosecond pulses in the range of 530-620 nm, suitable for excitation of the individual vibronic levels selected for this study. The synchronously pumped dye laser was equipped with a unidirectional device and a two-plate birefrigent filter which allowed for tuning to preselected wavelengths. The average power was 20 mW at 550 nm, while the pulse width was practically Heisenberg limited and dependent upon the excitation laser bandwidth. The bandwidths varied from 99.99; Aldrich Gold label, lot no. 0897) of surface area 20 m2/g was used for the photo(4) Barber, R. A,; de Mayo, P.; Okada, K. J. Chem. Soc., Chem. Commun. 1982, 1073.
(5) Kodama, S.; Yabuta, Y.; Kubokawa, Y. Chem. Lett. 1982, 1671. ( 6 ) AI-Ekabi, H.; de Mayo, P. J. Chem. Soc., Chem. Commun. 1984, 1231. (7) Taylor, T.W.; Murray, A. R. J . Chem. SOC.1938, 2078. (8) Kistiakowsky, G. B.; Smith, W. R. J. Am. Chem. Soc. 1936,58, 2428.
0 1985 American Chemical Society
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