10666 J. Phys. Chem. 1992, zyxwvu 96, 10666-10671 Single Vibronlc Level Relaxation In Photoreactlve 9- tert-Butylanthracene Satoshi Hirayam,* Mitsuhiko Kono, Yoshiharu Kubo, Laboratory of Chemistry, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan Fuji0 Tanaka, College of Integrated zyxwvutsrq Arts and Sciences, University of Osaka Prefecture, Cakuencho, Sakai 593, Osaka, Japan and Kosuke Shobatake Institute for Molecular Science, Myodaiji. Okazaki 444, Japan (Received: March 19, 1992) The laser induced fluorescence (LIF) excitation spectrum of photoreactive 9-tert-butylanthracene (9TBA) in supersonic free jets shows a rich band structure in the range 25 500-26 300 cm-I and was found to be characterized by three distinct features: namely, a low intensity of the electronic band origin, a progression of a low-frequency torsional vibration mode (a77 cm-I) of the tert-butyl group, and an absence of fluorescence when excited above 26 300 cm-' (an excess energy of 790 cm-I). The fluorescence lifetime measured for the electronic origin at 25 51 1 cm-' is 34.9 ns and remains approximately constant over a few torsional vibration bands. The fluorescence lifetimes decrease significantly above an excess energy of 482 cm-I. The electronic origins of the 1:1 van der Waals complexes of 9TBA with Ar and Xe were observed at 25 467 cm-' and 25 436 cm-l, respectively, but the LIF excitation spectra are dominated by two broad bands arising from complex formation. zy In the spectra of both complexes, one of these two broad bands is superposed on the highly structured bands associated with the bare molecule, and the other broad band appears in the range 26 700-27 400 cm-I where there is no measurable fluorescence intensity from the bare molecule. The appearance of this second broad band is ascribed to the photofragmentation of the van der Waals complexes. Excitation in this broad band resulted in fluorescence decay curves which are nonexponential and whose longest-lived component has a fluorescence lifetime comparable with that measured for the bare molecule at the electronic origin. 9-tert-Butyl- 1Gcyanoanthracene was found to be nonfluorescent in supersonic free jets, despite the large increase in the fluorescence quantum yield in the condensed phase in going from room temperature to 77 K. 2-tert-Buty- lanthracene was found to be fluorescent in supersonicjets, in the gas phase, and in the condensed phase, indicating that the tert-butyl group located at the &position in the anthracene ring does not quench fluorescencein this molecule. Other examples of anthracene derivatives which do not fluoresce in supersonic free jets are also reported. Introduction Very weakly fluorescent molecules can provide a wide variety of examples of extremely fast intramolecular radiationless tran- sition processes.'v2 The effects of ~olvent,~ temperature," or pressures on electronic relaxation processes on such molecules have been examined in detail in an effort to elucidate the nature of these fast processes. The technique of supersonic free jet expansion has developed rapidly over the past 10 years and has been shown to be an extremely powerful technique for investigating intramo- lecular electronic relaxation processes in Most of the anthracene derivatives whose fluorewmce has zyxwvu been examined using laser induced fluorescence (LIF) spectroscopy of supersonic free jets to date have been reasonably fluorescent in solution. Except for a few cases such as 9-bromoanthracene,12 anthracene derivatives which show only very weak fluorescence or no fluorescence in solution have not been examined extensively using supersonic free jets despite their potential importance in providing new information regarding very fast intramolecular relaxation processes. The weakly fluorescent anthracene derivative, 9-tert-butylanthracene (9TBA),I6 was chosen in this work since it has been found1620 that this molecule undergoes intramolecular bond isomerization in solution to form a Dewar anthracene, the formation of which is readily detected by the loss of the absorption band (]La band) characteristic to the anthracene moiety. The photochemically produced Dewar anthracene then undergoes a back reaction in the dark to form 9TBA. Intramolecular radia- tionless processes related to this bond isomerization are retarded upon lowering the temperature, and the fluorescence quantum yield reaches a value of unity at 77 K. Considering this large effect of temperature on the fluorescence properties of 9TBA, it was anticipated that the single vibronic level (SVL) excitation of molecules of this type in supersonic free jets would provide new information about vibronic level-specific radiationless transition processes which may be associated with the bond photoisomer- ization. A number of anthracene derivatives that are practically *To whom correspondence should be addressed. nonfluorescent in solution have been studied in this work, and the results obtained for these compounds are also presented. Experimental Section 9-tert-Butylanthracene (9TBA) was synthesized according to the literature2' and purifioed by recrystallization from ethanol (mp 107 "C). 9-tert-Butyl-10-cyanaanthracene (TBCA) (mp 157 "C) was a generous gift from Dr. Y. Itoh (Kyoto University). 2- tert-Butylanthracene (ZTBA), 9-acetylanthracene, 9-(perfluoro- acetyl)anthracene, and 2-ethylanthracene (Aldrich Chemical zy Co.) were purified on a thin layer chromatographic plate before being used for the fluorescence spectral and lifetime measurements in the condensed phase. 9-BenzoylanthraceneZ2 and 9-cyano- 10- benzoylanthracenez2 were synthesized according to the literature and were purified by recrystallization from ethanol followed by sublimation. The generation of supersonic free jets and the measurements of LIF excitation spectra and fluorescence lifetimes were essentially the same as reported previously.loJ' The diameter of the con- tinuous wave stainless-steel nozzle was 150 pm. The chamber was evacuated by using an Anelva 4000 10-in. diffusion pump (pumping speed zyxwvu 4OOO dm3 s-l) backed by an Ulvac D-650 rotary pump. The temperature of the beam source was kept below 100 'C for 9TBA since this compound is very easy to sublime. An excimer-laser-pumped dye laser (Lambda Physik EMG MSC 104 + FL3002 (QUI and DMQ in dioxane)) was used as the excitation source. The laser beam crossed the supersonic free jet 7 mm downstream from the nozzle. The fluorescence emission was focused onto the entrance slit of a monochromator (Nikon G250) and detected using a photomultiplier (Hamamatsu R331). The integrated output signal (PAR Model 162 boxcar averager) was digitized and then transferred to a personal computer (NEC E 9 8 0 1 VM). The LIF excitation spectra were not corrected for the variation of the intensity of the excimer-pumped dye laser with wavelength. For the fluorescence lifetime measurements, the emitted photons dispersed by the monochromator (Nikon G250) were detected with a cooled photomultiplier (Hamamatsu R 928) 0022-365419212096- 10666$03.OO/O 0 1992 American Chemical Society