10713
J. Phys. Chem. 1992,96, 10713-10719
Low-Frequency Vibrations of Triptycene Alan Furlan,+ Thomas Fischer,t Peter Flueltiger,* Hans-Ulrich Ciidel,t Samuel Leutwyler,*-t Mark J. Riley3 and Jaquw Webers Hans Peter UM,* Institut fftr anorganische, analytische. und physikalische Chemie, Universitdt Bern, Freiestr. 3, 3000 Bern 9, Switzerland; Interdisziplinlires Projektzentrum f i r Supercomputing, E TH-Zentrum, 8092 Zftrich, Switzerland; and Dgpartement de Chimie Physique, Universitz de Genbe, 30 quai Ernest-Ansermet, 1211 Ge&e 4, Switzerland (Received: July 30, 1992)
The ground-state vibrational spectra of triptycene (9,10-dihydro-9,10[1',2']benzenoanthracene) were studied by fluorescence emission and IR and Raman spectroscopies, as well as by semiempirical (AMI) and ab initio (HartreeFock) quantum chemical calculations. Comparison of experimental vibrational frequencies and intensities in the range 60-1oOO cm-l with the semiempirical and the ab initio values is made. Excellent agreement is found between experiment and ab initio calculation with respect to vibrational frequencies. Agreement with the frequencies predicted by the semiempirical AM1 calculation is less satisfactory. The interpretation and assignment of the lowest frequency vibrational modes is also discussed in terms of a vibrational excimer model in a basis of symmetry-adapted combinations of local benzene ring coordinates. The two lowest frequency modes at 64 and 21 1 cm-'are identified by shape and symmetry (e' and a i , respectively), which is very important for the understanding of the Jahn-Teller effect in the first excited state 'E' of triptycene.
1. Introduction
The electronic structure and spectroscopy of molecules containing weakly coupled *-aromatic chromophores has received considerable attention.'-' Triptycene (9,10-dihydro-9,10[1',2']benzenoanthracene) can be viewed as a trimer of three o-xylene molecules covalently bound by methine bridgehead carbons of the type found in [2.2.2]bicyclooctane. The molecule has Djhsymmetry. Following the pioneering study of Wilcox4 on the electroluc absorption spectrum, triptycene was studied with respect to its photophysics and photochemi~try.~-'Due to the through-space T / T electronic interactions of the three benzene rings, which by symmetry must have an odd number of antibonding overlaps, triptycene is also an example of a M6bius system and for this reason was also studied by photoelectron spectroscopy and semiempirical theoretical methods.8-'0 In the first excited state, triptycene exhibits an interesting excitonic coupling of the three constituent o-xylene SIelectronic states.24 In fact, our recent investigation of the Jahn-Teller (JT) effect in the first electronically excited SI('E') state of this system" was one of the motivations for analyzing the low-frequency ground-state modes. In the SI Soelectronic spectrum, the SIstate vibronic coupling leads to the appearance of a very low-frequency e' vibrational mode and irregular level structure. 18 vibronic transitions are observed in this JT system within 350 cm-I above the SI Soelectronic origin. Dispersed fluorescence emission spectra from these 18 vibronic e' levels show very extended vibrational progressions in the analogous e' vibration of the electronic ground state, with a frequency of 64.2 m-'. Further low-energy vibrational frequencies were found in the dispersed fluorescence spectra in the frequency range 100-800 cm-I, but the symmetry and form of t h a c modes were not identified in that work. Information on other low-frequency vibrations is also necessary in order to study intermode coupling effects in the JT-active SIexcited state, i.e., a possible E' @ (e' e a i ) vibronic co~p1ing.l~ In contrast to the above-mentioned studies of the electronic structure, excited state, and ion states, the ground-state vibrations of triptycene have not been studied at all. This paper is concemed with the low-frequency vibrational modes of triptycene, as studied by three complementary techniques described in section 2: fluorescence emission and IR and Raman spectroscopies. The low-frequency (
