Low-frequency large-amplitude vibrations of julolidine in the S₀ and S₁ states

Abstract

The vibrational modes of julolidine in the ground- and the first-excited states have been investigated by laser-induced fluorescence (LIF), multiphoton ionization (MPI) and dispersed emission spectroscopy. In the observed LIF excitation spectrum the mode $\text{140}\text{cm}{}^{\mathrm{-1}}$ appears and is assigned to the N-inversion mode of the S₁ state which gets coupled with the ring puckering modes of the fused saturated six-membered ring. A progression of $\text{148}\text{cm}{}^{\mathrm{-1}}$ intervals appears in the dispersed emission spectra, which on the basis of semi-empirical calculations has been identified as the N-inversion mode of the ground state.

Introduction

Spectroscopic investigations of low-frequency large-amplitude vibrations are of relevance to the determination of the labile structures of molecules. The classical well studied examples of the low-frequency modes are the umbrella inversion of sp³ N-atom as in ammonia, aniline and substituted N-methyl anilines (e.g., indoline) [1], [2], [3], [4], [5], [6], the puckering of cyclic ring systems as in cyclopentanone, cyclopentene, indane, phthalan, tetralin, tetrahydroquinoline (THQ), tetrahydroisoquinoline (THIQ) [7], [8], [9], [10], [11], [12], [13], [14], and the butterfly motion as in fluorobenzene, dihydroanthracene, xanthene and octafluoronaphthalene [15], [16], [17], [18], [19]. The low-frequency torsional mode usually appears as a long progression of overtones in biphenyl and dihydrophenanthrene, from which the structures of the concerned molecules [20], [21], [22] have been predicted. Indeed, the analysis of high-resolution spectral data provides information on not only different structures of a molecule in S₀ and S₁ states, but also on the barrier energies for inter-conversion from one conformer to another.

The high-resolution spectroscopy of amino-derivatives of benzene is of interest to chemists. The planarity of the benzene molecule is perturbed by the introduction of non-planar substituents, such as –NH₂, NMe₂, and –CH₃ groups; it may be expected that the spectra of such benzene derivatives might lead to a better understanding of the coupling of the out-of-plane torsional and inversion modes. A detailed vibrational analysis has been made by Brand et al. for the aniline vapour spectrum [2], [23]. They assigned several strong bands to the inversion of the –NH₂ group, which was found to be strongly anharmonic in the ground-electronic state. Several studies of dimethylaniline (DMA) and dimethylaminobenzonitrile (DMABN) cooled in a supersonic jet expansion have been attempted. The observed spectra exhibit a weak origin and long Frank–Condon progression of a low-frequency vibrational modes. A jet-cooled REMPI study of DMABN and DMA assigned the observed low-frequency modes in the S₀→S₁ spectrum to the torsion of the whole dimethylamino-substituent about the C_aryl–N bond [24]. Based on the Frank–Condon analysis, the authors concluded that the minimum energy configuration of the amino-group in DMA and DMABN is twisted by about 30° in the S₁ state relative to the minimum energy configuration of the S₀ state. Their fluorescence excitation (FE) and SVL spectra provide evidence of coupling between methyl torsion and inversion modes in the S₀ state [25], [26].

In this Letter we investigate the vibrational modes of julolidine in the S₀ and S₁ states by FE, SVL and multiphoton ionization (MPI) measurements. The structure of the molecule julolidine has similarity with that of coumarin 153 (C-153) and coumarin 337 (C-337). Topp et al. [28] considered only two conformers in C-153, one syn and the other anti. However, additional conformers are possible in the anti-form depending on the out-of-plane alignment of the substituents on the benzene ring. The present case of julolidine is a simpler one where it is possible to focus only on the syn–anti conformers whose relative energies remain unmodified by substituents. Semi-empirical calculations have been performed for the two low-energy conformers in order to correlate the experimental results.