Raman spectroscopic study on photochromic reaction of a diarylethene derivative

Abstract

Photochromic reaction of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene has been studied by FT-Raman spectroscopy. The Raman bands in the $\text{1300–1700}\text{cm}{}^{\mathrm{-1}}$ region of the open-ring isomer of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene are clearly distinguished from those of the closed-ring isomer. The vibrational assignment has been made by measuring the polarized Raman spectrum and density functional theory calculations at the B3LYP/6-31G** level.

Introduction

Organic photochromic molecules have been of considerable interests in its potential for the applications to optical memory and light switches. Various types of organic photochromic molecules have been studied. Spiropyran derivatives [1], [2] and azobenzene derivatives [3], [4] are thermally unstable and return to the initial isomers in the dark. In contrast, furylfulgide derivatives [5], [6] and diarylethene derivatives [7], [8], [9], [10] undergo thermally irreversible photochromic reactions. Among these molecules diarylethene derivatives are the most promising material for applications to optical devices because of their excellent thermal stability, rapid response, and high sensitivity [7], [8], [9].

The cyclization/cycloreversion of diarylethenes have been investigated by using steady-state and time-resolved absorption spectroscopy [11], [12], [13], [14], [15], [16] and fluorescence spectroscopy [17]. The diarylethenes are classified into three types depending on the substitution position of the thiophene rings to the ethylene moiety, 2 and 3 positions [10]. When the thiophene rings are bound to the ethelene moiety at position 2, it provides strong fluorescence from its open-ring conformer. Very recently, Fukaminato et al. [17] observed the absorption and fluorescence spectra of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene. This molecule undergoes the cyclization reaction by the excitation of the open-ring isomer with UV light and the cycloreversion reaction with visible light (Scheme 1). Three-dimensional optical recording has been demonstrated with a single crystalline 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene [17]. Thus, 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene is a good model system to investigate the photochromic reaction by using fluorescence spectroscopy [18], [19]. Very recently, we have measured the fluorescence excitation and dispersed fluorescence spectra of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene in a supersonic free jet. It has been shown that the initially excited state of the open-ring isomer is not the lowest excited S₁ state but the S₂ state [20]. This experimental result is in agreement with theoretical prediction [21], [22]. Unfortunately, fluorescence from the closed-ring isomer of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene is too weak to be detected, therefore, another spectroscopic techniques are necessary to investigate excited-state dynamics of both the cyclization and cycloreversion reactions.

Steady-state Raman spectroscopy is a familiar technique to investigate the normal modes of polyatomic molecules, however, to our knowledge no Raman spectrum has been reported for diarylethene derivatives so far, though vibrational frequencies and Raman intensities for some diarylethene derivatives were calculated by Majumbar et al. [23]. The observation of Raman spectrum of both the open-ring and closed-ring isomers may be very useful to investigate the changes in the structure by the irradiation of UV and visible light. In the present communication we will report the FT-Raman spectrum of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene. We have succeed in separating the Raman bands of the open-ring and closed-ring isomers. The assignment of the vibrations has been made by measuring the polarized Raman spectra and density functional theory (DFT) calculations on the normal modes.