Evidence by Optical Spectroscopy and EPR for the Light-Induced Conformational Isomerization of 2-Naphthylphenylcarbene in n-Heptane Shpolskii Matrix

Abstract

The quasi-line fluorescence and excitation spectra of 2-naphthylphenylcarbene (2-NPC) in n-hexane and n-heptane Shpolskii matrices at 4.2 K are composed of several spectra with origins shifted by 30-300 cm-' from each other. They have been attributed to the triplet-triplet transition of the syn and anti conformers of 2-NPC, each one being trapped in several different sites of the matrix. This attribution has been confirmed by the EPR experiments performed at 77 K. Furthermore, the fluorescence and EPR spectra reveal the lightinduced transformation of the syn 2-NPC into the anti 2-NPC conformer in the n-heptane matrix. The fluorescence decays of 2-NPC in both matrices at 4.2 K are nonexponential and attributed to the emission from different sublevels of the first excited triplet state TI. The zero-field splitting parameter D(T1) = 0.01 1 cm-' in the TI state of 2-NPC has been estimated from the magnetic field effect on the fluorescence decays. This value is much smaller than the D(T0) = 0.41 cm-' value in the ground state TO of 2-NPC determined by EPR, which indicates the delocalization of the z electron density onto the aromatic rings upon excitation.

Conformational dependence of electronic spectra and zero-field splitting parameters in 2-naphthylphenylcarbene

“…The energy barriers for the conversion of these forms into each other are 395 cm-' (pseudo E/trans + pseudo Ucis) and 458 cm-I (pseudo Zlcis + pseudo E/trans). Our previous experimental results (16) indicated that, in the n-heptane Shpolskii matrix, the species fluorescing at 589.5 nm converts thermally at 90 K (corresponding to 60 cm-I) into the species fluorescing at 600.5 nm. Therefore, the actual energy barrier should be smaller than our theoretical SCF value.…”

“…In the case of 2-NPC, the second excited triplet state T2 is separated from the TI state by 0.7 1 eV (5727 cm-l) for the pseudo-E/trans conformer and by 0.72 eV (5808 cm-I) for the pseudo-Ucis conformer. This large separation explains why the quasi line fluorescence spectra of both conformers exhibit mirror-image symmetry with the corresponding excitation spectra, as far as vibrational frequencies up to 1700 cm-' are concerned (16). Such a mirror-image symmetry does not exist for DPC (see Fig.…”

“…As a consequence, we assign the compound having the largest experimental D(To) value (12,16) to the pseudo-Zcis conformer of 2-NPC. This attribution is consistent with the fact reported in ref.…”

“…The fluorescence spectra of 2-NPC in 11-hexane and n-heptane Shpolskii matrices at 4.2 K are composed of several quasi line spectra with origins shifted by 3&300 cm-' from each other (16). Furthermore, the fluorescence and EPR spectra reveal that there is a light-induced transformation of the conformer emitting at 600.5 nm into the one emitting at 589.5 nm in the n-heptane matrix.…”

“…Furthermore, the fluorescence and EPR spectra reveal that there is a light-induced transformation of the conformer emitting at 600.5 nm into the one emitting at 589.5 nm in the n-heptane matrix. Based on preliminary estimates of the ZFS parameter D (12), these spectra have been attributed to the planar E/trans (syn) and Zlcis (anti) conformers of 2-NPC, each one being trapped in several different sites of the matrix (16). To check these interpretations on a theoretical basis, the transition energies and the ZFS parameters D of 2-NPC in several geometries have been calculated.…”