IDENTIFICATION OF SVL'S OF TWO ISOMERS OF GUAIAZULENE BY POPULATION LABELING SPECTROSCOPY

Abstract

In a supersonic expansion, 1,4-dimethyl-7-isopropylazulene (guaiazulene) forms two stable conformers upon the cooling of the rotational degree of freedom of the isopropyl $group^{1}$. This is reflected in the $S_{2}-S_{0}$ fluorescence excitation spectrum which shows two strong origin bands, that of rotamer A being blue-shifted by $106 cm^{-1}$ from the origin of rotamer B. In order to analyze the vibrational structure of the $S_{2}$ state of guaiazulene, a methodology is needed to distinguish the vibrations of each rotamer. Identification by searching the spectrum for pairs of transitions separated by $106 cm^{-1}$ is not always correct or useful. Utilizing jet conditions, we have employed an optical-optical double resonance technique. A pulsed (5 nsec) dye laser $(\nu_{1})$ is resonant with the $O_{0}^{0}$ band of one rotamer, say rotamer B. This depopulates the $S_{0} O_{0}$ level of rotamer B via electronic excitation. Another pulsed dye laser $(\nu_{2})$ is tuned to an SVL in the $S_{2}\leftarrow S_{0}$ excitation spectrum, an SVL whose molecular conformation is unknown. The OODR signal from this SVL is reported as the difference of sample ($\nu_{1}$ off, $\nu_{2}$ on) and blank ($\nu_{1}$ on $\nu_{2}$ on) fluorescence resulting from the $\nu_{2}$ excitation. When the unknown SVL is a member of the rotamer A manifold, there is no distinction between sample and blank; the fluorescence intensity of an SVL of A is independent of the concentration (or ground state population) of rotamer B. However, when the SVL belongs to the rotamer B structure, differences of up to 60\% are observed. This is due to the depopulation of the $O_{0}$ level of B by $\nu_{1}$, resulting in less fluorescence intensity derived from $\nu_{2}$. This population labeling experiment unambiguously assigns each SVL and also show that only tow conformers exist under our experimental conditions.