HIGH RESOLUTION OPTICAL SPECTROSCOPY OF A SIMPLE POLYENE SCHIFF BASE

Author Institution:We have obtained highly resolved electronic spectra of a simple polyene Schiff Base in n-alkane hosts at $4.2^\dag K$. The n-butyl dodecapentaenylidene Schiff Base ($DDPSB, CH_{3}-(CH=CH)_{5}HC=N-C_{4}H_{9})$ is taken as a model for the retinylidene Schiff Bases used in vision and other photobiological processes. Fluorescence and fluorescence and excitation spectra of DDPSB in n-tetradecane can be interpreted in terms of a single, mixed crystal site. Analyses of the detailed vibronic structure show that the dominant C=C stretching frequency decreases from $1587 cm^{-1}$ in the ground state $(1^{\prime}$Ag ) to $1444 cm^{-1}$ in the lowest excited singlet state $(2^{\prime}$Ag). The C-C strength increases from $1167 cm^{-1}$ to $1216 cm^{-1}$ upon electronic excitation to $2^{\prime}$Ag. These observations are consistent with simple notions of bond order redistribution in polyene excited states. The frequency changes are not, however, consistent with results previously obtained for polyene hydrocarbons. Thus, for example, in 2,12- Dimethyltridecahexaene $((CH_{3})-_{2}C=CH-(HC=CH)_{4}- C(CH_{3})=CH_{2})$ the C=C stretching frequency increases from $1576 cm^{-1}$ in $1^{\prime}$Ag to $1779 cm^{-1}$ in $2^{\prime}$Ag