Effects of hydrogen bonding on the low-lying electronic states of a model polyene aldehyde

Abstract

Absorption spectra of crotonaldehyde in several alcohols demonstrate linear relationships between the energies of the lowest lying I T T * and Ins* transitions and the solvent pK,, with the lrr*-lnr* energy difference decreasing from 14 800 to 8500 cm-' in changing from ethanol (pK, = 16.0) to perfluoro-tert-butanol (pK, = 5.2). These results indicate that the pKHB for hydrogen-bond formation is proprotional to the pK, of the alcohol solvent in both the ground and excited states. The ability of strong hydrogen bonders to differentially shift low lying %T* and 'nr* states in polyene aldehydes has been used to invert the r*/'nn* ordering and induce fluorescence in 2,4,6,8-decatetraenaI, a model compound which does not emit in hydrocarbon environments. Absorption, fluorescence, and fluorescence excitation spectra of decatetraenal have been obtained in several hydrogen-bonding solvents over a range of temperatures. The fluorescence intensities are strongly temperature dependent and can be fit to a simple model involving a low-temperature (T I 77 K) equilibrium between a nonemitting, hydrogen-bonded complex and a fluorescent species which is either protonated or strongly hydrogen bonded. The changes in the optical spectra with temperature and the pX, of the hydrogen bonder lead to a model in which the allowed transition between the hydrogen-bonded ground (1' and excited (l'Bu) states is followed by relaxation into an excited 'TT* state than Inr*, thus accounting for the observed fluorescence. Molecular orbital calculations support this model and also indicate an inversion of the llBu and 2'A, energy levels upon protonation. that at low temperatures favors a strongly 9) ydrogen bonded or protonated form. Protonated decatetraenal has I T T * lower