Substitution and Hydrogen Bonding Effects on Fluorescence and Intersystem Crossing in Naphthaldehydes and Their Derivatives

Abstract

Absorption and fluorescence spectra, fluorescence and phosphorescence quantum yields and lifetimes of 1-and 2-naphthaldehyde and their derivatives have been measured in rigid glass solution at 77 K. Naphthaldehydes and acetonaphthones are non-fluorescent, whereas naphthoyl chlorides and methyl naphthoates are fluorescent. Upon formation of a hydrogen bond with ethyl alcohol, 2-naphthaldehyde and 2-acetonaphthone become fluorescent, and 1-naphthoyl chloride increases in fluorescence quantum yield by a factor of about 65. The rate constants for intersystem crossing in these three molecules decrease by about two orders of magnitudes as a result of hydrogen bonding. The decrease of the rate constants is attributed to elevation of the (n, π*) level and the resulting change in its position relative to the (n, π*) level. The experimental results are discussed in relation to the current theories of radiationless transition, and are found to conform to the theory in which both spin-orbit coupling and nuclear kinetic-energy operators are treated as the perturbation causing intersystem crossing.