Effects of substitution and excited wavelength on the photochemistry of benzaldehydes studied by CIDEP

Abstract

Chemically induced dynamic electron polarization (CIDEP) spectra of transient radicals were measured upon photolysis of benzaldehydes (BAs) after 266 nm laser excitation. The substituent effects on CIDEP were studied for a series of substituted derivatives (CH3-, F-, Cl-, Br- and CN-) of BA in 2-propanol at room temperature. For F-, Cl-, and CN-substituted BAs, CIDEP spectra were observed that were due to combinations of, firstly, the triplet mechanism (TM) with emissive (E) phase patterns and, secondly, the S-T,0 mechanism (ST0M) of radical pairs with emissiveoabsorptive (E/A) phase patterns. In contrast, for CH3-substituted BAs, CIDEP spectra due only to the ST0M with E/A phase patterns were observed. The contribution of the TM-induced CIDEP shows an increasing trend for BAs with electron-withdrawing substituents (F-, C1-, and CN-) and a decreasing trend for BAs with an electron-donating substituent (CH3-). These substituent effects on the TM-induced CIDEP were found to be parallel to the effects on the reaction rate of hydrogen abstraction of excited BA molecules: both effects are strongly influenced by the (n, π*) and (π, π*) characteristics in the lowest excited triplet states. Furthermore, clear CIDEP signals due to the benzoyl and alpha-hydroxybenzyl radicals, which are produced by self-quenching of triplet BA, were newly obtained upon photolysis of a concentrated acetonitrile solution of BA (5 × 10-2 mo1 dm-3 ) by a 266 nm laser. This result is not consistent with the proposed wavelength effect on the BA photochemistry, but it is consistent with a normal radical reaction mechanism that shows no wavelength effect.