Triplet State of Benzaldehyde

Abstract

The phosphorescence spectra of benzaldehyde-h6, −1d1, −4d1, −3, 5d2 and −d6 have been studied optically in close packed matrices (acetophenone and methyl benzoate) and relaxed matrices (methylcyclohexane and perfluoro-n-hexane) at 4.2°K. All the spectra were sharp and could be analyzed vibrationally. In the close packed matrices, of the 11 possible out-of-plane (a″) fundamentals, nine are observed in Herzberg—Teller borrowing of intensity. The strongest modes are the aldehyde-H wag, the CHO torsion and CHO wag, and their intensities were found to be stronger than those of any totally symmetrical bands including the origin band. 2 and 3 quanta of aldehyde-H-wag and CHO-torsional modes are found with intensity alternation. In addition to this intensity alternation, the lack of long sequences of any nontotally symmetric mode and the analysis of the Franck—Condon envelope show that the triplet benzaldehyde does not deviate far from the ground state geometry. We conclude it is coplanar or nearly so. Deuteration effects on the vibronic intensities are remarkable. The long lifetime, ∼4×10−2sec at 4.2°K, supports a planar geometry for the triplet. The large change of lifetime and spectral intensity on substitution of the aldehyde hydrogen by deuterium shows that the aldehyde H plays an important role in the vibronic interaction. In the relaxed matrices most of the out-of-plane fundamentals are too inactive to be identified in the phosphorescence. The only out-of-plane mode clearly identified is the fundamental CHO-torsional mode and its overtone. Higher overtones are not found. Most of the intensity is in the 0–0 band and the ν (C=O) progression which is longer than in the close packed matrices. Also active are in-plane modes such as δ(Φ-CHO), ν(Φ-CHO), ν1, ν9a, and ν8a. The short lifetime in the relaxed matrices (fully 20 times shorter than in the close packed matrices), ∼2×10−3sec at 1.68°K, without appreciable deuteration effect establishes that the triplet benzaldehyde is highly nonplanar with the aldehyde group twisted out of plane. We conclude from the lifetime that the twist angle exceeds 60°. The vapor phosphorescence resembles the relaxed matrix phosphorescence, but with still reduced lifetime. We conclude that in the free molecule the aldehyde group in the benzaldehyde triplet is also strongly out of plane.

Dioxetane chemiluminescence. The effect of deuterium substitution on the thermal decomposition of trans-3,4-diphenyl-1,2-dioxetane

“…The lifetime of carbonyl triplet excited states in rigid media at low temperature has been found to be dependent upon isotopic substitution.31 The lifetime of benzaldehyde triplet in EPA at 77 K was found to depend upon isotopic substitution at the aldehyde carbon.32 Furthermore, spectroscopic investigation of benzaldehyde has revealed that the aldehydic hydrogen plays an important, if not dominant, role in the radiationless deactivation of the electronically excited triplet state. 33 The effect of deuterium substitution on the lifetime of triplet excited states has been attributed to the difference in the Franck-Condon overlap factors for the hydrogen and deuterium substituted compounds. 34 That is, at constant vibrational excitation energy, the deuterium substituted compound would be at a considerably higher vibrational quantum number than the hydrogen substituted compound.…”

“…32 Furthermore, spectroscopic investigation of benzaldehyde has revealed that the aldehydic hydrogen plays an important , if not dominant, role in the radiati onless deactivation of the electronically exci ted triplet state. 33 These effects of deuteriuli substituti on have been attributed to the di fference in the spacing of the vibrational l evel s of the carbon-hydrogen and carbon deuterium bonds . 3• That is, at constant vibrational exci tation energy, the deuterium substi tuted compound would be at a considerably higher vibrational quantum number than the hydrogen substi tuted compound.…”

Photo‐Organocatalysis of Atom‐Transfer Radical Additions to Alkenes

“…[13] In contrast, the organic halides 2, the olefins 3, and 2,6-lutidine do not absorb within this frequency region. [17] The photochemistry of 1 a is dominated by reactions of the lowest electronically excited triplet state, [13] which possesses a relatively long lifetime [18] along with a triplet energy (E T ) of 300 kJ mol À1 . We did not detect any groundstate association between the reaction components by spectroscopic and absorption measurements.…”

Photo‐Organocatalysis of Atom‐Transfer Radical Additions to Alkenes

“…Detection of such photolysis products [16] suggests that 1 a can be capable of sensitizing the alkyl halides 2 to the excited triplet state upon illumination, thus inducing an n!s * transition which would result in a rapid homolytic dissociation of the CÀX bond in 2. [17] The photochemistry of 1 a is dominated by reactions of the lowest electronically excited triplet state, [13] which possesses a relatively long lifetime [18] along with a triplet energy (E T ) of 300 kJ mol À1 . This energetic value is in the range of the carbon-halogen bond dissociation energies of the alkyl halides used in this ATRA methodology (260-300 kJ mol À1 , see Table S1 in the Supporting Information), which is congruent with a homolytic cleavage of 2 induced by an energy-transfer mechanism.…”