Investigation of Singlet → Triplet Transitions by the Phosphorescence Excitation Method. III. Aromatic Ketones and Aldehydes

Abstract

The single -, triplet absorption spectra of 16 photochemically active aromatic ketones and aldehydes have been investigated by the phosphorescence excitation method. Both n,r and r,r triplet states have been located in a number of molecules. The relation between our experimental findings and a recent theoretical interpretation of intersystem-crossing mechanisms in carbonyl compounds is discussed. The intensity of singlet + triplet transitions in carbonyl compounds is examined theoretically, and it is predicted that the S-T,,, transitions in aromatic compounds should be enhanced by the presence of the > G O substituent. This prediction is experimentally verified with several examples. Within the accuracy of our experiments, there was no external heavy atom effect on the intensity of the %Tu,, transitions, although the addition of ethyl iodide to the solvent did enhance the somewhat weaker S-rT,,, transitions by about a factor of 2. This difference in sensitivity to external heavy atoms provides a useful criterion for distinguishing ST,,, and S+T,,, transitions. The relation of the spectroscopic results to the interpretation of the photochemical properties of these molecules is discussed. nterest in the triplet-state properties of organic (15) S. K. Lower and M. A.

The interrelation between polarographic half-wave potentials and the energies of electronic excited states

“…Previously the determination of the triplet energies of organic compounds has relied on measuring emission spectra (phosphorescence) in solid matrices at low temperature (16) or from singlet-triplet absorption techniques, phosphorescence excitation (17), or oxygen-intensification (18). However, in general, reported triplet energies are scarce compared to the singlet energies of organic compounds.…”

Time-resolved infrared spectra and structures of the excited singlet and triplet states of fluorenone

“…Such characteristics of excited states lead to the diversity of photophysical and photochemical properties of aromatic carbonyl compounds. To identify the nature of the lowest excited state, several criteria have been proposed such as absorption intensity, transition energy shift with increasing solvent polarity, direction of transition moment, vibrational structure, fluorescence lifetime and quantum yield, phosphorescence lifetime and quantum yield, singlet-triplet splitting, external heavy atom effect on the S 0 fi T 1 transition and electron spin resonance (esr) [1][2][3][4].…”

Ordering, Interaction, and Reactivity of the Low‐Lying nπ* and ππ* Excited Triplet States of Acetophenone Derivatives

“…Considering the fact that the entropy differences are likely to be small in AP and Me-AP, as in the case of MeO-AP, the present results are consistent with previous studies, which predict that the energy levels of the lowest np* and pp* states of AP and Me-AP lie closer than those of MeO-AP. [7,9] Time-resolved mid-infrared spectra at 0-1 ms of photoexcited acetophenone derivatives in carbon tetrachloride are shown for the 900-1900 cm À1 region in Figure 5. The timeresolved near-infrared spectra of acetophenone derivatives in carbon tetrachloride (not shown) are very similar to those in benzene, thus showing that the T 1 and T 2 energy gaps in the two solvents do not differ significantly.…”