Definition of the nature of ketone triplet states on the basis of singlet oxygen generation efficiency

Abstract

The efficiency of singlet oxygen generation (SA) was measured for acetophenone, p-aminobenzophenone, and Michler's ketone by the time-resolved 02(*Ag) phosphorescence method in various solvents. The SA value for acetophenone is 0.3-0.4 in benzene, cyclohexane, acetonitrile, and alcohols (practically independent of solvent); the lowest triplet state of acetophenone is therefore probably pure n,r* in all solvents. The values for Michler's ketone and p-aminobenzophenone are 0.24-0.4 in all solvents investigated (except in ethanol for Michler's ketone) and are also typical for ketones with n,?r* triplet states. For Michler's ketone in ethanol, SA = 0.65; the triplet state therefore probably has substantial C T character. Amino-substituted benzophenones do not undergo photoreduction in alcohols, which is explained partly by a low quantum yield of triplet and mainly by the considerable negative charge at the carbonyl oxygen because of intramolecular charge transfer, especially in alcohols. As a result, the oxygen atom in the 3n,r* state is considerably less electrophilic than in benzophenone.Internal conversion is the main deactivation channel for the excited singlet state of Michler's ketone and p-aminobenzophenone and probably occurs by fast back electron transfer with deactivation to the ground state; the rate of this process increases strongly with decreasing energy of the CT state.In tr od u c ti 0 nThe efficiency of singlet oxygen generation (SA) is very sensitive to the electron density distribution in the lowest state of ketones.For T,T* triplet states, SA -0.8-1.0, while for n,r* triplets SA -0.3-0.4.1-12 Intramolecular triplet charge-transfer states are similar in character to 3r,r* statesI3J4 and would therefore also be expected to have SA values near 1. For example, Chattophadhyay et al. showed that that the efficiency of 02(lAs) generation from oxygen quenching of the metal-to-ligand CT triplet state of a tris(bipyridine)ruthenium(II) complex is approximatelyThese results suggest that it should be possible to define the nature of the lowest triplet state of ketones by measuring the S A values. On the other hand, introduction of different substituents or changing the medium can lead toinversion of the ordering of the n,r* and r,r* or CT triplet states in ketones (see ref 13 for a review). For acetophenone, analysis of spectra, phosphorescence lifetimes,lS and triplet-triplet absorption spectra in various solventsI6 led to the conclusion that the 3n,?r* and 3~,~* states have very similar energies and that, in nonpolar hydrocarbons, the 3n,T* is the lowest state, in cyclohexane they are isoenergetic, but in acetonitrile or hydrogen-bonding solvents such as alcohols, the lowest state is %r,r*. Porter and Suppan (and many later investigators) have shown that 3n,?r* ketones characteristically abstract hydrogen atoms from donors very efficiently and have phosphorescence spectra with characteristic vibrational structure, shortphosphorescence lifetimes, and a small SI*-T ~p l i t t i n g .~~J~ Moreover, the dipole ...

Application of LC‐MS and LC‐MS‐MS to the analysis of photo‐decomposed crystal violet in the investigation of cultural heritage materials aging

“…Both mass and UV-Vis chromatograms showed further small peaks in the range 25-29 min (peaks n. 18,19,21,22,27,29). Corresponding molecular formula obtained by isotopic pattern and exact mass (see Table 1) show that all of them contain an oxygen atom.…”

“…In previous literature, it has already been demonstrated how MK in solution is involved in singlet oxygen generation [21] and how the latter is involved in further degradation of triphenylmethane dyes on a clay surface. [22] We therefore suggest that these three mechanisms might explain the observed faster degradation of CV on paper in the presence of MK.…”

Ultrafast electron diffraction: Excited state structures and chemistries of aromatic carbonyls

“…30,31 Like for benzaldehyde, the prolific study of acetophenone is largely due to its intense phosphorescence facilitated by efficient intersystem crossing from S 1 into the triplet manifold. The photophysics of acetophenone have been studied in the gas phase, [6][7][8][9]20,26,[43][44][45][46][47] solution, 21,48 and in matrices. 22,23,44 Phosphorescence detected in the vapor phase is seen to decrease in intensity with increasing photon energy and exhibits a drop-off as excitation approaches S 2 .…”

Singlet Sigma: The “Other” Singlet Oxygen in Solution

“…The quenching of excited-state organic molecules by ground-state oxygen is one of the most important, and consequently well-studied, reactions in photochemistry (55,57,(77)(78)(79)(80)(81)(82)(83)(84)(85)(86). Despite years of both experimental and theoretical efforts, many aspects of the quenching mechanism remain poorly understood.…”