Effect of sensitizer heavy atoms on singlet oxygen generation efficiency

Abstract

3723this correlation. The data points, plotted as k!s/k:,, are compared with the predicted behavior of eq 7 (solid line). Also shown in this figure is the hard-sphere rate constant, represented by the dashed line of zero slope and intercept one. This plot illustrates graphically our observation made earlier that the rotational relaxation proceeds more efficiently than the hard-sphere rate constant since all data p i n t s lie below the dashed line. It is also shown in Figure 9 that the measured rate constants for most of the ga.w significantly exceed the Lennard-Jones collision rate since they lie below the predicted LJ line. This suggests that the interaction occurs over larger distances than allowed for in the LJ potential; i.e., the attractive part of the potential is more long range. ConclusionsIn this work we have measured collision-induced rotational relaxation rates corresponding to destruction of an initial, narrow rotational distribution within the 6l vibrational level of SI benzene vapor by six collision partners. The collision partners studied were H2, He, N2, Ar, Cd-IH,(So), and cyclohexane. All experiments were performed at ambient temperature (293 K).The relaxation rates exceed the hard-sphere rate for all collision partners and for most also exceed the Lennard-Jones collision rate.Presumably, the intermolecular potential has a longer range attractive force than is provided by the Lennard-Jones potential. The rotational relaxation rate constants determined represent state-to-field values, as individual relaxation pathways could not be seen in this study.Rotational relaxation by So benzene was found to be less than the value measured by Coveleskie and Parmenter for the 00 level.We suggest that this is a consequence of a reduced contribution from electronic energy transfer in our experiments on 6I than were present in Coveleskie and Parmenter's work on 00. We thus believe that our value gives an improved estimate of rotational relaxation due to So benzene collisions.There are comparatively few studies that report rotational relaxation in excited electronic states for polyatomic molecules. This study significantly extends the range of collision partners measured for a large polyatomic in an excited electronic state.The quenching rate constants of triplets by molecular oxygen (k 3 and the efficiency of singlet oxygen generation per quenching encounter (SA) were measured for various r,r* and n,r* tridet sensitizers containing heavy atoms using nanosecond laser photolysis and time-resolved 02('Ag) phosphorescence. For r,r* triplets (tetrahalo-p-benzoquinones and haloanthracenes), introduction of the heavy atom has practically no influence on SA. For n,r* triplets (haloacetophenones), SA values decrease sharply with increasing number of heavy atoms; for example, for 4-bromo-and 2,4'-dibromoacetophenone in benzene, SA = 0.10 and 0.032, respectively. An analysis of the mechanism of triplet quenching by oxygen was carried out, taking into account intersystem crossing between singlet and triplet states of the ...

Interaction of singlet oxygen with peroxy and acylperoxy radicals

“…The T-T absorption spectrum and ketyl radical absorption spectrum overlap strongly, as in the cases of benzophenone, biacetyl, and camphr~quinone. '~ The hT value for benzil in ethanol took into account the ketyl radical buildup (see, for example, ref 12).…”

Temperature dependence of the oxygen quenching of .pi..pi.*-singlet and .pi..pi.*-triplet states of singlet oxygen sensitizers

“…Together with eq 7 this yields the expressions for P I and P3: (9) For kd the value k: = 3.2 X 1Olo M-I s-l found for oxygen quenching of the SI state of CHR will be used in the following evaluations.…”

Electronic spectra and intersystem spin‐orbit coupling in 1,2‐ and 1,3‐squaraines

“…This effect is generally achieved by the introduction of heavy atoms into the molecular structure, resulting in spin‐orbit perturbation. [ ] Nevertheless, different authors [ ] have recently suggested the development of a new generation of PDT photosensitizers, which can absorb two photons simultaneously in the same quantum event (TPA). The main feature of two‐photon absorbing photosensitizers is that they show higher tissue penetration and photons with half energy with respect to the corresponding one‐photon transition can be used.…”