Electron transfer reactions of triplet 9-arylxanthenium and 9-arylthioxanthenium cations

Abstract

Triplet excited states of 9-arylxanthenium and 9-arylthioxanthenium cations have been characterized using laser flash photolysis. The triplet cations absorb at -300 nm and are long-lived ( T > 5-10 ps) in the absence of electron donors. They undergo efficient electron transfer reaction with a variety of aromatic donors, including biphenyl, naphthalene, substituted benzenes, and the precursor alcohols, to give the corresponding xanthenyl or thioxanthenyl radicals and the radical cation of the aromatic donor. Both the radical and radical cations are readily detectable by transient techniques. The rate constants for electron transfer quenching of the triplet cations decrease with increasing oxidation potential of the donor, as expected from the Rehm-Weller equation. The triplet cations are somewhat poorer oxidizing agents than the singlet excited states of the same cations due to their lower excitation energy. However, the triplet cations are much more useful sensitizers in that they give substantially higher yields of cage escape from the initial geminate pair than the corresponding singlets, based on a lower limit of 25% for biphenyl radical cation formation from the triplet 9-phenylxanthenium cation.

Photophysics and Photochemistry of Conformationally Restricted Triarylmethanes: Application as Photoredox Catalysts

“…However in molecules with electron poor aromatic substituents this photoinduced electron transfer process is shut down due to the electron deficient aryl rings and hence CRTs with electron poor aryl rings has higher fluorescence quantum yield compared to the CRTs with electron-rich substituents. Moreover, due to higher excitation energy of the singlet state the singlet excited state is more oxidizing compared to the triplet excited state [58].…”

Laser flash photolysis study on 9-phenylxanthenium tetrafluoroborate: Identification of new features due to the triplet state

“…However, the yield of net electron transfer (ф et ) process is found to be very small (≤ 0·02) due to efficient back electron transfer reaction between the products of the photo-induced electron transfer reactions, i.e., 9-phenylxanthenyl radical and radical cation of the corresponding reactant. Even though the singlet excited state of PX+ has been studied extensively by monitoring the fluorescence of the system, barring a couple of papers by Johnston and Wong, 17,18 there is hardly any literature dealing with the triplet state of this system. In these papers, the authors have characterized the triplet-triplet absorption spectra of PX+ by generating this species from PX in the presence of varied concentrations (1-10 mM) of trifluoroacetic acid.…”

“…In these papers, the authors have characterized the triplet-triplet absorption spectra of PX+ by generating this species from PX in the presence of varied concentrations (1-10 mM) of trifluoroacetic acid. 17,18 They found that *For correspondence the triplet-triplet absorption due to PX+ shows maximum xat < 300 nm and is characterized by a lifetime of several microseconds. The exact lifetime depends on the concentration of PX in the medium (which can be varied by adjusting the concentration of the added acid) as PX+ triplet undergoes electron transfer reaction with PX and produces 9-phenylxanthenyl radical, which can be characterized by the 340 nm band in the transient absorption spectrum.…”

“…The exact lifetime depends on the concentration of PX in the medium (which can be varied by adjusting the concentration of the added acid) as PX+ triplet undergoes electron transfer reaction with PX and produces 9-phenylxanthenyl radical, which can be characterized by the 340 nm band in the transient absorption spectrum. 17,18 This triplet state of PX+ can also undergo electron transfer reaction with several arene molecules. 18 The lack of studies involving the triplet state of PX+ can partly be attributed to strong fluorescence quantum yield of the species, which is reported to be 0·8 in 2,2,2-trifluoroethanol, 18 a value much higher than 0·42 reported by Minto and Das (in acetonitrile containing 8% H 2 SO 4 ).…”

“…17,18 This triplet state of PX+ can also undergo electron transfer reaction with several arene molecules. 18 The lack of studies involving the triplet state of PX+ can partly be attributed to strong fluorescence quantum yield of the species, which is reported to be 0·8 in 2,2,2-trifluoroethanol, 18 a value much higher than 0·42 reported by Minto and Das (in acetonitrile containing 8% H 2 SO 4 ). 8 A high fluorescence quantum yield of the system not only implies a low yield of the triplet, but the fact that the intense and broad fluorescence covers a large portion of the visible region makes observation of transient absorption due to triplet, if any, in this region a difficult exercise indeed.…”