Excited Singlet State Reactions of Triphenylpyrylium Ion with Electron Donors: Evidence for Electron Transfer and the Observation of Marcus Inverted Region for the Charge Shift in the Radical Pair

Fluorescence Quenching Study on Electron Transfer from Certain Amines to Excited State Triphenylpyrylium Ion (TPP+)

The fluorescence quenching of excited singlet state of 2,4,6-triphenylpyrylium tetrafluoroborate (TPPBF4 or TPP+), a very good electron acceptor by amines were investigated in a acetonitrile solution using steady state technique. The bimolecular quenching rate constants lie in the range 2.11–10.26 × 1010 M-1 s-1. The driving force (ΔGet) for electron transfer process was calculated from the oxidation potential of amines and the reduction potential of TPP+. The observed k q values correlated well with the driving force for the electron transfer reactions. Aromatic amines show higher k q than aliphatic amines. From the oxidation potential of amines and the quenching rate constant values, a mechanism involving photoinduced electron transfer from amines to excited state TPP is suggested.

Photoinduced Electron Transfer between Triphenylpyrylium Ion (TPP+) and Certain Phenols

The photoinduced interaction between triphenylpyrylium ion (TPP+) and certain phenols were studied in acetonitrile medium by using absorption, steady state and time resolved fluorescence spectroscopic methods. The quenchers used were phenol, o-aminophenol, o-cresol, o-hydroxyphenol, o-chlorophenol, salicylic acid and salicylaldehyde. Linear Stern-Volmer plots were obtained from both steady state and time resolved measurements and also the calculated values of bimolecular quenching rate constants (k q) were matched well. The free energy change (ΔG et) has been evaluated by using Rehm-Weller equation for the confirmation of electron transfer from phenols to TPP+ ion.

Photosensitized oxidation of sulfides: discriminating between the singlet-oxygen mechanism and electron transfer involving superoxide anion or molecular oxygen

The oxidation of diethyl and diphenyl sulfide photosensitized by dicyanoanthracene (DCA), N-methylquinolinium tetrafluoroborate (NMQ(+)), and triphenylpyrylium tetrafluoroborate (TPP(+)) has been explored by steady-state and laser flash photolysis studies in acetonitrile, methanol, and 1,2-dichloroethane. In the Et(2)S/DCA system sulfide-enhanced intersystem crossing leads to generation of (1)O(2), which eventually gives the sulfoxide via a persulfoxide; this mechanism plays no role with Ph(2)S, though enhanced formation of (3)DCA has been demonstrated. In all other cases an electron-transfer (ET) mechanism is involved. Electron-transfer sulfoxidation occurs with efficiency essentially independent of the sulfide structure, is subject to quenching by benzoquinone, and does not lead to Ph(2)SO cooxidation. Formation of the radical cations R(2)S(*+) has been assessed by flash photolysis (medium-dependent yield, dichloroethane>>CH(3)CN>CH(3)OH) and confirmed by quenching with 1,4-dimethoxybenzene. Electron-transfer oxidations occur both when the superoxide anion is generated by the reduced sensitizer (DCA(*-), NMQ(*)) and when this is not the case (TPP(*)). Although it is possible that different mechanisms operate with different ET sensitizers, a plausible unitary mechanism can be proposed. This considers that reaction between R(2)S(*+) and O(2)(*-) mainly involves back electron transfer, whereas sulfoxidation results primarily from the reaction of the sulfide radical cation with molecular oxygen. Calculations indeed show that the initially formed fleeting complex RS(2)(+)...O-O(*) adds to a sulfide molecule and gives strongly stabilized R(2)S-O(*)-(+)O-SR(2) via an accessible transition state. This intermediate gives the sulfoxide, probably via a radical cation chain path. This mechanism explains the larger scope of ET sulfoxidation with respect to the singlet-oxygen process.

Charge Shift and Triplet State Formation in the 9-Mesityl-10-methylacridinium Cation

The target donor-acceptor compound forms an acridinium-like, locally excited (LE) singlet state on illumination with blue or near-UV light. This LE state undergoes rapid charge transfer from the acridinium ion to the orthogonally sited mesityl group in polar solution. The resultant charge-transfer (CT) state fluoresces in modest yield and decays on the nanosecond time scale. The LE and CT states reside in thermal equilibrium at ambient temperature; decay of both states is weakly activated in fluid solution, but decay of the CT state is activationless in a glassy matrix. Analysis of the fluorescence spectrum allows precise location of the relevant energy levels. Intersystem crossing competes with radiative and nonradiative decay of the CT state such that an acridinium-like, locally excited triplet state is formed in both fluid solution and a glassy matrix. Phosphorescence spectra position the triplet energy well below that of the CT state. The triplet decays via first-order kinetics with a lifetime of ca. 30 micros at room temperature in the absence of oxygen but survives for ca. 5 ms in an ethanol glass at 77 K. The quantum yield for formation of the LE triplet state is 0.38 but increases by a factor of 2.3-fold in the presence of iodomethane. The triplet reacts with molecular oxygen to produce singlet molecular oxygen in high quantum yield. In sharp contradiction to a recent literature report, there is no spectroscopic evidence to indicate the presence of an unusually long-lived CT state.

The Role of Aromatic Radical Cations and Benzylic Cations in the 2,4,6-Triphenylpyrylium Tetrafluoroborate Photosensitized Oxidation of Ring-Methoxylated Benzyl Alcohols in CH2Cl2 Solution

A steady-state and laser flash photolysis (LFP) study of the TPPBF(4)-photosensitized oxidation of ring-methoxylated benzyl alcohols has been carried out. Direct evidence on the involvement of intermediate benzyl alcohol radical cations and benzylic cations in these reactions has been provided through LFP experiments. The reactions lead to the formation of products (benzaldehydes, dibenzyl ethers, and diphenylmethanes) whose amounts and distributions are influenced by the number and relative position of the methoxy substituents. This behavior has been rationalized in terms of the interplay between the stabilities of benzyl alcohol radical cations and benzyl cations involved in these processes. A general mechanism for the TPPBF(4)-photosensitized reactions of ring-methoxylated benzyl alcohols has been proposed, where the alpha-OH group of the parent substrate acts as the deprotonating base promoting alpha-C-H deprotonation of the benzyl alcohol radical cation (formed after electron transfer from the benzyl alcohol to TPP) to give a benzyl radical and a protonated benzyl alcohol, precursor of the benzylic cation. This hypothesis is in contrast with previous studies, where formation of the benzyl cation was suggested to occur from the neutral benzyl alcohol through the Lewis acid action of excited TPP(+) (TPP).

Cyclic transmembrane charge transport mediated by pyrylium and thiopyrylium ions

Transient spectroscopy revealed that 2,4,6-trimethylpyrylium, 2,4,6-triphenylpyrylium, and 2,4,6-triphenylthiopyrylium ions oxidatively quench excited triplet [5,10,15,20-tetrakis(4-sulfonatophenyl)porphinato]zinc(II) to form the corresponding neutral radicals and the zinc porphyrin pi-cation. The measured quenching rate constants were proportional to the pyrylium one-electron reduction potentials, that is, the reaction driving force. In the presence of anionic dihexadecyl phosphate vesicles, only the fraction of pyrylium not bound to the vesicle was capable of reacting with the photoexcited zinc porphyrin. Nonetheless, the pyrylium radicals mediated highly efficient transmembrane reduction of tris(2,2'-bipyridine)cobalt(III) contained within the inner aqueous core of the vesicles with apparent quantum yields that approached unity. Permeability coefficients (P) determined for the pyrylium radicals, pyrylium cations, and the proton were 10(-4)-2 x 10(-5) cm/s, 10(-10) cm/s, and < 5 x 10(-7) cm/s, respectively, so that only the neutral radicals are membrane-permeable on the time scale of the transmembrane redox reactions. However, each electron carrier was demonstrated to transport up to 200 electrons, at which point the internal pool of electron acceptors was exhausted. Since the cations are membrane-impermeable, a reaction cycle is proposed that includes hydrolysis of the pyrylium cations formed within the aqueous core to the corresponding 1,5-diketones which, as neutral molecules, can diffuse across the bilayer. According to this mechanism, while undergoing redox cycling the pyrylium ions function as cyclical antiporters of OH(-) and the electron, thereby maintaining electroneutrality in the reaction compartments.

Quenching mechanism of quinolinium-type chloride-sensitive fluorescent indicators

Quinolinium based Cl- sensitive fluorescent indicators have been used extensively to measure intracellular Cl- activity. To define their fluorescence quenching mechanism, a series of N-methyl quinolinium derivatives were synthesized, including N-methylquinolinium (Q), 6-methylQ, 6-methoxyQ, 6-chloroQ, 3-bromoQ, 6-aminoQ and N-methylisoquinolinium. Stern-Volmer plots for quenching by Cl-, Br-, SCN-, I-, F-, OAc- and CO3(2-) from both intensity and lifetime measurements were linear. Bimolecular quenching rate constants (kq) decreased with increasing anion oxidation potentials and increased with increasing quinolinium reduction potentials. The free energy change for charge transfer (deltaG), calculated from indicator spectral and electrochemical properties, was found to correlate with log kq. These results suggest that quenching of quinolinium fluorescence in water by anions involves a charge-transfer quenching mechanism. Understanding the mechanism facilitates structure-based predictions of the anion sensitivities of quinolinium indicators to design improved Cl- indicators with tailored properties.