Direct Observation and Structural Characterization of the Encounter Complex in Bimolecular Electron Transfers with Photoactivated Acceptors

Abstract

The encounter complex between photoexcited quinones Q* and various aromatic donors (ArH) is observed directly by time-resolved ps spectroscopy immediately before it undergoes electron transfer to the ion-radical pair [Q•-, ArH•+]. The encounter complex (EC) is spectrally characterized by distinctive (near IR) absorption bands, and its temporal evolution is established by quantitative kinetics analysis. The structural characterization of the 1:1 encounter complex [Q*, ArH] identifies the cofacial juxtaposition of the donor and acceptor moieties for optimal overlap of their π-orbitals. Further comparisons of the (excited-state) encounter complex with the corresponding (ground-state) EDA complex of aromatic donors and quinones establish its charge-transfer character, which directly relates to electron transfer within the encounter complex. The mechanistic significance of the encounter complex to bimolecular electron transfer is discussed (Scheme ).

Spin dynamics and zero-field splitting constants of the triplet exciplex generated by photoinduced electron transfer reaction between erythrosin B and duroquinone

“…The solid lines are obtained from the nonlinear least squares fits based on the modified Table 1 Kinetic parameters and intrinsic enhancement factors of DQ Bloch equation (discussed later). The observation of the unusual net-A polarization suggests the production of the triplet exciplex as the reaction intermediate, because the SOC interaction due to heavy atoms is a short-range interaction [1][2][3][4][5][6][7][8][9]. The intermediate would be a nearly pure charge transfer complex or contact RP in the polar solvent.…”

“…We examined the acceptor concentration dependence of the k 1st obs values because a nonlinear relationship is the diagnostic of a pre-equilibrium intermediate between the donor and acceptor [7][8][9]27]. For concentrations up to 0:05 mol dm À3 in 1-PrOH at room temperature, the kinetics plots revealed the linear dependence of the k 1st obs on the DQ concentration.…”

“…The dynamics and reaction mechanisms of triplet exciplexes have received much attention in the field of photochemistry for a long time [1][2][3][4][5][6][7][8][9]. Significant progress has recently been made in understanding the character of triplet exciplexes.…”

“…The MFE is attributed to the sublevel selective back electron transfer (ET) reaction due to the SOC interaction from the triplet exciplex to the ground state. Recently, transient absorption bands observed in the infrared region were assigned to triplet exciplexes of quinones and aromatic molecules [7][8][9]. The rate constants of the exciplex formation and ET in the exciplexes were determined from the buildup and decay rates of the bands.…”

Spectroscopic and Thermodynamic Studies of Charge- Transfer Complexes Formation Between Cytosine, Uracil and Thymine With Electronic Accepters

“…Thermodynamic parameters ΔG°, ΔH° and ΔS° are the measure of the stability of the CT complexes as evident in Table ( [27,28]. This expected trend from the Mulliken theory that there should be a slight admixture of CT states so that D and A aquivers slight positive and negative charge respectively [29].…”