Photodriven electron transfer in triad molecules: a two-step charge-recombination reaction

Abstract

Excitation of carotenoid-porphyrin-quinone (C-P-Q) triad molecules initiates a two-step electron transfer to yield a final charge separated state of the form C*+-P-Q•". This state has a very long lifetime in solution (10~7-10-6 s), and the nature of the ultimate charge recombination reaction has not previously been investigated. Nanosecond transient absorption spectroscopic studies have been performed on a series of such triad molecules wherein the nature of the linkages joining the porphyrin to the quinone and/or carotenoid moieties is varied systematically. The results reveal that charge recombination in C'+-P-Q" does not occur in a single step but rather via an unusual two-step electron transfer involving an intermediate C-P'+-Q'" species. The temperature dependence is consistent with the formation of C-P*+-Q" via a thermally activated process, and measurements over the range 221-296 K yield AH* = 2.7 kcal/mol and AS* = -20 cal/(deg-mol). The transient absorption measurements also reveal that the quantum yield of the C"+-P-Q*~s tate is a function of three electron transfer rate constants and that it can therefore be maximized by controlling the ratios of these rate constants to one another and to the rates of other pathways which depopulate the relevant excited states.

Spin-selective recombination reactions of radical pairs: Experimental test of validity of reaction operators

“…In some quinone-based triad molecules (CPQ) at and a little below room temperature, charge recombination is a two-step sequential process, C •+ P Q •− → C P •+ Q •− → CPQ, in which the porphyrin to carotenoid electron transfer is thermally activated and rate limiting. 37 In CPF triads, only single-step recombination has been observed. 38 For the present case, another triad with the same carotenoidporphyrin type and linkage but different porphyrin-fullerene connection 30, 31 recombines much faster than the molecule studied here, an observation that is also consistent with a single step process.…”

Benzene-Templated Model Systems for Photosynthetic Antenna−Reaction Center Function

“…Charge recombination by a two-step mechanism has been observed in other porphyrinbased donor-acceptor systems. [37][38][39] Alternatively, P Zn •+ -P 2H -C 60 •-could recombine directly to give one or more of the available triplets, which could then equilibrate so as to populate mainly the lowest lying triplet, P ZN -3 P 2H -C 60 . Additional studies will be necessary to unravel this situation.…”

Efficient Multistep Photoinitiated Electron Transfer in a Molecular Pentad

“…In this connection, it should be noted that in simpler triad systems, charge recombination occurs by a two-step reaction involving an intermediate species (8). A related mechanism may be operating here, although more studies will be necessary in order to elucidate the details of the process.…”

“…Transient absorption studies on the nanosecond time scale (8,9) reveal that this state is indeed produced. Excitation of the freebase porphyrin moiety of 1 in chloroform solution at 650 nm resulted in the observation of a long-lived transient (Fig.…”