Ultrafast Charge Separation and Driving Force Dependence in Cyclophane-Bridged Zn−Porphyrin−Quinone Molecules

Abstract

The rate of charge separation in a series of cyclophane-bridged Zn-porphyrin-quinone systems has been investigated in nonpolar and polar solvents by means of fluorescence upconversion. In all systems with driving forces in the range 0.3-1.3 eV, ultrafast charge separation occurs with a rate constant of (2-5) × 10 12 s -1 . In combination with previous investigations on free-base porphyrin-quinone systems the driving force dependence can be probed from the (slightly endoergic) normal to the moderately inverted region for the rate of charge separation alone. The (more limited) data for charge recombination in these systems are reasonably well reproduced by the same reorganization energies and electronic couplings that result from the analysis of the charge separation. The data allow, for the first time, a satisfactory quantum mechanical analysis of the driving force dependence in porphyrin-quinone systems of the charge separation alone and, consequently, the testing of the assumption on the comparability of charge separation and recombination by experimental means.

First-principles study of photoinduced electron-transfer dynamics in a Mg–porphyrin–quinone complex

“…In particular, we note that in cyclophane-bridged Zn-porphyrinquinone [28,37], ET kinetic constants between 3 × 10 12 and 5 × 10 12 have been measured, which corresponds to exponential decay times between 330 and 250 fs. In these cases, the estimated diabatic coupling was around 100 cm −1 , and the ET process was considered barrierless, which is in close correspondence with the main features of our ab initio based model.…”

“…Thus, the overall reorganization energy is 4328 cm −1 , about 0.54 eV, in surprisingly good agreement with values obtained from the measurement of ET kinetics in cyclophane-bridged porphyrin-quinone molecules. [28] It is worth noting that more than 80% of the total reorganization energy of the system can be ascribed to just five vibrational modes of benzoquinone. These modes are sketched in figure 2.…”

“…This assumption, albeit ad hoc, has a sound physical basis, as barrierless ETs have been widely documented in porphyrin-quinone type complexes. [2,28] The diabatic coupling, V , between the excited state, P * Q , and the CT state, P + Q − , computed by the diabatization procedure explained in section 2, is 120 cm −1 . Empirical estimates based on Marcus theory of ET in metallo-porphyrin-quinone systems with a similar edge-to-edge distance suggest values between 100-160 cm −1 , and similar values can be obtained using the Hopfield semiempirical formula.…”

Quantum dynamics of electron-transfer reactions: photoinduced intermolecular electron transfer in a porphyrin–quinone complex

“…Of course, only totally symmetric modes can give rise to linear vibronic couplings, whereas non-totally symmetric modes contribute to the Hamiltonian only through quadratic vibronic couplings, values have been reported in porphyrin-quinone type complexes. [2,44] Finally, the diabatic coupling, V , between the excited state, P * Q , and the CT …”

Oxidation and Reduction of Cerium(IV) Sandwich Complexes with Porphyrin Ligands Linked by Aliphatic Diether Bridges of Variable Chain Length

“…Most of these contain covalent links between the individual functional groups or chromophores [1] . The (porphyrin-quinone)zinc systems prepared by Staab et al [2] were especially stimulating for our work devoted to (porphyrin)cerium(IV) double-deckers in which the two porphyrin systems were linked by a quinone-containing bridge [3] . In the course of that work, cerium(IV) doubledeckers of the type Ce[ttpO(CH 2 ) n Ottp] (1) [4] , namely 1b and 1c, were constructed in which the two porphyrin rings are bridged by aliphatic diether chains containing 9 or 10 methylene groups (n ϭ 9 or 10), respectively.…”