Photoinduced Electron Transfer from Tetrasulfonated Porphyrin to Benzoquinone Revisited. The Structural Volume-Normalized Entropy Change Correlates with Marcus Reorganization Energy

Abstract

Time-resolved laser-induced optoacoustic spectroscopy was used for the determination of the enthalpy, DeltaTH, and structural volume changes, DeltaTV, concomitant with triplet state formation upon excitation of meso-tetrakis(4-sulfonatophenyl)porphyrin, TSPP(4-), as well as with the triplet state electron-transfer (ET) quenching by benzoquinone, BQ (DeltaRH and DeltaRV). The values of DeltaTH and DeltaTV for (3)TSPP(4-) formation in the presence of different cations (Li+, Na+, K+, NH4+, and Cs+) correlated with each other and afforded a value of DeltaTG = 140 +/- 20 kJ mol(-1), equal to the value of E+ at 77 K, but much larger than the DeltaTH values in solution at room temperature, due to the large entropic factor in solution. The influence of the cations on DeltaTH and DeltaTV (a contraction ranging from 5.4 to 3.8 cm3 mol(-1)) is attributed to changing chromophore-water interactions in the ground and triplet states. Upon quenching of 3TSPP(4-) by BQ, the quantum yield of free radical formation, PhiR = 0.66 +/- 0.04, is the same in the solutions of the five cations. The values of DeltaRH and DeltaRV are small and have a large error. The energy level of the free radicals formed is thus very similar to that of 3TSPP(4-). TDeltaRS and X = TDeltaRS/DeltaRV, i.e., the structural volume change-normalized entropy change for free radicals formation, were derived using average values of DeltaRH and DeltaRV together with the calculated DeltaRG degrees . The measured Marcus reorganization energy, lambda, and X fall into the lambda vs X linear dependence we previously found for the radical formation upon ET quenching of triplet flavins (3FMN and 3FAD) by amines and amino acids. Thus, X = TDeltaRS/DeltaRV in aqueous solutions is a property of the particular donor-acceptor pair linearly correlated to the corresponding Marcus reorganization energy. The value of X is much larger than the predicted value applying the electrostriction concepts in view of the noncontinuum nature of the aqueous solutions.

Spectral Properties and Deactivation Processes of Anionic Porphyrin Coupled with TiO2 Nanostructure

“…The energetic distance of the T 1 → S 0 transitions for this porphyrin is 1.43 eV [13]. It has been shown that the conduction band potential of the TiO 2 semiconductor is −0.5 V versus anatase (NHE), and the ground oxidation potential of TSPP porphyrin is 0.98 V in N,N -dimethylformamide [11]. Hence, from simple calculations it is possible to conclude that for TSPP coupled with TiO 2 both singlet and triplet states lie above the conduction band of TiO 2 , which suggests the electron transfer is energetically favorable from singlet and triplet states of TSPP involved in the complex, assuming that the singlet-triplet energy gap is the same as for TSPP (0.48 eV).…”

“…Using photoacoustic (PAS) spectroscopy [9], the heat produced by TiO 2 and TiO 2 -TSPP powdered samples were recorded. To record the time-resolved photothermal signals, the laser-induced optoacoustic spectroscopy (LIOAS) method [10,11] was applied. The excitation source was the nitrogen-dye laser (Photon Technology Int., GL-3300/GL301), and the detection system was operated using a piezoelectric transducer (Panametric Inc.,V1030).…”

Enthalpy, Entropy, and Volume Changes of Electron Transfer Reactions in Photosynthetic Proteins

“…Pulsed time-resolved photoacoustics provides an unique way to probe the molecular mechanism of electron transfer and proton transfer events in chemical reactions (Braslavsky & Heibel, 1992;Chen & Dibold, 1996;Borsarelli & Braslavsky, 1999; Andres et al, 2006;Crovetto et al, 2006;Davies et al, 2008) and photosynthetic systems (Malkin, 2000;Herbert et al, 2001;Delosme, 2003;Mauzerall, 2006). For a given photoreaction, the accessible parameters include the molecular volume change secondary to conformational change or electrostriction, and enthalpy changes.…”

Methodology of Pulsed Photoacoustics and Its Application to Probe Photosystems and Receptors

“…Pulsed time-resolved photoacoustics provides an unique way to probe the molecular mechanism of electron transfer and proton transfer events in chemical reactions [1–7] and in biological systems, including photosynthetic complexes [8–16] and retinal-based pigments such as halorhodopsins [17,18], bacteriorhodopsins [19–24] and rhodopsins [25,26]. For a given photoreaction, the accessible parameters include the molecular volume change secondary to conformational change or electrostriction, and enthalpy and entropy changes.…”