Does Bimolecular Charge Recombination in Highly Exergonic Electron Transfer Afford the Triplet Excited State or the Ground State of a Photosensitizer?

Abstract

The investigations were made on photoinduced electron transfer (ET) from the singlet excited state of rubrene (1RU*) to p-benzoquinone derivatives (duroquinone, 2,5-dimethyl-p-benzoquinone, p-benzoquinone, 2,5-dichloro-p-benzoquinone, and p-chloranil) in benzonitrile (PhCN) by using the steady state and time-resolved spectroscopies. The photoinduced ET produces solvent-separated type charge-separated (CS) species and the charge-recombination (CR) process between RU radical cation and semiquinone radical anions obeys second-order kinetics. Not only the CS species but also the triplet excited state of RU (3RU*) is seen in the transient absorption spectra upon laser excitation of a PhCN solution of RU and p-benzoquinone derivatives. The comparison of their time profiles clearly suggests that the CR process between RU radical cation and semiquinone radical anions to the ground state is independent from the deactivation of 3RU*. This indicates that the CR in a highly exergonic ET occurs at a longer distance with a large solvent reorganization energy, which results in faster ET to the ground state than to the triplet excited state that is lower in energy than the CS state. Photoinduced ET from 3RU* in addition from 1RU* also occurs when p-benzoquinone derivatives with electron-withdrawing substituents were employed as electron acceptors.

Photoinduced energy and electron transfer in rubrene–benzoquinone and rubrene–porphyrin systems

“…The k et is within the reported diffusion constant of Ru in PhCN is 5.6 × 10 9 M −1 s −1 [46] and BQ in DMF is ∼10 10 M −1 s −1 at 25 • C [47], supporting the notion that the quenching of Ru by BQ occurs mainly according to a diffusion-controlled mechanism under our experimental conditions and over the concentration range applied. It is worth pointing out that the dynamic of electron transfer from Ru to BQ is in agreement with that reported in literature [43]. Moreover, in this study the rate constant of intermolecular back electron transfer from BQ • to Ru • + has been determined from a slope of the second-order plot for the slow component to be in order of 10 9 M −1 s −1 [43].…”

“…It is worth pointing out that the dynamic of electron transfer from Ru to BQ is in agreement with that reported in literature [43]. Moreover, in this study the rate constant of intermolecular back electron transfer from BQ • to Ru • + has been determined from a slope of the second-order plot for the slow component to be in order of 10 9 M −1 s −1 [43]. the range of 500-700 nm.…”

“…Analogously, similar bands are identified for the Ru in the presence of BQ ( Figure 3B) but with fast decay and recovery for the excited state and ground-state bleaching, respectively. It should be noted that BQ • absorbs over the range 400-450 nm, which overlaps with the Ru • + TA band in the same range and makes it difficult to detect the radical anion signal [24,43]. Moreover, the concentrations of BQ and Ru are too low to provide a detectable signature of their radical ion pair.…”

“…To support the PET scenario, we estimated the G o by the Rehm-Weller equation [44]: [43]. The k et is within the reported diffusion constant of Ru in PhCN is 5.6 × 10 9 M −1 s −1 [46] and BQ in DMF is ∼10 10 M −1 s −1 at 25 • C [47], supporting the notion that the quenching of Ru by BQ occurs mainly according to a diffusion-controlled mechanism under our experimental conditions and over the concentration range applied.…”

Fullerene as Spin Converter

“…The triplet manifold may be reached via indirect secondary routes such as intersystem crossing from the first excited state (S 1 ) and charge separation and recombination between radical ions [70][71][72][73]. This kind of recombination may require orbital contact via flexible link or by a rigid spacer [74][75][76][77][78][79].…”