Hole Tunneling and Hopping in a Ru(bpy)32+-Phenothiazine Dyad with a Bridge Derived from oligo-p-Phenylene

Superexchange in the fast lane - intramolecular electron transfer in a molecular triad occurs by conformationally gated superexchange

Photoinduced electron transfer via hopping is generally considered to have a stronger temperature dependence than electron transfer via superexchange. However, in this work, an opposite trend of the temperature dependence is observed. This unexpected result is rationalized by considering the specific geometrical and electronic structure of the Ru-bis(terpyridine) photosensitizer.

Photoinduced Intercomponent Processes in Selectively Addressable Bichromophoric Dyads Made of Linearly Arranged Ru(II) Terpyridine and Expanded Pyridinium Components

Three new linearly arranged bichromophoric systems 1-3 have been prepared, and their photophysical properties have been studied, taking also advantage of femtosecond pump-probe transient absorption spectroscopy. The three compounds contain the same chromophores, that is a Ru(II)-terpy-like species and a fused expanded bipyridinium (FEBP) unit, separated by three different, variously methylated biphenylene-type bridges. The chromophores have been selected to be selectively addressable, and excitation involving the Ru-based or the FEBP-based dyes results in different excited-state decays. Upon Ru-based excitation at 570 nm, oxidative photoinduced electron transfer (OPET) takes place in 1-3 from the ³MLCT state; however, the charge-separated species does not accumulate, indicating that the charge recombination rate constant exceeds the OPET rate constant. Upon excitation of the organic dye at 400 nm, the FEBP-based ¹π-π* level is prepared, which undergoes a series of intercomponent decay events, including (i) electron-exchange energy transfer leading to the MLCT manifold (SS-EnT), which successively decays according to 570 nm excitation, and (ii) reductive photoinduced electron transfer (RPET), leading to the preparation of the charge-separated (CS) state. Reductive PET, involving the FEBP-based singlet state, is much faster than oxidative PET, involving the MLCT triplet state, essentially because of driving force reasons. The rate constant of CR is intermediate between the rate constants of OPET and RPET, and this makes 1-3 capable to selectively read the 400 nm excitation as an active input to prepare the CS state, whereas excitation at wavelengths longer than 480 nm is inefficient to accumulate the CS state. Moreover, intriguing differences between the rate constants of the various processes in 1-3 have been analyzed and interpreted according to the superexchange theory for electron transfer. This allowed us to uncover the role of the electron-transfer and hole-transfer superexchange pathways in promoting the various intercomponent photoinduced decay processes occurring in 1-3.

Direct observation of hole shift and characterization of spin states in radical ion pairs generated from photoinduced electron transfer of (phenothiazine)(n)-anthraquinone (n = 1, 3) dyads

Photoinduced intramolecular electron transfer of dyad PTZ3-PTZ2-PTZ1-B-AQ consisting of phenothiazine trimer (PTZ3-PTZ2-PTZ1), bicyclo[2.2.2]octane (B), and anthraquinone (AQ) was investigated. After excitation (∼20 ps) of the AQ moiety in THF, a metastable radical ion pair (RIP) PTZ3-PTZ2-PTZ1(+)-B-AQ(-) appeared at ∼620 nm. From 500 ps to 6 ns the spectrum changed to a new absorption (∼950 nm), which was assigned to the hole-shifted stable RIP state PTZ3-PTZ2(+)-PTZ1-B-AQ(-). The time constant of the hole-shift process was determined to be 6.0 ns. The hole-shifted RIP state had a lifetime (τ) of 250 ns and was characterized by spin-polarized signals as a spin-correlated radical pair (SCRP) by means of time-resolved ESR. These results were compared with those for the phenothiazine monomer analog PTZ-B-AQ, which also produced the RIP state PTZ(+)-B-AQ(-) with τ = 1.9 μs. Time-resolved ESR showed an all emission signal pattern showing the triplet mechanism of PTZ-B-(3)AQ* → (3)[PTZ(+)-B-AQ(-)]. The origin of the difference in the lifetimes between the trimer and the monomer RIP states was discussed from various points of view, including free energy difference in the RIP states, reorganization energy difference in the charge recombination process, and the spin-state difference. Of these, the spin-state difference effect provided the most reasonable explanation.

Multistage complexation of fluoride ions by a fluorescent triphenylamine bearing three dimesitylboryl groups: controlling intramolecular charge transfer

A propeller-shaped boron-nitrogen compound (NB(3)) with three binding sites for fluoride anions was synthesized and investigated by optical absorption, luminescence, and ((1)H, (11)B, (13)C, (19)F) NMR spectroscopy. Binding of fluoride in dichloromethane solution occurs in three clearly identifiable steps and leads to stepwise blocking of the three initially present nitrogen-to-boron charge transfer pathways. As a consequence, the initially bright blue charge transfer emission is red-shifted and decreases in intensity, until it is quenched completely in presence of large fluoride excess. Fluoride binding constants were determined from global fits to optical absorption and luminescence titration data and were found to be K(a1) = 4 × 10(7) M(-1), K(a2) = 2.5 × 10(6) M(-1), and K(a3) = 3.2 × 10(4) M(-1) in room temperature dichloromethane solution. Complexation of fluoride to a given dimesitylboryl site increases the electron density at the central nitrogen atom of NB(3), and this leads to red shifts of the remaining nitrogen-to-boron charge transfer transitions involving yet unfluorinated dimesitylboryl groups.