Time-dependent density functional theory (TD-DFT) calculations have been used to investigate low-energy singlet and triplet excited state potential energy surfaces (PES) of two prototype RuII-bistridentate complexes: [RuII(tpy)2]2+ (tpy is 2,2’:6’,2’’-terpyridine) and [RuII(dqp)2]2+ (dqp is 2,6-di(quinolin-8-yl)pyridine). Solvent effects were considered using a self-consistent reaction field scheme. The calculations provide information about the excited state manifold along pathways for activated decay of metal-to-ligand charge-transfer (MLCT) excited states via metal-centered (MC) states for the two complexes. Significant differences in the energy profiles of the investigated PESs are explained through characterization of the electronic properties of the involved states calculated by the TD-DFT calculations. Finally, implications of the computational results for the design of octahedral metal complexes utilizing ligand field splitting (LFS) strategies for efficient light-harvesting in photochemical applications such as artificial photosynthesis are discussed.

Calculated triplet excited state potential energy surfaces are presented for a set of three bis-tridentate Ru(II)-polypyridyl dyes covering a wide range of room temperature excited state lifetimes: [Ru(II)(tpy)(2)](2+), 250 ps; [Ru(II)(bmp)(2)](2+), 15 ns; and [Ru(II)(dqp)(2)](2+), 3 μs (tpy is 2,2':6',2″-terpyridine, bmp is 6-(2-picolyl)-2,2'-bipyridine, and dqp is 2,6-di(quinolin-8-yl)pyridine). The computational results provide a multidimensional view of the (3)MLCT-(3)MC transition for the investigated complexes. Recently reported results of significantly prolonged (3)MLCT excited state lifetimes of bis-tridentate Ru(II)-complexes, for example [Ru(II)(dqp)(2)](2+), are found to correlate with substantial differences in their triplet excited state multidimensional potential energy surfaces. In addition to identification of low-energy transition paths for (3)MLCT-(3)MC conversion associated with simultaneous elongation of two or more Ru-N bonds for all investigated complexes, the calculations also suggest significant differences in (3)MLCT state volume in the multidimensional reaction coordinate space formed from various combinations of Ru-N bond distance variations. This is proposed to be an important aspect for understanding the large differences in experimentally observed (3)MLCT excited state lifetimes. The results demonstrate the advantage of considering multidimensional potential energy surfaces beyond the Franck-Condon region in order to predict photophysical and photochemical properties of bis-tridentate Ru(II)-polypyridyl dyes and related metal complexes.

Conjugated polymer/fullerene-based solar cells are, due to their potential flexibility, cost efficiency and low weight, interesting as future alternatives to today’s silicon cells [1]. In these cells, both the dissociation of the polymer exciton into extractable charges, i.e. charges that are free and mobile, and the recombination of the free charges are expected to be crucial in order to obtain high light-to-charge conversion efficiencies. The exciton dissociation is believed to strongly depend upon the driving force [2] while the driving force dependence of the recombination has not been thoroughly investigated. Therefore, we have studied the charge formation and recombination in blends of alternating polyfluorene copolymers (APFO polymers) and PCBM with different driving force. By varying the polymer in the blends, a driving force range of 0.05–0.77 eV has been covered.

We have studied the influence of three different fullerene derivatives on the charge generation and recombination dynamics of polymer/fullerene bulk heterojunction (BHJ) solar cell blends. Charge generation in APFO3/[70]PCBM and APFO3/[60]PCBM is very similar and somewhat slower than charge generation in APFO3/[70]BTPF. This difference qualitatively matches the trend in free energy change of electron transfer estimated from the LUMO energies of the polymer and fullerene derivatives. The first order (geminate) charge recombination rate is significantly different for the three fullerene derivatives studied and increases in the order APFO3/[70]PCBM < APFO3/[60]PCBM < APFO3/[70]BTPF. The variation in electron transfer rate cannot be explained from the LUMO energies of the fullerene derivatives and single-step electron transfer in the Marcus inverted region and simple considerations of expected trends for the reorganization energy and free energy change. Instead we suggest that geminate charge recombination occurs from a state where electrons and holes have separated to different distances in the various materials because of an initially high charge mobility, different for different materials. In a BHJ thin film this charge separation distance is not sufficient to overcome the electrostatic attraction between electrons and holes and geminate recombination occurs on the nanosecond to hundreds of nanoseconds time scale. In a BHJ solar cell, we suggest that the internal electric field in combination with polarization effects and the dynamic nature of polarons are key features to overcome electron-hole interactions to form free extractable charges.

A series of bistridentate ruthenium(II) polypyridyl-type complexes based on the novel 2,6-di(quinolin-8-yl)pyridine (dqp) ligand have been synthesized and their photophysical properties have been studied. The complexes are amenable to substitution in the 4-position of the central pyridine with conserved quasi-C2v symmetry, which allows for extension to isomer-free, rod-like molecular arrays for vectorial control of electron and energy transfer. DFT calculations performed on the parent [Ru(dqp) 2](2+) complex (1) predicted a more octahedral structure than in the typical bistridentate complex [Ru(tpy)2](2+) (tpy is 2,2':6',2"-terpyridine) thanks to the larger ligand bite angle, which was confirmed by X-ray crystallography. A strong visible absorption band, with a maximum at 491 nm was assigned to a metal-to-ligand charge transfer (MLCT) transition, based on time-dependent DFT calculations. 1 shows room temperature emission (Phi = 0.02) from its lowest excited ((3)MLCT) state that has a very long lifetime (tau = 3 micros). The long lifetime is due to a stronger ligand field, because of the more octahedral structure, which makes the often dominant activated decay via short-lived metal-centered states insignificant also at elevated temperatures. A series of complexes based on dqp with electron donating and/or accepting substituents in the 4-position of the pyridine was prepared and the properties were compared to those of 1. An unprecedented (3)MLCT state lifetime of 5.5 micros was demonstrated for the homoleptic complex based on dqpCO2Et. The favorable photosensitizer properties of 1, such as a high extinction coefficient, high excited-state energy and long lifetime, and tunable redox potentials, are maintained upon substitution. In addition, the parent complex 1 is shown to be remarkably photostable and displays a high reactivity in light-induced electron and energy transfer reactions with typical energy and electron acceptors and donors: methylviologen, tetrathiofulvalene, and 9,10-diphenylanthracene. This new class of complexes constitutes a promising starting point for the construction of linear, rod-like molecular arrays for photosensitized reactions and applications in artificial photosynthesis and molecular electronics.

A bistridentate RuII-polypyridine complex [Ru(bqp)2]2+ (bqp = 2,6-bis(8'-quinolinyl)pyridine) has been prepared, which has a coordination geometry much closer to a perfect octahedron than the typical Ru(terpyridine)2-type complex. Thus, the complex displays a 3.0 mus lifetime of the lowest excited metal-to-ligand charge transfer (3MLCT) state at room temperature. This is, to the best of our knowledge, the longest MLCT state lifetime reported for a RuII-polypyridyl complex at room temperature. The structure allows for the future construction of rod-like, isomer-free molecular arrays by substitution of donor and acceptor moieties on the central pyridine units. This makes it a promising photosensitizer for applications in molecular devices for artificial photosynthesis and molecular electronics.