Photoinduced Electron Transfer in an Anthraquinone–[Ru(bpy)3]2+–Oligotriarylamine–[Ru(bpy)3]2+–Anthraquinone Pentad

Photoinduced Electron Transfer in Organized Assemblies—Case Studies

In this review, photoinduced electron transfer processes in specifically designed assembled architectures have been discussed in the light of recent results reported from our laboratories. A convenient and useful way to study these systems is described to understand the rules that drive a light-induced charge-separated states and its subsequent decay to the ground state, also with the aim of offering a tutorial for young researchers. Assembled systems of covalent or supramolecular nature have been presented, and some functional multicomponent systems for the conversion of light energy into chemical energy have been discussed.

Room-Temperature Stable Noncovalent Charge-Transfer Dianion Biradical to Produce Singlet Oxygen by Visible or Near-Infrared Light Photoexcitation

Noncovalent interaction between small molecules can generate a charge-transfer (CT) state, achieving the effect of a conjugated large molecule as well as a transition-metal complex. Herein, we demonstrate a room-temperature stable dianion biradical conveniently produced by noncovalent intermolecular CT interaction between anthraquinone (AQ) and potassium tert-butoxide (KOtBu). Essentially, CT from KOtBu to AQ boosts absorption bands from the UV to visible and near-infrared (NIR) range, enabling AQ-KOtBu to have new absorption bands around 400, 550, and 900 nm. The absorption bands of AQ-KOtBu are dramatically enhanced after blue-to-green or NIR light excitation. Interestingly, both ground state AQ-KOtBu (C₍₁₎) and photoexcited AQ-KOtBu (C₍₂₎) are quenched by oxygen to produce singlet oxygen. Furthermore, C₍₁₎ can be photoactivated by purged nitrogen in solution, and C₍₂₎ can be regenerated after the photoexcitation and purged nitrogen in solution, which may serve as a photosensitizer under visible and NIR light excitation.

Evaluating the Role of Molecular Heredity in the Optical and Electronic Properties of Cross-Conjugated Benzo[1,2-d:4,5-d']bisoxazoles

A series of eight benzo[1,2-d:4,5-d']bisoxazole (BBOs) were synthesized using the heredity principle as a design motif, whereby we investigated which characteristics of the linear parents were inherited by their cross-conjugated children. Four linear parents bearing 4-tert-butylbenzene (P) or 1,3-bis(4-tert-butylphenyl)benzene (M) at either the 2,6- or 4,8-position on the BBO and four cross-conjugated children bearing various combinations of the two isoelectronic aryl substituents were evaluated. Due to the bulky nature of the M substituent compared to that of the P substituent, the influence of steric hindrance along the BBO axes was explored theoretically and experimentally. The optical and electronic properties of each molecule were investigated in the solution and solid state using density functional theory (DFT) and time-dependent DFT (TD-DFT) and characterized using ultraviolet photoelectron spectroscopy (UPS), ultraviolet-visible (UV-vis) spectroscopy, and photoluminescence (PL) spectroscopy. The well-correlated theoretical and experimental results showed that the selective tuning of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels was possible through the strategic placement of substituents without impacting the H → L transition energy. Specifically, the theoretical results demonstrated that for the BBO children the HOMO and LUMO energy levels were inherited from the 4,8- and 2,6-parents, respectively. Each molecule was found to exhibit emission maxima ≤451 nm, making them ideal candidates for blue organic light-emitting diode (OLED) materials.

Charge-Separated Mixed Valency in an Unsymmetrical Acceptor-Donor-Donor Triad Based on Diarylboryl and Triarylamine Units

In this study, we report the generation of new mixed-valence (MV) subspecies with charge-separated (CS) characters from an unsymmetrical acceptor-donor-donor (A-D-D) triad. The triad was synthesized by attaching a dimesitylboryl group (A) to a D-D conjugate that consisted of triarylamine (NAr₃) units. The MV radical cation, obtained by chemical oxidation of the triad, exhibited a strong intervalence charge transfer (IVCT) absorption derived from the bis(NAr₃)^•+ moiety in the near-IR region. The charge-separated MV (CSMV) state, obtained by photoexcitation of the triad, caused a blue shift in IVCT energy in the femtosecond transient absorption spectra, reflecting a bias of positive charge distributions to the D end site. This resulted from increased electron density at the A site and restructuring of the central D site from NAr₃ to NAr₂ sites. Interestingly, any shift in the IVCT energy that was caused by the polarity of the solvent was minimal, reflecting the unique characteristics of the CSMV state. These findings represent the first detailed analysis of the CSMV state, including a comparison with conventional MV states. Therefore, this work provides new insights into counterion-free MV systems and their applications in molecular devices.

Excitation Power Modulates Energy-Transfer Dynamics in a Supramolecular RuII -FeII -RuII Triad

Multichromophoric arrays are key to light harvesting in natural and artificial photosynthesis. A trinuclear, symmetric Ru^II -Fe^II -Ru^II triad may resemble a light-harvesting model system in which excitation energy from donor units (Ru-terpyridine fragments) is efficiently transferred to the acceptor (the Fe-terpyridine fragment). The photoinduced dynamics after simultaneous excitation of more than a single chromophoric unit (donor/acceptor) at varying excitation fluence is investigated in this contribution. Data suggests that energy transfer is decelerated if the acceptor states (on the Fe^II unit) are not depopulated fast enough. As a consequence, the lifetime of a high-lying excited state (centered on either of the Ru^II units) is prolonged. A kinetic model is suggested to account for this effect. Although the proposed model is specifically adopted to account for the experimental data reported here, it might be generalized to other situations in which multiple energy or electron donors are covalently linked to a single acceptor site, a situation of interest in contemporary artificial photosynthesis.

π-Expansive Heteroleptic Ruthenium(II) Complexes as Reverse Saturable Absorbers and Photosensitizers for Photodynamic Therapy

Five heteroleptic tris-diimine ruthenium(II) complexes [RuL(N^N)₂](PF₆)₂ (where L is 3,8-di(benzothiazolylfluorenyl)-1,10-phenanthroline and N^N is 2,2'-bipyridine (bpy) (1), 1,10-phenanthroline (phen) (2), 1,4,8,9-tetraazatriphenylene (tatp) (3), dipyrido[3,2-a:2',3'-c]phenazine (dppz) (4), or benzo[i]dipyrido[3,2-a:2',3'-c]phenazine (dppn) (5), respectively) were synthesized. The influence of π-conjugation of the ancillary ligands (N^N) on the photophysical properties of the complexes was investigated by spectroscopic methods and simulated by density functional theory (DFT) and time-dependent DFT. Their ground-state absorption spectra were characterized by intense absorption bands below 350 nm (ligand L localized ¹π,π* transitions) and a featureless band centered at ∼410 nm (intraligand charge transfer (¹ILCT)/¹π,π* transitions with minor contribution from metal-to-ligand charge transfer (¹MLCT) transition). For complexes 4 and 5 with dppz and dppn ligands, respectively, broad but very weak absorption (ε < 800 M^-1 cm^-1) was present from 600 to 850 nm, likely emanating from the spin-forbidden transitions to the triplet excited states. All five complexes showed red-orange phosphorescence at room temperature in CH₂Cl₂ solution with decreased lifetimes and emission quantum yields, as the π-conjugation of the ancillary ligands increased. Transient absorption (TA) profiles were probed in acetonitrile solutions at room temperature for all of the complexes. Except for complex 5 (which showed dppn-localized ³π,π* absorption with a long lifetime of 41.2 μs), complexes 1-4 displayed similar TA spectral features but with much shorter triplet lifetimes (1-2 μs). Reverse saturable absorption (RSA) was demonstrated for the complexes at 532 nm using 4.1 ns laser pulses, and the strength of RSA decreased in the order: 2 ≥ 1 ≈ 5 > 3 > 4. Complex 5 is particularly attractive as a broadband reverse saturable absorber due to its wide optical window (430-850 nm) and long-lived triplet lifetime in addition to its strong RSA at 532 nm. Complexes 1-5 were also probed as photosensitizing agents for in vitro photodynamic therapy (PDT). Most of them showed a PDT effect, and 5 emerged as the most potent complex with red light (EC₅₀ = 10 μM) and was highly photoselective for melanoma cells (selectivity factor, SF = 13). Complexes 1-5 were readily taken up by cells and tracked by their intracellular luminescence before and after a light treatment. Diagnostic intracellular luminescence increased with increased π-conjugation of the ancillary N^N ligands despite diminishing cell-free phosphorescence in that order. All of the complexes penetrated the nucleus and caused DNA condensation in cell-free conditions in a concentration-dependent manner, which was not influenced by the identity of N^N ligands. Although the mechanism for photobiological activity was not established, complexes 1-5 were shown to exhibit potential as theranostic agents. Together the RSA and PDT studies indicate that developing new agents with long intrinsic triplet lifetimes, high yields for triplet formation, and broad ground-state absorption to near-infrared (NIR) in tandem is a viable approach to identifying promising agents for these applications.

Design and Syntheses of Novel Fluoroporphyrin-Anthraquinone Complexes as Antitumor Agents

A novel fluoroporphyrin-anthraquinone hybrid with dipeptide link and its metal complexes were synthesized and evaluated for anti-proliferation activity in human cancer cell line HeLa. The preliminary results demonstrated that all the compounds showed moderate to excellent antitumor activities. Among the active compounds, compound 3 which contains fluorinated porphyrin-anthraquinone and zinc ion exhibited the highest potency with IC50 value of 8.83 µM, indicating that it was a promising antitumor candidate.

Light-Driven Electron Accumulation in a Molecular Pentad

Accumulation and temporary storage of redox equivalents with visible light as an energy input is of pivotal importance for artificial photosynthesis because key reactions, such as CO2 reduction or water oxidation, require the transfer of multiple redox equivalents. We report on the first purely molecular system, in which a long-lived charge-separated state (τ≈870 ns) with two electrons accumulated on a suitable acceptor unit can be observed after excitation with visible light. Importantly, no sacrificial reagents were employed.

Unusual distance dependences of electron transfer rates

There are regimes in which electron transfer rates increase with increasing donor–acceptor distance, leading to rate maxima at large donor–acceptor separations.