The electron accepting character of a dithienylethene-fused p-benzoquinone derivative is significantly reduced upon ring-closing isomerization. Visible light unlocks the π-electronic conjugation of the quinone so it can be utilized for a light-driven oxidation reaction.

The components of a 4:1 mixture of Rh(III)Cl tetrakis(4-methylphenyl)porphyrin 1 and a bowl-shaped tetra(4-pyridyl)cavitand 4 self-assemble into a 4:1 complex 14•4 via Rh–pyridyl axial coordination bonds. The single-crystal X-ray diffraction analysis and variable-temperature (VT) 1H NMR study of 14•4 indicated that 14•4 behaves as a quadruple interlocking gear with an inner space, wherein (i) four subunits-1 are gear wheels and four p-pyridyl groups in subunit-4 are axes of gear wheels, (ii) one subunit-1 and two adjacent subunits-1 interlock with one another cooperatively, and (iii) four subunits-1 in 14•4 rotate quickly at 298 K on the NMR time scale. Together, the extremely strong porphyrin-Rh–pyridyl axial coordination bond, the rigidity of the methylene-bridge cavitand as a scaffold of the pyridyl axes, and the cruciform arrangement of the interdigitating p-tolyl groups as the teeth moiety of the gear wheels in the assembling 14-unit make 14•4 function as a quadruple interlocking gear in solution. The gear function of 14•4 was also supported by the rotation behaviors of other 4:1 complexes: 24•4 and 34•4 obtained from Rh(III)Cl tetrakis[4-(4-methylphenyl)phenyl]porphyrin 2 or Rh(III)Cl tetrakis(3,5-dialkoxyphenyl)porphyrin 3 and 4 also served as quadruple interlocking gears, whereas 14•5 obtained from 1 and tetrakis[4-(4-pyridyl)phenyl]cavitand 5 did not behave as a gear. The results of activation parameters (ΔH⧧, ΔS⧧, and ΔG⧧) obtained from Eyring plots based on line-shape analysis of the VT 1H NMR spectra of 14•4, 24•4, and 34•4 also support the interlocking rotation (geared coupled rotation) mechanism.

To shed a light on fundamental molecular functions of photoinduced charge conductions by organic photovoltaic materials, it is important to directly observe molecular geometries of the intermediate charges just after the photoinduced electron-transfer reactions. However, highly inhomogeneous molecular environments at the bulk heteojunction interfaces in the photoactive layers have prevented us from understanding the mechanism of the charge conductions. We have herein investigated orbital geometries, electronic couplings, and hole-dissociation dynamics of photoinduced charge-separated (CS) states in a series of poly(3-hexylthiophene)–fullerene linked dyads bridged by rigid oligo-p-phenylene spacers by using time-resolved EPR spectroscopy. It has been revealed that one-dimensional intramolecular hole-dissociations exothermically take place from localized holes in initial CS states, following bridge-mediated, photoinduced charge-separations via triplet exciton diffusions in the conjugated polymer-backbones. This molecular wire property of the photoinduced charges in solution at room temperature demonstrates the potential utility of the covalently bridged polymer molecules applied for the molecular devices.

To elucidate how local molecular conformations play a role on electronic couplings for the long-range photoinduced charge-separated (CS) states in protein systems, we have analyzed time-resolved electron paramagnetic resonance (TREPR) spectra by polarized laser irradiations of 9,10-anthraquinone-1-sulfonate (AQ1S–) bound to human serum albumin (HSA). Analyses of the magnetophotoselection effects on the EPR spectra and a docking simulation clarified the molecular geometry and the electronic coupling of the long-range CS states of AQ1S•2–-tryptophan214 radical cation (W214•+) separated by 1.2 nm. The ligand of AQ1S– has been demonstrated to be bound to the drug site I in HSA. Molecular conformations of the binding region were estimated by the docking simulations, indicating that an arginine218 (R218+) residue bound to AQ1S•2– mediates the long-range electron-transfer. The energetics of triad states of AQ1S•2––R218+–W214•+ and AQ1S––R218•–W214•+ have been computed on the basis of the density functional molecular orbital calculations, providing the clear evidence for the long-range electronic couplings of the CS states in terms of the superexchange tunneling model through the arginine residue.

Recent progress is overviewed on experimental elucidations of fundamental molecular functions of the light–energy conversions by the photoactive layers of the organic photovoltalic (OPV) cells by means of the time-resolved electron paramagnetic resonance spectroscopy. Positions and orientations of the unpaired electrons and electronic coupling matrix elements are clarified in photoinduced, primary charge-separated (CS) states. Connections between the molecular geometries and the electronic couplings have been characterized for the initial CS states to elucidate how the structure, orbital delocalization, and molecular libration play roles on exothermic carrier dissociation via a vibrationally relaxed charge-transfer complex with prevention of the energy-wasting charge recombination. Superior functions to biological molecules are presented for the efficient photocurrent generations induced by orbital delocalization and by shallow trap depths at polymer-stacking domains. The above structural and electronic characteristics of the primary electron–hole pairs are essential to evaluations, designs, and developments of the efficient solar cells using organic molecules.

To elucidate how cofactor geometries after photoinduced primary charge-separations influence electronic couplings (VCR) for primary charge-recombination (CR) processes in the photosynthetic reaction center, we have analyzed time-resolved electron paramagnetic resonance (TREPR) spectra both of the primary charge-separated (CS) state (P+•HA–•) and its charge-recombined triplet state (3P*) of the special pair in Rhodobacter sphaeroides R26. To determine the CS-state geometry, quantum mechanical modeling has been performed on the spin polarization of the 3P* generated by the spin dynamics due to the anisotropies of the hyperfine and the spin–spin dipolar interactions in the primary CS state. From transverse magnetizations of the primary CS state, we have also determined the VCR value leading to the singlet excited state (1P*) of the special pair. The above analyses have revealed that while the primary charge separation does not largely modulate the cofactor conformations, it leads to significant enhancement in the VCR of the singlet recombination processes with respect to the triplet CR. This enhanced coupling is demonstrated by the larger orbital overlap between 1P* and an electron-accepting orbital of chlorophyll (BA) than between 3P* and BA, caused by the charge-transfer electronic character of 1P* in which the electron is locally distributed at a side of the bacteriochlorophyll (PM) situated in close proximity to BA.

Nanosecond time-resolved electron paramagnetic resonance (TREPR) spectroscopy has been utilized at T = 77 K to characterize alkyl side-chain effects on geometries and on the electronic couplings (VCR) of transient charge-separated (CS) states in the photoactive layers fabricated by the spin-coating of mixed solutions of regioregular polyalkylthiophenes (RR-P3AT) and [6,6]-C61-butyric acid methyl ester (PCBM). By increasing the alkyl side-chain number from 6 to 12 in P3AT, a highly distant and long-lived CS state has been obtained. This result is explained by a coupling of the hole dissociation to the polymer librations by the side-chains. From an exponential decay of VCR with respect to the CS distance, the attenuation factor (βe) has been determined to be βe = 0.2 Å–1. Such a long-range tunneling feature is explained by the generations of the shallowly trapped, delocalized electron–hole pairs by the dissociation of the hole toward π-stacking directions at the organic photovoltaic interface.Keywords: charge separation; electron tunneling; entropy; organic thin film solar cell; spin correlated radical pair; trap depth;