Prof. Imahori has been the forerunner of photoinduced charge separation in molecular donor-acceptor (D-A) linked systems. Especially, he has made remarkable achievement in organic solar cells (OSCs) as well as porphyrin-fullerene systems. The following is a description of his major achievements.
1. Fundamentals of photoinduced charge separation in donor-acceptor linked systems
To convert solar energy into electricity and chemical energy efficiently, it is pivotal to elucidate the basic principle of photoinduced charge separation. Prof. Imahori has made outstanding contribution to photoinduced charge separation. He proposed and demonstrated, for the first time, that the acceleration of photoinduced charge separation and the deceleration of charge recombination in donor-fullerene systems arise from intrinsic small reorganization energies of fullerenes as acceptors. This finding allows us to produce long-lived charge-separated (CS) state with a high quantum yield in donor-fullerene systems. The breakthrough has led him to a number of world records in D-A linked systems as well as their assembled ones on electrodes.
In this context, the importance of dynamic vibrations, rotation, and fluctuations in molecular D-A systems, which are involved in the excited-state generation, charge separation, and charge dissociation, has gradually been revealed. By focusing on these time-dependent dynamic effects on electron and spin behavior entangled with atomic nucleus and their collective movements and adopting them into molecular design, Prof. Imahori has opened up a new molecular science and creative innovation. In particular, he discovered that efficient transformation from a singlet CS state to a triplet CS state via a triplet charge-transfer (CT) state is possible in porphyrin-fullerene linked systems, resulting in the formation of the final long-lived, triplet CS state with an extremely high quantum yield of 99%. This notable conversion is rationalized by enhanced spin-orbit coupling between the two-dimensional porphyrin and three-dimensional fullerene through the dynamic effects. Photoinduced charge separation and subsequent charge dissociation were found to occur in regioregular monodisperse polythiophene-fullerene linked systems. The positive charge dissociation along the polythiophene backbone was explained by the entropy effect arising from collective motion of alkyl side chains to overcome the Coulomb barrier at the D-A interface. These pioneering works on the manipulation of locally excited state, CT state, and CS state are expected to expand "the world of photoinduced charge separation" by rational molecular design, realizing high performance in OSCs, organic light-emitting diodes, artificial photosynthesis, and photoredox organic reactions.
2. Elucidation of relationship between molecular structure and photovoltaic properties of organic solar cells
Prof. Imahori was among the first to advance the field of large π-aromatic dye-sensitized solar cells (DSSCs). In this regard, porphyrins have drawn much attention because of their extraordinary high molar absorption coefficients of Soret and Q-bands around 400-600 nm. If the absorption is averaged out at the wavelength of 400-800 nm, such porphyrins could be extremely promising in term of light-harvesting. Indeed, he has successfully developed versatile π-expanded porphyrins exhibiting enhanced light-harvesting in visible and near-infrared regions and resultant high power conversion efficiencies (PCEs). In particular, a combination of D-π-A structure, substituted methylene thiophene-fused structure, and co-sensitization of complementary dye led to the record PCE ever reported for DSSCs with fused porphyrin dyes. Meanwhile, he focused on self-assembly of organic molecules on metal and semiconductor surfaces. They tend to be tilted to their surfaces even if they are densely packed. Along this line, he disclosed the close correlation between dye geometry on TiO2, electron transfer (ET) kinetics, and photovoltaic properties. The ET on TiO2 occurs via through-space interaction rather than through-bond one, leading to fast electron injection to TiO2 and fast charge recombination in more tilted dyes on TiO2.
Prof. Imahori has elucidated the importance of fullerene isomers on their photovoltaic performance in bulk heterojunction (BHJ) OSCs. The degree of fullerene isomer aggregation by substituted patterns on the fullerene surface was found to have a large impact on their photovoltaic properties. Non-fullerene acceptors (NFA) have also been explored by rational molecular design. He exemplified that use of self-associating diazacoronene as the donor in A-D-A structure of NFA leads to the formation of very long-lived excited singlet state in films, which would become an useful strategy for minimizing the loss of open-circuit voltage in BHJ OSCs.
3. Exploration of cell engineering by light
Prof. Imahori has been the pioneer who utilizes a giant dipole moment produced from D-A linked molecules for regulating the membrane potential of living cells by light. In fact, depolarization and inhibition of potassium ion channel were observed in neuronal model cells. This is the first example of the use of the CS state in optogenetics. The highest yield of photoinduced charge separation (88%) was attained in artificial lipid membrane by using a multi-cationic D-A linked molecule. His unique approach is highly promising for the development of neuronal therapy by light.
Thus, Prof. Imahori's great contributions deserve well to the Chemical Society of Japan (CSJ) Award.