Molecular Photoinduced Charge Separation: Fundamentals and Application

Abstract

We have designed and synthesized a number of donor-acceptor linked systems to elucidate the science of photoinduced charge separation. In particular, porphyrin-fullerene and polymer-fullerene linked molecules have been found to be very useful as model systems for addressing issues in excited-state generation, charge separation, and charge dissociation. We have also proposed a new concept, “dynamic exciton” to establish new comprehensive photochemistry dealing with manipulation of locally excited state, charge-transfer state, and charge-separated state involving mutual transformation. Simultaneously, it is pivotal to adopt time-dependent dynamic effects on electron and spin behaviors entangled with movement of atomic nuclei (i.e., vibration, rotation, and fluctuation) and their collective motion into molecular donor-acceptor systems. Versatile dyes for dye-sensitized solar cells as well as donor/acceptor molecules for bulk heterojunction organic solar cells have also been tailored toward better understanding the underlying mechanism as well as improving their photovoltaic performances. Meanwhile, giant dipole moment produced from donor-acceptor linked molecules by light has been successfully utilized to regulate the membrane potential of living biological cells. This is the first example of the use of charge-separated state in optogenetics and is promising as neuronal therapy by light.

Molecular designs, synthetic strategies, and properties for porphyrins as sensitizers in dye-sensitized solar cells

“…This trend could be explained through the increase in their dye aggregation, consistent with the higher G values, and charge-recombination effects owing to the introduction of the strong electron-drawing BTD moiety. As we demonstrated before, [47][48][49] adsorbed Zn-porphyrins on TiO 2 are heavily tilted to the surface, even if they are well-packed on TiO 2 (40°∼70°from the surface normal). The tilt angle of Znporphyrins on TiO 2 depends on the dye-loading amount, molecular length, bulkiness, interaction between specically incorporated atoms (N, F, S atoms) and TiO 2 , and the adsorption conditions (time, immersion solvent, etc.).…”

“…The lower PCE of ZnPNTz may have resulted from the inefficient electron injection derived from its inferior ET parameters as well as much faster CR, as a result of the longer molecular length of ZnPNTz than that of XW17. [47][48][49] In contrast to trans-A 2 BC-type push-pull porphyrin dyes, cis-A 2 BC-type push-pull porphyrin dyes with two donor moieties are more appealing because of the more favourable orbital distribution in the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for ET processes as well as improved light-harvesting ability by their asymmetrical molecular structure (vide infra). Despite the prospect of cis-A 2 BC-type push-pull porphyrin dyes, it is a synthetic challenge to introduce three different substitutions at desired meso-positions.…”

“…Meanwhile, an increase in the length of the linkers between a porphyrin core and TiO 2 and closer HOMO orbital distribution to TiO 2 by the introduction of the thiophene to the acceptor side would lead to fast CR because of the tilted geometry and close orbital distribution to TiO 2 . [47][48][49] The more tilted geometry also resulted in a lower G value of AfZnP (AfZnP (4.8 × 10 −11 mol cm −2 ) < DfZnP-iPr (7.5 × 10 −11 mol cm −2 ) z DfZnP (7.6 × 10 −11 mol cm −2 )). Therefore, AfZnP bearing the thiophene ring fused on the acceptor side exhibited a lower PCE than those for DfZnP and DfZnP-iPr containing the fused structure on the donor side.…”

“…The decrease in the PCE may be ascribed to the more tilted geometry of ZnPTD on TiO 2 and thereby the faster CR between the injected electrons in the CB of TiO 2 and the porphyrin radical cation. [47][48][49] Given that the ethynyl-linked porphyrin dimers exhibit excellent light-harvesting abilities, the suppression of dye aggregation and/or control of dye geometry on TiO 2 by a co-adsorbent/co-sensitizer and/or by the pretreatment of TiO 2 may boost their PCEs.…”

“…The reason for the decreased PCE obtained by the XW60-based DSSC may be due to the unfavourably tilted adsorption of the Z2 component caused by the short bridging group. [47][48][49] On the other hand, the fact that nearly the same PCEs were achieved by the device with XW61-63 indicated that the longer bridging group was unnecessary for highly efficient CC dyes. Furthermore, the co-adsorption of XW61 with CDCA afforded a record PCE of 12.4% for iodine electrolyte-based DSSCs.…”