Some Photochemical Reactions Including Intramolecular Charge Transfer And Their Applications

The excited-state character of tridurylboron derivatives, such as thermodynamics and conformational transition in its intramolecular charge transfer (ICT) process, has been studied. Moreover, a series of moleculars have been synthesized for application in dye-sensitized solar cells and antennae. In addition, we have discussed the photocyclization of2,2’-disubstituted bixanthenylidenes (BAEs), as follows:In chapter1, a general introduction to ICT was simply presented, such as the status of research, potential application and open questions about ICT.In Chapter2, the synthesis, ground-state structures and excited-state properties of three series of pyrene-(PB1-3),3-carbazol-(CBC1-3) and N-carbazolphenyl (CBN3a-c) substituted tridurylboron derivatives are reported, and these compounds are constructed as conjugated donor (D)-acceptor (A) molecules with different electron-accepting/donating ability. Twisted D-A molecular structures of these tridurylboron derivatives have been demonstrated by experimental and theoretical studies. Solvatochromic effects, fluorescence quantum yields (Φf) and fluorescence spectra in rigid solutions at77K reveal three types of excited states, a locally excited (LE) state, an intramolecular charge transfer (ICT) state and a twisted ICT (TICT) state, in their photochemical processes. The fluorescence spectra of these tridurylboron derivatives in binary solvent mixtures, or in the temperature range of293-77K, display the equilibria and the interconversions between the three excited states, and dual or triple fluorescence emission under a certain condition. Three types of excited states have been characterized by a non-dipole LE state with a small diving force (ΔG>0.5eV), whose energy level is independent on solvent and temperature, a highly fluorescent ICT state with an intermediate driving force (0.1eV<ΔG<0.5eV) and a more planar conformation, and a more twisted TICT state with a large driving force (ΔG<0.1eV), as compared to that of parent ground state. The formation of the TICT state undergoes a larger energy barrier of conformational change over that of the ICT state. Therefore, the excited-state character of these tridurylboron derivatives is controlled by two important factors, thermodynamics, and kinetics of conformational transition, of its ICT process.In Chapter3, a series of new metal-free organic dyes with either a boron dipyrromethene (BODIPY)-phenylene or-thiophene as a π-conjugated bridge have been synthesized for application in dye-sensitized solar cells. The photophysical and electrochemical properties of these dyes were investigated and their performance as sensitizers in dye-sensitized solar cells has been measured. A combination of a methoxy modified donor and BODIPY-thiophene bridge possesses a stronger electron-donating ability and longer wavelength absorption band, and as a sensitizer reveals the best properties of DSSCs, whose conversion efficiency was2.26%.In Chapter4, three porphyrin-cored dendrimers with non-conjugated coumarins as dendrons have been synthesized and characterized. The photophysical properties of the title compounds were investigated by means of UV/Vis absorption and fluorescence spectroscopy in dilute CH2Cl2solutions and in thin neat films. The intramolecular energy transfer from the coumarin units to the porphyrin core clearly reveals two factors influencing energy-transfer efficiency. Firstly, a better spectral overlap between the absorption spectrum of porphyrin core and the emission spectrum of the coumarin moiety results in high energy-transfer efficiency. Secondly, a long alkyl side-chain improves solubility of dendrimers, but also prevents the coumarins from self-quenching. Hence, the dendrimer with N-octy1groups possesses a higher efficiency than that with N-ethy1groups. The dendrimers emit red light with higher fluorescence quantum yields over the free porphyrin.In Chapter5, we have demonstrated that the efficiency and regioselectivity in the photocyclization of2,2’-disubstituted bixanthenylidenes (BAEs) depend on both the steric hindrance effects and electronic effects of the substituent groups. In contrast to the reversible photochemistry of the2,2’-substituted bixanthenylidenes (1a-f), the photocyclization of2,2’-diacyl bixanthenylidenes (1g-j) reveals an irreversible process where the initial cyclic intermediate C(E) can undergo a rapid [1,11] hydrogen shift to form stable isomer C’(E) in a degassed solution, which cannot revert to the starting compound, but can be rapidly oxidized to helixanthen2(E) in the presence of oxygen, so giving a highly efficient and regiospecific photocyclization. This implies that the photoconversion efficiency is independent of the oxygen concentration during the irradiation period. It can be expected that the clean photoconversion has a potential application as a convenient chemical actinometry on the radiation measurement in UV wavelength region of250-400rm.