Chi-Wen Cheng

Synthesis and electron-transfer properties of benzimidazole-functionalized ruthenium complexes for highly efficient dye-sensitized solar cells

Novel heteroleptic ruthenium complexes--RD1, RD5, RD10 and RD11--with ligands based on benzimidazole were synthesized and characterized for application to dye-sensitized solar cells (DSSC); the remarkable performance of RD5-based DSSC is understood for its superior light-harvesting ability and slower charge-recombination kinetics.

Effects of Hydrogen Bonding on Internal Conversion of GFP-like Chromophores. II. The meta-Amino Systems

To rationalize the efficient quenching of the fluorescence and the Z → E photoisomerization of m-ABDI, the meta-amino analogue of the green fluorescent protein (GFP) chromophore, in protic solvents, the femtosecond time-resolved fluorescence and transient infrared (TRIR) spectra of m-ABDI in CD3CN, CH3OH, and CD3OD are determined. For solutions in CD3CN, the fluorescence decay lifetime is ∼7.9 ns and IR absorption lines near 1513, 1531, 1557, and 1613 cm(-1) of m-ABDI in its electronically excited state were observed with a decay time >5 ns. For solutions in CH3OH, the fluorescence decay is double exponential with time constants of ∼16 and 62 ps. In addition to IR absorption lines of m-ABDI in its electronically excited state with a decay time of ∼16 ps, new features near 1513, 1532, 1554, and 1592 cm(-1) were observed to have a rise time of ∼19 ps and a decay constant of ∼58 ps, indicating formation of an intermediate. The assignments for the IR spectra of the ground and excited states were assisted with DFT and TDDFT calculations, respectively. We conclude that the torsion of the exocyclic C═C bond (the τ torsion) is responsible for the nonradiative decay of electronically excited m-ABDI in CD3CN. However, in CH3OH and CD3OD, the solute-solvent hydrogen bonding (SSHB) interactions diminish significantly the barrier of the τ torsion and induce a new pathway that competes successfully with the τ torsion, consistent with the efficient fluorescence quenching and the diminished yield for Z → E photoisomerization. The new pathway is likely associated with excited-state proton transfer (ESPT) from the solvent to m-ABDI, particularly the carbonyl group, and generates an intermediate (ESPT*) that is weakly fluorescent.

Theoretical Investigation of Molecular Properties of the First Excited State of the Phenoxyl Radical

A theoretical study of molecular, electronic, and vibrational properties of the first excited state of the phenoxyl radical, A 2B2, is presented. The calculated molecular geometries, vertical and adiabatic excitation energies, and harmonic vibrational frequencies are compared with analogous results obtained for the ground state. The calculated excitation energies correspond well to experimental data. The harmonic vibrational frequencies of the A 2B2 and the ground state are similar except for modes involving the vibrations of the CO bond.

Rovibronic bands of the à B22←X̃ B21 transition of C6H5O and C6D5O detected with cavity ringdown absorption near 1.2 μm

We recorded several rovibronic bands of C(6)H(5)O and C(6)D(5)O in their A (2)B(2)<--X (2)B(1) transitions in the range 1.14-1.31 microm with the cavity ringdown technique. While the electronic transition is forbidden, several vibronic bands are observed. By comparison of rovibronic contours of observed and simulated bands to determine their types of transition, and by consideration of vibrational wavenumbers of the upper state based on quantum-chemical calculations, we were able to provide vibronic assignments of observed bands and derive several experimental vibrational wavenumbers (given as nu in unit of cm(-1) in this paper) for the A (2)B(2) state, namely, nu(12)=947, nu(13)=793, nu(14)=417, nu(15)=964, nu(16)=866, nu(17)=723, nu(18)=680, and nu(19)=499 for C(6)H(5)O, and nu(12)=772, nu(13)=626, nu(14)=365, nu(15)=812, nu(17)=599, nu(18)=532, and nu(19)=436 for C(6)D(5)O. Transitions involving vibrationally excited levels of nu(20) were also observed; nu(20) of the A state is greater by 50 cm(-1) than the X state of C(6)H(5)O. A weak origin at 7681 cm(-1) for the A<--X transition of C(6)H(5)O (7661 cm(-1) for C(6)D(5)O) with a c-type contour was observed. Observed isotopic ratios of vibrational wavenumbers for the A state of C(6)H(5)O to those of C(6)D(5)O are in good agreement with the predictions from quantum-chemical calculations at the B3LYP/aug-cc-pVTZ level.

Theoretical interpretation of the UV-vis spectrum of the CS2/Cl complex in the spectral region 320-550 nm.

Accurate multireference configuration interaction and time-dependent density functional calculations have been performed to interpret the experimental UV-vis spectrum of the CS(2)/Cl complex in the spectral region 320-550 nm. The molecular structure of the complex responsible for the previously observed UV-vis spectrum is recognized as ClSCS, not ClCS(2). Two low-lying excited states of ClSCS, responsible for its optical absorption, have been identified and analyzed. Optical excitation of ClSCS leads to the excitation-specific bond elongation that may lead to photofragmentation of the molecule. In addition, experimental conditions for verifying the presence of ClCS(2) are identified and detailed characterization of its optically active excited states with possible photofragmentation pathways is given.

Extrinsic charge traps in disordered organic materials

The effects of p-diethylaminobenzaldehyde diphenylhydrazone, a 0.22 eV trap, on the charge transport properties of disordered organic materials consisted of a mixture of N,N′-diphenyl N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine and polycarbonate are described. Trap-perturbed, trap-limited, and trap-controlled (trap-to-trap) transports were observed, where the trap is less effective at very low trap concentrations but appears to have increasing trap depth with respect to the concentration increase in the trap-limited regime. Most photocurrent transients showed non-dispersive transport behaviors; however, some were dispersive, especially at low electric fields or near the cross-over region between the trap-limited and trap-controlled transports. Field dependency of the mobilites at the trap-limited regime is higher than that of the trap-controlled regime, suggesting some type of superexchange phenomenon or field induced detrapping for the latter transport. For most known systems, charge transport ...