Experimental and Theoretical Studies on the Intramolecular Charge-Transfer Emission of Phenyldisilanes

Altering intra- to inter-molecular hydrogen bonding by dimethylsulfoxide: A TDDFT study of charge transfer for coumarin 343

DFT and TDDFT methods were carried out to investigate the influences of intramolecular and intermolecular hydrogen bonding on excited state charge transfer for coumarin 343 (C343). Intramolecular hydrogen bonding is formed between carboxylic acid group and carbonyl group in C343 monomer. However, in dimethylsulfoxide (DMSO) solution, DMSO 'opens up' the intramolecular hydrogen bonding and forms solute-solvent intermolecular hydrogen bonded C343-DMSO complex. Analysis of frontier molecular orbitals reveals that intramolecular charge transfer (ICT) occurs in the first excited state both for C343 monomer and complex. The results of optimized geometric structures indicate that the intramolecular hydrogen bonding interaction is strengthened while the intermolecular hydrogen bonding is weakened in excited state, which is confirmed again by monitoring the shifts of characteristic peaks of infrared spectra. We demonstrated that DMSO solvent can not only break the intramolecular hydrogen bonding to form intermolecular hydrogen bonding with C343 but also alter the mechanism of excited state hydrogen bonding strengthening.

Direct observation of the solvent reorientation dynamics in the “twisted” intramolecular charge-transfer process of cyanophenyldisilane–water cluster by transient infrared spectroscopy

The solvent reorientation dynamics of the intramolecular charge-transfer (ICT) process of the (p-cyanophenyl)pentamethyldisilane-H(2)O (CPDS-H(2)O) cluster was investigated by transient infrared (IR) absorption spectroscopy. Transient IR bands of two distinct charge-transfer (CT) states appeared in both the OH and the CN-stretching vibration regions. Analyses of the IR spectra and the time profiles of the transient bands revealed that the ICT process of the CPDS-H(2)O cluster proceeds in two steps. The first step is a transition from a photo-prepared locally excited (LE) state to the CT state, which is accompanied by a minor reorientation of the H(2)O moiety. In contrast, the second step is an extensive reorientation process of the H(2)O molecule in the CT state. These two reorientation processes exhibit very distinct pico- and nano-second time scales. In the latter case, a relatively slow time constant of 2 ns was related to a large geometric change in the orientation.

Determination of the Equilibrium Structure of the Charge-transfer State of (p-Cyanophenyl)pentamethyldisilane by Means of Transient Infrared Spectroscopy

An equilibrium structure of the charge-transfer (CT) state of (p-cyanophenyl)pentamethyldisilane was determined by transient infrared absorption spectroscopy of its CH stretching vibration region, and by the spectral simulation with quantum chemical calculations. It was found that a pattern of the CH stretching vibration bands of the CT state is substantially different from that of the S0 state. This band feature of the CT state was well reproduced assuming the planar sigma(Si-Si)pi* state, where the disilanyl group and the phenyl ring lie in the same plane. Considering that the disilanyl group in the S0 and the locally excited pipi* states lie in the plane perpendicular to the phenyl ring, an occurrence of a twisting-type structural change during the ICT process was experimentally identified in the present study.

Spectroscopic and Excited-State Properties of Tri-9-anthrylborane I: Solvent Polarity Effects

Spectroscopic and excited-state properties of tri-9-anthrylborane (TAB), showing unique absorption and fluorescence characteristics originating from p(boron)-pi(anthryl group) orbital interactions, were studied in 12 solvents. Although the absorption maximum energy (nu(a)) of TAB which appeared at around 21 x 10(3) cm(-1) (band I) was almost independent of the solvent polarity parameter, f(X) (f(X) = (D(s) - 1)/(2D(s) + 1) - (n(2) - 1)/(2n(2) + 1) where D(s) and n represent the static dielectric constant and the refractive index of a solvent, respectively), the fluorescence maximum energy (nu(f)) showed a linear correlation with f(X). The f(X) dependence of the value of nu(a) - nu(f) demonstrated that the change in the dipole moment of TAB upon light excitation was approximately 8.0 D, indicating that absorption band I was ascribed to an intramolecular charge-transfer transition in nature. The excited electron of TAB was thus concluded to localize primarily on the p orbital of the boron atom. Furthermore, it was shown that the fluorescence lifetime and quantum yield of TAB varied from 11.8 to 1.1 ns and from 0.41 to 0.02, respectively, with an increase in f(X). The present results indicated that the nonradiative decay rate constant (k(nr)) of TAB was influenced significantly by f(X). Excited-state decay of TAB was understood by intramolecular back-electron (charge) transfer from the p orbital of the boron atom to the pi orbital of the anthryl group, which was discussed in terms of the energy gap dependence of k(nr). Specific solvent interactions of TAB revealed by the present spectroscopic and photophysical studies are also discussed.

Intramolecular charge-transfer fluorescence of 1-phenyltridecamethylbicyclo[2.2.2]octasilane

1-Phenyltridecamethylbicyclo[2.2.2]octasilane (1) is prepared by the reaction of the corresponding 1-chlorotridecamethylbicyclo[2.2.2]octasilane with phenyllithium in the presence of N,N,N',N'-tetramethylethylenediamine and the molecular structure of is determined by X-ray crystallography. Phenyloligosilane shows dual fluorescence even in non-polar hexane though the TICT-like mechanism is not applicable.

Intramolecular charge-transfer process of jet-cooled (p-cyanophenyl)pentamethyldisilane: roles of the torsional motion and the Si-Si bond change

To investigate the intramolecular charge-transfer (ICT) process of (p-cyanophenyl)pentamethyldisilane (CPDS), laser-induced fluorescence, dispersed fluorescence, and two-color resonance enhanced two-photon ionization spectra were measured in a jet-cooled isolated condition. Dual fluorescence of CPDS was observed from a ground vibrational level in the locally excited pipi state. Similar to an emission from the charge-transfer (CT) state in solution, one of the dual emissions of the isolated molecule in the jet was assigned as the CT emission. A significant vibrational dependence on the ICT process was found as exciting vibronic levels of the molecule. It was identified that the promoting mode of the ICT process is a torsional motion of the disilanyl group with respect to the phenyl ring. It was also revealed that an effective appearance energy of the CPDS cation via the CT state is much lower than that via the locally excited pipi state suggesting that the electronic configuration of the CT state is similar to that of the cation. On the basis of an electronic configuration of the cationic state, that of the CT state was suggested to be of the (sigma(Si)(-)(Si), 2ppi) type.