First Observation of Intramolecular Charge-Transfer Emission from Jet-Cooled (p-Cyanophenyl)pentamethyldisilane in an Isolated Molecular Condition

Steric effects on the intramolecular charge transfer fluorescence of benzo[b]thiophene-1,1-dioxide bridged macrocages

Intramolecular charge transfer (ICT) fluorescence has been widely investigated and exploited in sensor molecules. However, steric effects on the ICT fluorescence properties have rarely been reported so far, although research in this area would promote an understanding of the basics of solvation. Herein, we report the detailed fluorescence properties of bis(trimethylsilyl)benzo[b]thiophene-1,1-dioxide (TMSBTO2) and its caged cyclophanes and non-cage isomers, which demonstrate ICT fluorescence in various solutions. The fluorescence band maxima for these benzo[b]thiophene-1,1-dioxides (BTO2s) showed a red-shift with increasing solvent polarity, confirming ICT fluorescence characteristics. The linearity of the Lippert-Mataga plots was confirmed for all ICT fluorescence measured in hexane, toluene, AcOEt, CH2Cl2, and EtOH. The slopes of the plots decreased in the following order: TMSBTO2, non-cage isomers, and caged BTO2s. It is concluded that the Onsager radii for these BTO2s were increased in the abovementioned order, assuming that the difference in the dipole moments between the excited and ground states for these BTO2s was identical.

Effects of Substituents on the Blue Luminescence of Disilane-Linked Donor‒Acceptor‒Donor Triads

A series of disilane-linked donor‒acceptor‒donor triads (D‒Si‒Si‒A‒Si‒Si‒D) was synthesized to investigate the effects of substituents on the photophysical properties. The triads were prepared by metal-catalyzed diiodosilylation of aryl iodides using a Pd(P(t-Bu)₃)₂/(i-Pr)₂EtN/toluene system that we previously developed. Optical measurements, X-ray diffraction analysis, and density functional theory calculations revealed relationships between the photophysical properties and molecular structures of these triads in solution and in the solid state. The compounds emitted blue to green fluorescence in CH₂Cl₂ solution and in the solid state. Notably, compound 2 showed fluorescence with an absolute quantum yield of 0.17 in the solid state but showed no fluorescence in CH₂Cl₂. Our findings confirmed that the substituent adjacent to the disilane moiety affects the conformations and emission efficiencies of compounds in solution and in the solid state.

Oligothienyl catenated germanes and silanes: synthesis, structure, and properties

The synthesis of two new groups of oligothienyl catenated silanes and germanes, Me5M2Thn (1a-b), Me5M2ThnM2Me5 (2a-c) (terminal), and ThnM2Me4Thn (3a-d) (internal) (M = Si, Ge; n = 2, 3; Th = 2- or 2,5-thienyl), is reported. The study of their structural parameters as well as of their spectral (NMR), electrochemical (CV) and optical (UV/vis absorbance, luminescence) properties has been performed in detail; in addition, the unexpected compound [Th2Si2Me4Th]2 (3a') is also studied. Theoretical investigations have been performed for model compounds in order to establish structure-property relationships. The molecular structures of 2a (Me5Si2Th2Si2Me5), 2b (Me5Ge2Th2Ge2Me5), 3a (Th2Si2Me4Th2) and 3b (Th2Ge2Me4Th2) have been investigated by X-ray diffraction analysis. An effective conjugation with flattening of both Th planes in terminal 2a and 2b was observed. The main trends in the dependence of the optical and electrochemical properties on the structural parameters have been established. All of the compounds studied exhibit a strong emission within the 378-563 nm range, and the maximal quantum yield (up to 77%) is observed for the Si derivative 3a'. For the majority of the compounds, the quantum yields (20-30%) are significantly larger than for 2,2'-bi- and 2,2':5',5''-terthiophenes. Due to their good emission properties, these compounds could be used to develop new materials with specific spectral properties.

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.

Characterization of Weak NH−π Intermolecular Interactions of Ammonia with Various Substituted π-Systems

Among the several weak intermolecular interactions pervading chemistry and biology, the NH-pi interaction is one of the most widely known. Nevertheless its weak nature makes it one of the most poorly understood and characterized interactions. The present study details the results obtained on gas-phase complexes of ammonia with various substituted pi systems using both laser vibrational spectroscopy and ab initio calculations. The spectroscopic measurements carried out by applying one-color resonant two-photon ionization (R2PI) and IR-vibrational predissociation spectroscopy in the region of the NH stretches yield the first experimental NH stretching shifts of ammonia upon its interaction with various kinds of pi-systems. The experiments were complemented by ab initio calculations and energy decompositions, carried out at the second-order Møller-Plesset (MP2) level of theory. The observed complexes show characteristic vibrational spectra which are very similar to the calculated ones, thereby allowing an in-depth analysis of the interaction forces and energies. The interaction energy of the conformers responsible for the observed vibrational spectra has the maximum contribution from dispersion energies. This implies that polarizabilities of the pi-electron systems play a very important role in governing the nature and geometry of the NH-pi interaction. The larger polarizability of ammonia as compared to water and the tendency to maximize the dispersion energy implies that the characteristics of the NH-pi interactions are markedly different from that of the corresponding OH-pi interactions.

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.

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.

Photochemistry of a Trisilane Substituted by a Pendant p-Cyanostilbene Electron Acceptor Chromophore

Photolysis of (E)-4-cyano-4'-(2-heptamethyltrisilanyl)stilbene (E)-3 in a medium polarity solvent (CH(2)Cl(2)) results in efficient intramolecular electron transfer, which converts the initial pi,pi excited state ((1)LE state) to the charge transfer (CT) excited state. The CT excited state fluorescesces, undergoes E,Z photoisomerization, and reacts with MeOH to produce hydrosilane 4 via Si-Si bond cleavage and protonation of the central silicon, as shown by deuterium labeling. The CT excited state assignments are consistent with the observed quadratic plots of 1/Phi(f) versus [MeOH] and 1/Phi(EZ)() versus [MeOH], which indicate that both the initial (1)LE state and the CT excited state are quenched by MeOH, with the CT excited state serving as the emissive state and the state primarily responsible for the E,Z photoisomerization. Although biacetyl triplet sensitized photolysis results in efficient E,Z isomerization, quenching of direct photolyses by azulene shows (Z)-3 is not a product of the lowest energy triplet excited state, populated from the CT state by back electron transfer. The azulene quenching also shows that hydrosilane 4 is formed from the same excited state that gives (Z)-3. The intermediacy of a complex of the CT excited state with MeOH accounts for the observed upward curvature in the plot of 1/Phi(SiH) vs 1/[MeOH]. The linear behavior of Phi(EZ)()/Phi(SiH) x [MeOH] versus 1/[MeOH] further suggests that this complex reacts with uncomplexed MeOH to give hydrosilane 4.