Direct Observation of Cascade of Photoinduced Ultrafast Intramolecular Charge Transfer Dynamics in Diphenyl Acetylene Derivatives: Via Solvation and Intramolecular Relaxation

Abstract

Interaction of light with electron donor-acceptor π-conjugated systems leading to intramolecular charge transfer (ICT) plays an essential role in transformation of light energy. Here the cascade of photoinduced ICT processes is directly observed by investigating the excited state relaxation dynamics of cyano and mono/di methoxy substituted diphenyl acetylene derivatives using femtosecond pump-probe spectroscopy and nanosecond laser flash photolysis. The femtosecond transient absorption spectra of the chromophores upon ultrafast excitation reveal the dynamics of intermediates involved in transition from initially populated Frank-Condon state to local excited state (LE). It also provides the dynamic details of the transition from the LE to the charge transfer state yielding the formation of the radical ions. Finally, the charge transfer state decays to the triplet state by geminate charge recombination. The latter dynamics are observed in the nanosecond transient absorption spectra. It is found that excited state relaxation pathways are controlled by different stages of solvation and intramolecular relaxation depending on the solvent polarity. The twisted ICT state is more stabilized (978 ps) in acetonitrile than cyclohexane where major components of transient absorption originate from the S1 state.

Molecular-Level Understanding of Excited States of N-Annulated Rylene Dye for Dye-Sensitized Solar Cells

“…As the transition from the second SADS to third SADS corresponds to the vibrational relaxation of the ICT state, it is probable that the energy dissipation to vibrational modes of solvent molecules mediates the vibrational relaxation. The agreement in the time scales of the excited-state relaxation of DND and the solvation implies that the formation of the ICT state is strongly coupled with solvent reorganization. ,, There have been studies ,,,− showing that the formation or relaxation of the ICT state is strongly correlated to solvation. For example, in TA studies on D–A dyads ,− or D−π–A−π–D triads, the dynamics of the ICT formation and relaxation are on the same time scale as C ( t ) extracted from the SE features in the TA spectra.…”

“…Many studies ,,,,, on push–pull dyes concluded that ICT is accompanied by structural changes. For example, on the basis of DFT calculations, it has been suggested that the lowest singlet excited state (that is, the ICT state) of D–A or D–A–D dyes possess twisted geometries, with larger , or smaller , donor–acceptor dihedral angles than the ground state.…”

“…For example, on the basis of DFT calculations, it has been suggested that the lowest singlet excited state (that is, the ICT state) of D–A or D–A–D dyes possess twisted geometries, with larger , or smaller , donor–acceptor dihedral angles than the ground state. Also, it was suggested that the transition from an ICT state with planar geometry to an ICT state with twisted geometry goes along with an increase of the dipole moment. , …”

Unsymmetrical Squaraine Dye-Based Organic Photodetector Exhibiting Enhanced Near-Infrared Sensitivity

“…The resultant time constants are given in Table , and the corresponding decay associated spectra are shown in Figure S7 in Supporting Information. The fast component (τ 1 = 412 fs in ACN and τ 1 = 2.5 ps in toluene) can be assigned to the solvation relaxation processes from the Franck–Condon state to the solvent-stabilized local excited state , based on the observation of the redshift of stimulated emission (∼763 nm) and blueshift of excited state absorption (∼525 nm) with increasing time. The time constant τ 2 , 10.78 and 114.8 ps, for ACN and toluene respectively, could be attributed to the dark state, resulting from the conformational changes of ANPHSQ , as result of a charge transfer stabilization in the polar solvent.…”

Confinement Effect of Micro- and Mesoporous Materials on the Spectroscopy and Dynamics of a Stilbene Derivative Dye

“…‘Push–pull’ systems, a class of molecules containing donor (D) and acceptor (A) moieties in their structure, undergo intramolecular charge-transfer (ICT) processes under light irradiation [13,14,15]. The excitation of the molecule usually leads at first to the S 1 -locally excited (LE) state whose electron density distribution and dipole moment values are similar to those of the ground-state, S 0 .…”