Exploring Solvent and Substituent Effects on the Excited State Dynamics and Symmetry Breaking of Quadrupolar Triarylamine End-Capped Benzothiazole Chromophores by Femtosecond Spectroscopy

Abstract

We investigate herein the excited state dynamics and symmetry breaking processes in three benzothiazole-derived two-photon absorbing chromophores by femtosecond fluorescence and transient absorption (fs-TA) spectroscopies in solvents of various polarity. The chromophores feature a quasi-quadrupolar D-π-A-π-D architecture comprised of an electron-withdrawing benzothiazole core and lateral triphenylamine donors (Qbtz-H), while the acceptor strength of the central unit is enforced by attached cyano groups (Qbtz-CN) and the electron-donating strength of the arylamine moieties by introduction of peripheral methoxy groups (Qbtz′-CN). Steady state spectroscopy reveals positive solvatochromism, which is mostly pronounced for Qbtz′-CN. Femtosecond spectroscopy of Qbtz-H reveals the coexistence of the Franck–Condon (FC) state and states populated after symmetry breaking (SB) in low-polarity solvents such as toluene and tetrahydrofuran, while the SB state becomes favorable in polar acetonitrile. For the other two molecules possessing a stronger electron-accepting unit and thus more polar excited state, SB takes place even in low-polarity solvents, as shown by fs-TA spectroscopy. Global fitting of the fs-TA spectra together with investigation of the evolution associated spectra (EAS) reveals the existence of an initial FC state in Qbtz-H, in all studied solvents, which relaxes toward Intermediate Charge Transfer (I-CT) and SB states. On the other hand, for Qbtz-CN and Qbtz′-CN in more polar solvents, the FC state undergoes ultrafast relaxation toward symmetry-broken charge transfer (SB-CT) states which in turn show very fast recombination to the ground state. Our measurements confirm that the extent of symmetry breaking is larger for D-π-A-π-D systems with the stronger acceptor core and increases further by increasing electron-donating strength of triarylamine moieties, giving rise to symmetry breaking in these nonionic quadrupolar molecules with ethynylene (triple bond) π-spacers also in less polar solvents.

Ultrafast Photophysics of Regioisomeric Triphenylamine Dyes Having Dipolar D‐π‐A‐A and Octupolar D‐(π‐A‐A)3 Character with a Benzothiazole Unit in Matched or Mismatched Orientation

“…The time‐correlated single photon counting (TCSPC) method has been employed in the ps‐ns timescale (Fluotime 200 equipment, Picoharp). A pulsed laser diode at 400 nm with 80 ps pulse duration was used as the excitation source while the whole system had an instrumental response function (IRF) of ∼80 ps [71] . After performing the measurements, the results were analyzed by using multi‐exponential functions taking into account the laser pulse (reconvolution fitting), thus allowing us to detect lifetimes down to a few tens of ps.…”

“…The goodness of the fit was determined by reducing the χ 2 below 1.1 and examining the residuals. For the fs‐ps fluorescence dynamics, a femtosecond upconversion method was employed [71] . A Ti:Sapphire mode‐locked femtosecond laser (800 nm, 80 fs pulse duration, 80 MHz repetition frequency) was used as the excitation source after passing through a β‐barium borate (BBO) crystal producing the second harmonic at 400 nm.…”

“…A pulsed laser diode at 400 nm with 80 ps pulse duration was used as the excitation source while the whole system had an instrumental response function (IRF) of ~80 ps. [71] After performing the measurements, the results were analyzed by using multi-exponential functions taking into account the laser pulse (reconvolution fitting), thus allowing us to detect lifetimes down to a few tens of ps. The goodness of the fit was determined by reducing the χ 2 below 1.1 and examining the residuals.…”

“…For the fs-ps fluorescence dynamics, a ChemPhysChem femtosecond upconversion method was employed. [71] A Ti:Sapphire mode-locked femtosecond laser (800 nm, 80 fs pulse duration, 80 MHz repetition frequency) was used as the excitation source after passing through a β-barium borate (BBO) crystal producing the second harmonic at 400 nm. To detect the dynamics of T-btz-2-H, the laser was tuned at 760 nm and the excitation beam was at 380 nm.…”

“…For the fs-TA measurements an amplified Ti:Sapphire laser (800 nm, 80 fs pulse duration, 1 kHz repetition frequency) was employed. [71,72] The samples were excited with the second-harmonic at 400 nm while the white light probe beam spanning from 330 nm to 680 nm was produced by focusing the 800 nm pulses into a CaF2 plate. As in the fluorescence measurements, the dynamics were detected under magic angle conditions.…”