Ultrafast Conformational Relaxation Dynamics in Anthryl-9-benzothiazole: Dynamic Planarization Driven Delocalization...

Conformational motion in the excited state of fluorophores critically governs their photophysical properties. Unveiling controlling parameters of photoinduced molecular motion in organic dyes is essential for optimization of light-triggered processes. Herein, we present ultrafast dynamics of conformational relaxation controlled photophysical properties of anthryl-9-benzthiazole (AnBT). The title compound is a bichromophore, consists of anthracene (AN) and benzothiazole (BT) units connected by a single bond, and exists in out of plane ground state conformation (dihedral angle of about 65?). Vibronic resolved structured absorption feature of anthracene localized excitation is lost in first excited singlet state displaying large Stokes-shifted fluorescence, characteristics of delocalized state involving both AN and BT unit. Ultrafast transient absorption spectroscopic studies revealed evolution of anthracene-localized Franck?Condon state to a delocalized excited state in a few picosecond time scale depending on solvent viscosity. A planarized motion of AN and BT units is proposed to be involved in excited state relaxation. However, protonation of BT unit is shown to induce significant intramolecular charge transfer facilitated by ultrafast torsional relaxation promoting nonradiative deactivation. Thus, depending upon protonation state, AnBT is shown to undergo either ultrafast planarized motion facilitating delocalized emissive excited state or perpendicular torsion rendering nonemissive twisted intramolecular charge transfer state. Experimental results were corroborated by quantum chemical calculation.