Singlet and Triplet Excited-State Dynamics of 3,7-Bis(arylethynyl)phenothiazines: Intramolecular Charge Transfer and Reverse Intersystem Crossing

Abstract

A series of 3,7-bis(arylethynyl)-substituted phenothiazine-based fluorophores (bearing benzene, naphthalene, methoxynaphthalene, anthracene, phenanthrene, and pyrene) were designed and synthesized via a Pd-catalyzed Sonogashira cross-coupling reaction. These molecules show in solution large Stokes shifts (generally 5500-6500 cm-1) and high quantum yields (40-82% depending on the aryl groups), as a consequence of the planarization of their butterfly-like structure occurring in the excited state. Moreover, significant two-photon absorption was revealed for these phenothiazine derivatives, whose cross section increases upon enhancing the molecular conjugation. The remarkable Stokes shifts, fluorescence quantum yields, and two-photon absorption make these molecules appealing as fluorescent probes for bioimaging applications. Significant emission was also detected in the solid state where anthracene-, phenanthrene-, and pyrene-substituted phenothiazines show red-shifted emission maxima compared to the solution, indicating considerable π-πstaking. The anthracene-substituted phenothiazine arouses particular interest, as it exhibits photoinduced intramolecular charge transfer (ICT) from the phenothiazine to the anthracene, revealed by its important fluorosolvatochromism and through ultrafast transient absorption experiments. The spectroscopic results are in line with the time-dependent density functional theory (TD-DFT) calculations and cyclic voltammetry measurements. The anthracene-substituted molecule also features a remarkable triplet production and hints of a delayed fluorescence behavior, allowed in nonpolar solvents by a fairly competitive reverse intersystem crossing pathway. These findings make the investigated molecules interesting as new organic active materials for optoelectronic devices. Copyright © 2020 American Chemical Society.