Dimethoxy substituted triphenylamine based donor-acceptor fluorophores: tunable solid-state emission and reversible thermofluorochromism

Abstract

A new series of donor-acceptor fluorescence molecules based on dimethoxy substituted triphenylamine donor and different acceptors (malononitrile, ethyl cyanoacetate, cyanoacetic acid, cyanoacetamide, indanedione and dimethyl barbituric acid) were synthesized and investigated their solid-state structural assembly and tunable and switchable fluorescence. Single crystal analysis revealed twisted non-planar molecular conformation with acceptor dependent structural organization and intermolecular interactions. DMT-MN, DMT-ECA and DMT-ID displayed intermolecular interactions facilitated dimer formation with opposite molecular orientation. DMT-CAA and DMT-BA showed network structure whereas DMT-CA exhibited complementary amide H-bonding. Solid-state fluorescence studies showed blue shifted emission for DMT-CA (λmax = 532 nm) and DMT-ID displayed red shifted fluorescence at 630 nm. DMT-MN exhibited highest solid-state fluorescence efficiency of 16.2%. Computational studies supported the fluorescence tuning by showing acceptor group dependent optical band gap. It also suggested locally excited (LE) state emission in DMT-CA and charge transfer (CT) state emission in DMT-ID. Stimuli-responsive fluorescence studies indicated that DMT-ECA exhibited reversible temperature dependent off-on fluorescence switching. The fluorescence intensity was strongly reduced by heating and reversed to initial state upon cooling. The reversible thermofluorochromism of DMT-ECA might be attributed to the flexible ethyl group and relatively weak intermolecular interactions. PXRD studies confirmed the structural stability of DMT-ECA before and after heating.DMT-MN showed crystallization solvent dependent tunable fluorescence. As-synthesized powder and crystals grown from ethyl acetate showed fluorescence at 554 nm whereas DCM-EtOH obtained crystals showed fluorescence at 590 nm. However, single crystal analysis did not show any polymorphism and exhibited only slight variation in the molecular conformation. Thus, the present work explored the structure-property of new donor-acceptor molecules and elucidated the impact of molecular structure for achieving thermofluorochromism.