Evaluation of electron or charge transfer processes between chromenylium-based fluorophores and protonated–deprotonated aniline

Abstract

Three functional dyes based on a chromenylium skeleton were prepared. The chromenylium-based fluorophores were regarded as acceptor parts and the aniline served as donor parts; their pH-dependent fluorescent responses were used to evaluate the photoinduced electron transfer (PET) or intramolecular charge transfer (ICT) processes between the chromenylium-based fluorophores and aniline. Two chromenylium–indole hybrid functional dyes (1a–b) respectively showed a fluorescence enhancement and decrease with increasing acidity, while chromenylium–coumarin hybrid dye (1c) gave an OFF–ON response towards a gradually decreasing pH. The three dyes gave emissions at 675–850 nm when they were excited at 650 nm, and the calculated pK_a values of 1a–c are 4.13, 3.15 and 2.48, respectively. The optical responses were also illustrated by (TD)DFT calculation; the OFF–ON emission of dye 1c was mainly controlled by the PET process, and both PET and ICT processes were found between the aniline parts and the chromenylium–indole parts in dyes 1a–b.