Design strategy of infrared 4-hydroxybenzylidene-imidazolinone-type chromophores based on intramolecular charge transfer: A theoretical perspective

Abstract

Partial genetically encoded 4-hydroxybenzylidene-imidazolinone (HBI)-type chromophores are new promising fluorescent probes, which are suitable for imaging and detection of living cells. However, the lack of infrared chromophores hinders the development seriously. Here more than 30 HBI-type chromophores with regular structure modifications were employed and typical spectral redshift change laws and mechanisms were investigated by quantum methods. Results show that both one-photon spectrum (OPS, absorption/emission) and two-photon absorption (TPA) can achieve large redshift via either extending conjugated lengths of frag-3 or enlarging conjugated areas of frag-1 of HBI skeleton. Spectral redshifts of all chromophores are highly related to intramolecular charge transfer (ICT), but neutral ones are closely related to the total ICT or electron-accepting-numbers of frag-3, and the high correlative factor of anions is the aromaticity of frag-2 bridge. The frag-2 bridge with high aromaticity can open a reverse charge transfer channel in anion relative to neutral, obtaining significant redshift. Based on analysis, a new 6-hydroxyl-naphthalene-imidazolinone (HNI) series, which have larger conjugated area in frag-1, are predicted. The OPS and TPA of anionic HNI ones acquire about 76–96 nm and 119–146 nm red-shift relative to traditional HBI series respectively as a whole. The longest emission of anionic HNI-4 realizes more 244 nm redshift relative to HBI-1. Our work clarifies worthy spectral regularities and redshift mechanisms of HBI-type chromophores and provides valuable design strategy for infrared chromophores synthesis in experiment.