title: Enhanced fluorescence of [[5'-(4-hydroxyphenyl)[2,2'-bithiophen]-5-yl]methylene]-propanedinitrile (NIAD-4): solvation induced micro-viscosity enhancement.
abstract: Excited state solvation plays a very important role in modulating the emission behavior of fluorophores upon excitation. Here, the solvation effects on the local micro-environment around a fluorophore are proposed by investigating the fantastic emission behavior of a novel amyloid fibril marker, NIAD-4, in different alcoholic and aprotic solvents. In alcoholic solvents, high solvent viscosity causes an obvious enhancement of fluorescence because of the restriction of torsion of NIAD-4, where the formation of a non-fluorescent twist intramolecular charge transfer (TICT) state is suppressed. In aprotic solvents, high solvent polarity leads to a remarkable redshift of the emission spectra suggesting strong solvation. Surprisingly, an abnormal fluorescence enhancement of NIAD-4 is observed with increasing solvent polarity of the aprotic solvents, whereas solvent viscosity plays little role in influencing the fluorescence intensity. We conclude that such an abnormal phenomenon is originated from a solvation induced micro-viscosity enhancement around the fluorophore upon excitation which restricts the torsion of NIAD-4. Femtosecond transient absorption results further prove such a micro-viscosity increasing mechanism. We believe that this solvation induced micro-viscosity enhancement effect on fluorescence could widely exist for most donor-π-acceptor (D-π-A) compounds in polar solvents, which should be carefully taken into consideration when probing the micro-viscosity in polar environments, especially in complex bioenvironments.
authors.affiliation: Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. andong@iccas.ac.cn guoqj@iccas.ac.cn.
authors.forename: Jiangpu
authors.initials: J
authors.lastname: Hu
authors.forename: Huaning
authors.initials: H
authors.lastname: Zhu
authors.forename: Yang
authors.initials: Y
authors.lastname: Li
authors.forename: Xian
authors.initials: X
authors.lastname: Wang
authors.forename: Renjun
authors.initials: R
authors.lastname: Ma
authors.forename: Qianjin
authors.initials: Q
authors.lastname: Guo
authors.forename: Andong
authors.initials: A
authors.lastname: Xia