Extreme Fluorescence Sensitivity of Some Aniline Derivatives to Aqueous and Nonaqueous Environments:  Mechanistic Study and Its Implication as a Fluorescent Probe

Abstract

Effects of solvent water on the photophysical properties of a series of meta- and para-substituted anilines have been investigated by means of time-resolved fluorescence, transient absorption, and photoacoustic measurements. Some aniline derivatives exhibit extremely short fluorescence lifetime (tau(f)) and small quantum yield (Phi(f)) in water (e.g., tau(f) = 45 ps and Phi(f) = 0.0019 for m-cyanoaniline (m-ANCN) in H(2)O), which is in marked contrast with their much larger values in nonaqueous solvents (tau(f) = 7.3 ns and Phi(f) = 0.14 for m-ANCN in acetonitrile). Photoacoustic and transient absorption measurements show that the remarkable fluorescence quenching of m-ANCN in water is attributed almost exclusively to fast internal conversion. The lifetime measurements of m-ANCN in H(2)O/acetonitrile binary solvent mixtures reveal that the quenching is related to variation of hydrogen-bonding interactions between the amino group and water molecules and the conformational change of the amino group upon electronic excitation. Similar fluorescence quenching due to solvent water is also found for N-alkylated m-ANCNs. The drastic differences in the fluorescence intensity and lifetime of m-ANCNs under hydrophobic and hydrophilic environments and also the large solvent polarity dependence of the fluorescence band position suggest the possibility that they can be utilized as fluorescent probes for investigating the microenvironment of biological systems. In suspensions of human serum albumin (HSA) in water, remarkable enhancement of the fluorescence intensity and lifetime is observed for m-ANCN and its N-alkylated derivatives, demonstrating that m-ANCNs can be a candidate for novel fluorescent probe with small molecular size.

Photophysics of a β-carboline based non-ionic probe in anionic and zwitterionic liposome membranes

“…Being sensitive to the environment and excited state interactions, fluorescence lifetime serves as an indicator to explore the environment around the fluorophore (Prendergast, 1991;Balabai et al, 1998;Chattopadhyay et al, 2002;Seth et al, 2005;Oshima et al, 2006). It also provides valuable information relating to the binding of the probe within the vesicles (Sujatha and Mishra, 1998;Seth et al, 2005).…”

Efficient Singlet‐State Deactivation of Cyano‐Substituted Indolines in Protic Solvents via CNHO Hydrogen Bonds

“…This trend was identical to that previously found for the substituted anilines. [9] The deuterium effect was consistent with the involvement of a hydrogen-bond interaction. There was little difference between the measured lifetimes and quantum yields of CI and CMI.…”

“…Tobita et al performed a systematic study of the photophysical properties of meta-and para-substituted anilines in various solvents. [9] Again, it turned out that the fluorescence lifetimes of all the studied derivatives were markedly reduced in aqueous solution. Based on their results and on MO calculations, Tobita attributed the quenching behavior to hydrogen-bond interactions between the amino nitrogen and water molecules.…”

“…This hypothesis will be corroborated below based on quantum-chemical calculations. However, since strong quenching effects have previously been found in aqueous solutions for other indoline [6] or aniline [9] derivatives as well, we first turn to a thorough analysis of the photophysical and photochemical behavior in this medium as well as in water-based mixed solvents. The fluorescence lifetimes of I and MI are, indeed, significantly reduced with respect to the organic solvents (Table 1), but the lifetimes of the cyanated derivatives are a further order of magnitude shorter.…”

“…This is at variance with the behaviour of N,N-dimethylcyanoanilines, which exhibit longer fluorescence lifetimes and smaller overall (intersystem crossing plus internal conversion) nonradiative deactivation rate constants than their non-methylated counterparts, an observation that led Tobita et al to the conclusion that hydrogen bonding at the amino nitrogen is crucial for the quenching effect in aqueous solution. [9] The triplet-state yields could not be reliably measured in aqueous solutions because the triplet-triplet absorptions were very small and overlapped with the absorptions of radical ions originating from the (one-or two-photon) ionization process discussed below. An estimation based on the pulse energy dependences at the wavelength of maximal T-T absorption suggested that the quantum yields of triplet formation of aqueous CI and CMI do not exceed 0.03.…”