Diffusive torsional dynamics of malachite green molecules in solid matrices probed by fluorescence decay

Abstract

The torsional dynamics of phenyl rings of malachite green molecules in the excited state is studied in polymeric and monomeric glass matrices by measuring the fluorescence decay time as a function of temperature. It is shown that the phenyl rings rotate diffusively in solid polymers (polymethyl methacrylate and polyvinyl alcohol) quite rapidly even at low temperatures. To analyze the experimental results, we used the concept of microviscosity which controls the diffusive rotational motion of phenyl rings of malachite green molecules in solid matrices. By using the reaction-rate theory, we show that a horizontal excited-state potential surface rather than a downhill potential surface for the rotation of phenyl rings can more reasonably explain the rotational motion in polymers. If we assume that the potential is horizontal, the temperature dependence of the microviscosity can be described by Andrade equation with a definite activation energy which is known to be valid for many liquids over a wide range of temperatures. This implies that the microscopic dynamics of small molecular rotations in a solid polymer resembles the behavior in many liquids. By monitoring the fluorescence decay of malachite green molecules doped in ethanol monomeric glass during its phase transition, we show that the effects of phase transition are well represented in the fluorescence decay time. We then propose to use malachite green molecules as sensitive optical microprobes of local dynamics in various solid matrices and their phase transitions, etc.

Microscopic dynamics of the glass transition investigated by time-resolved fluorescence measurements of doped chromophores

“…The fact that the nonradiative decay time of MG in the glassy samples is of Arrhenius-type temperature dependence is consistent with our previous studies. 33,34 Dhinojwala et al also found that below T g the rotational reorientation time of disperse red 1 in polystyrene shows an Arrhenius temperature dependence. 31 Below T g , the thermally activated hopping stops and molecules are frozen in a certain configuration so that they cannot move around.…”

“…It is well known that the internal conversion at the sink is very fast for MG and the diffusive rotational motion is the rate-limiting process; then, the nonradiative decay time of MG is proportional to the microscopic viscosity of the surrounding solvent or host matrix molecules. 18,[32][33][34] Furthermore, since the internal potential for the phenyl-ring rotation is known to be barrierless and flat, and the required amount of rotation for the internal conversion to occur is very small, about 10°, 32 the dynamics of solvents in the supercooled state can be probed in the mesoscopic time window of 10 Ϫ11 -10 Ϫ8 s and on a length scale of several angstroms. 18 This feature is quite different from most of other molecular probes.…”

“…It is known that the rotational motion of the three phenyl rings around the central carbon bonds is the rate-limiting process of the decay of the excited state S 1 , and the fluorescence lifetime varies by several orders of magnitude with the viscosity of the solvent or host matrix. 18,[32][33][34] The internal potential for the rotational motion of the phenyl rings has been shown to be barrierless and flat when a MG molecule is in the S 1 excited state. [32][33][34] This special nature makes MG a very sensitive molecular probe, since the friction given by the surrounding molecules is the only impediment to the rotational motion of the phenyl rings.…”

“…18,[32][33][34] The internal potential for the rotational motion of the phenyl rings has been shown to be barrierless and flat when a MG molecule is in the S 1 excited state. [32][33][34] This special nature makes MG a very sensitive molecular probe, since the friction given by the surrounding molecules is the only impediment to the rotational motion of the phenyl rings. Furthermore, the required rotational angle of the phenyl rings for the internal conversion from the S 1 excited state to the ground state is very small ͑about 10°͒, which assures MG to serve as a sensitive probe on a microscopic length scale, i.e., on the order of several angstroms.…”

Functional Assembly of Aptamer Binding Sites by Single-Molecule Cut-and-Paste

“…12,13 An increase in the quantum yield up to several orders of magnitude upon binding of this molecule to either BSA films, 14 specific antibody binding pockets 15 or suitable RNA structures 16,17 was reported in the literature. Particularly interesting appears the work by Kolpashchikov 18 where it was shown that a MGaptamers may be split in two and used as DNA sensors with single mismatch resolution.…”