Environment-sensitive behavior of fluorescent molecular rotors

Mark A Haidekker,Emmanuel A Theodorakis

doi:10.1186/1754-1611-4-11

Abstract

Molecular rotors are a group of fluorescent molecules that form twisted intramolecular charge transfer (TICT) states upon photoexcitation. When intramolecular twisting occurs, the molecular rotor returns to the ground state either by emission of a red-shifted emission band or by nonradiative relaxation. The emission properties are strongly solvent-dependent, and the solvent viscosity is the primary determinant of the fluorescent quantum yield from the planar (non-twisted) conformation. This viscosity-sensitive behavior gives rise to applications in, for example, fluid mechanics, polymer chemistry, cell physiology, and the food sciences. However, the relationship between bulk viscosity and the molecular-scale interaction of a molecular rotor with its environment are not fully understood. This review presents the pertinent theories of the rotor-solvent interaction on the molecular level and how this interaction leads to the viscosity-sensitive behavior. Furthermore, current applications of molecular rotors as microviscosity sensors are reviewed, and engineering aspects are presented on how measurement accuracy and precision can be improved.

Highlights

The term molecular rotor is commonly used to describe a fluorescent molecule that has the ability to undergo an intramolecular twisting motion in the fluorescent excited state
Relaxation from the twisted intramolecular charge transfer (TICT) state occurs in one of two ways: In the case of DMABN, the S1 - S0 energy gap in the twisted state is large enough to allow photon emission when the molecule returns to the ground state in a twisted conformation
When the TICT energy gap is much smaller than the locally excited (LE) energy gap, nonradiative relaxation occurs from the TICT conformation

Summary

Introduction

The term molecular rotor is commonly used to describe a fluorescent molecule that has the ability to undergo an intramolecular twisting motion in the fluorescent excited state. In the case of molecular rotors that exhibit nonradiative relaxation from the twisted state, the fluorescent quantum yield increases in higher-viscosity solvents [15]. It is noteworthy that the rigorous derivation by Förster and Hoffmann - under the assumption of rotational friction according to the DSE model - and the more empirical derivation by Loutfy and Arnold under the assumption of a power-law microfriction behavior - lead to the same relationship between quantum yield and bulk viscosity. When the intramolecular rotation rate becomes very high in solvents of low viscosity, additional effects, such as hydrogen bond formation, excimer formation, and polar-polar interaction are no longer negligible and cause significant deviations from established models that describe the relationship between quantum yield and viscosity.

Conclusion

19. McMillan DE

30. Loutfy RO

44. Lukac S

Findings

53. Guilbault GG