What is it about?

An organic molecule is like a microscopic detector receiver with oscillating length of inductance coil. It is a radiotechnical analogue of a vibronic π-conjugated system of this molecule. The resonant LC-circuit absorbs electromagnetic waves at a frequency modulated by parametric oscillation of the coil length. It can be used in molecular electronics and optical chemosensorics to determine the properties of surrounding media by analyzing the shape of the vibronic spectral lines of fluorescent molecular probes. It was demonstrated that an organic molecule with extended π-electron system can be considered to be a molecular optical parametric oscillator.

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Why is it important?

The shape of rovibronic absorption and the emission bands are determined not only by the statistical distribution of the ensemble of molecules in the solution over their energy levels (inhomogeneous broadening) but also by the fact that each individual rovibronic transition of each molecule has its own wide spectrum due to zero-point oscillations and thermal fluctuations. In this work, for the first time, it was shown theoretically that although the magnitude of the broadening of the individual rovibronic transition energy spectrum in the molecule is smaller than a rovibronic bandwidth, it has the same order of magnitude. Notwithstanding, until now, it was conventionally assumed that the homogeneous broadening is several orders of magnitude less than the inhomogeneous one. Numerous computations of the spectra of the molecules are performed by special methods for smoothing a series of individual transitions represented by delta functions. All these methods have one thing in common. Such methods smooth out the quasi-continuum of closely spaced vibronic transitions, approximately calculating the common envelope of all vibronic transitions, and thus form a continuous spectral band. They do not work with the broadening of single rovibronic transition. In the framework of harmonic approximation of the potential energy surfaces, the new theory is proposed here describes the optical spectrum broadening of any single rovibronic transition due to zero-point oscillations and thermal fluctuations around equilibrium positions. Franck–Condon diagrams with slanting equidistant vibrational levels are proposed to consider this effect.

Perspectives

I hope this article demonstrates how the fundamental principles of quantum mechanics act in a molecular system. They manifest in the connection of the light emitted and absorbed by the molecule with its vibrations. This makes it possible to design molecular devices of a new type.

Peter Lebedev-Stepanov
A.V. Shubnikov Institute of Crystallography, FSRC “Crystallography and Photonics” RAS

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This page is a summary of: Parametric broadening of the molecular vibronic band due to zero-point oscillations and thermal fluctuations of interatomic bonds, AIP Advances, March 2021, American Institute of Physics, DOI: 10.1063/5.0047044.
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