System-bath interactions in molecules and molecular assemblies

This project is related to the following Fields, Subjects and Techniques:

A chemical reaction involves motion along a specific path in the potential energy landscape. Completing the reaction usually involves passing reaction barrier, the height of which largely determines the rate of the reaction. The solvent molecules surrounding a solute are essential in determining the dynamics of the system and the motions of the solvent provide additional degrees of freedom allowing the solute to overcome reaction barriers, dissipate energy and reorganise its electronic structure. The solvent can also be very effective in preventing a specific motion of the solute that is necessary to complete the reaction (by altering the friction experienced by certain molecular motions, for example in isomerization reactions). The solvent thus acts as a bath mediating solute dynamics that are not possible in the gas phase. This very fundamental interaction between molecules has a strong impact on chemical reactions as well as relaxation and transport processes and is responsible for the richness of condensed phase dynamics.

Probing and characterising the motions of the bath is thus important in order to understand the dynamics in molecular system. The motions of the molecules in a liquid are very complex and take place on a wide range of timescales and broad scale of amplitudes. Fluctuations of the position and orientation of the solvent molecules will affect the electronic transition frequency of a solute through electrostatic interactions. Probing the time-evolution of the transition frequency of the solute thus offers insight into the underlying dynamics of the bath. The autocorrelation of the fluctuation of the transition frequency of the system, C(t), reflects the timescales and amplitude of the liquid motions and it is at the heart of the theory of condensed phase dynamics. This function and its Fourier transform counterpart, the spectral density, is of central importance to understand the dynamics of molecules in the condensed phase.

The dynamical modulation of the transition frequency results in line broadening in the frequency domain. The environment (bath) may consist of both intra- and intermolecular nuclear motions and from point of optical spectroscopy no distinction between the different nuclear motions is not necessary. On the other hand, partitioning the response of the environment into solute and solvent properties is desirable since it separates out the pure properties of the liquid. . However, this distinction is not always straight forwardly obtained from the experimental data, especially at short times. Obtaining the dynamical information about the bath motions from a linear absorption spectra is hopeless due to the presence of slow fluctuations of the transition frequency. The static (or slow) contributions scramble the dynamical information and techniques that directly can project out the different timescales of the fluctuations are needed.Two such techniques are Fluorescence Line narrowing and Three pulse photon echo . Both these techniques are capable of removing the inhomogeneous fluctuations of the transition frequency and reveal the true dynamical interaction between the electronic subsystem and the environment.

This project aims to investigate system bath interaction and their influence on the dynamics on systems ranging from dye molecules in solution to the dynamics in larger molecular aggregates with photosynthetic function. The recent work in progress has mainly been concerned with solvation and analysing the contribution of the decay of the correlation function in terms of intra- and intermolecular nuclear motions. One of the directions currently perused is the role of solute and solvent dependence in polar solvation dynamics.

Another project concerns the intermolecular vibrations often observed in the peak shift. The direct comparison of 3PEPS and FLN enables a direct comparison of the vibrational modes that contribute to the measured correlation functions. These results will be coupled to quantum chemistry calculations (hyperlink) to determine the predictive power of the ability to directly calculate the strength of the modes and the vibrational frequencies of medium sized molecules.