Coppola, Federico (2020) Development of theoretical-computational methods for the study of photophysics and photoreactivity occurring far from the Franck-Condon region. [Tesi di dottorato]

Preview

Text FCoppola_XXXII_ScienzeChimiche.pdf Download (31MB) | Preview

Collection description

The main vibrational spectroscopy techniques are based on the processes of infrared radiation absorption and Raman scattering, which are daily used by chemist to provide deep information on chemical structures and physical phases of matter even in complex chemical environment. Laser technology has contributed greatly to the evolution of spectroscopy, introducing even the time domain (from as to ps) associated with the frequency domain increasing the resolution and overcoming some limitations of ordinary radiative sources allowing the observation of ultrafast and transient phenomena in real time. Such advanced time resolved and non-linear vibrational spectroscopies such as, i.e., Femtosecond Stimulated Raman spectroscopy and Transient absorption which are part of the present work, can give further insight at atomistic/ molecular level on very short time scale about excited state structural dynamics, proton and electron transfer reactions, isomerizations and so on. The interpretation of vibrational spectra and the subtle relationships between the experimental data and the molecular dynamics can be very challenging to unravel because of numerous concomitant effects (ultrafast transient phenomena, broadening, vibrational couplings, complex matrices) for this reason it is source of debate. In this regard, the atomistic-level description provided by a reliable and careful theoretical-computational approach can be a valuable aid in interpretation, prediction and further clarification of experimental evidence. The aim pursued in this PhD project is to study the pivotal phenomena behind photophysical and reactive events affecting photoactive molecules in condensed phase, using a reliable and accurate theoretical-computational methodology. In this work we focus on photoinduced charge transfer reactions of two non-covalent p-stacked dimers, which have been recently characterized by means of Femtosecond Stimulated Raman Spectroscopy that show a complex and intriguing photophysical behavior both in the ground and excited states. In order to unravel the vibrational dynamics of key vibrational modes and relaxation processes occuring far from Franck-Condon region we propose a non-canonical theoretical approach based on the Wavelet Analysis of suitable time dependent signals obtained through ab-initio molecular dynamics simulations, within methods rooted in Density Functional Theory (DFT) and it’s time dependent version TD-DFT - as the best compromise between accuracy and computational demands - within the adiabatic approximation for the ground and electronic excited state description. Our aim is to know what happens on the ultrafast time scale of electronic excited states translating the inherently anharmonic nuclear motion in terms of vibrational couplings and relaxation processes along with the time domain. We also linked vibrational fingerprints to electronic properties, as ultimate goal. We were able to careful assign the ground and excited state vibrational frequencies, to quantify the anharmonic vibrational couplings among those vibrational modes that play a key role in the photophysics at stake. By analyzing the temporal evolution of some selected structural parameters, we have identified the vibrational modes that most affect the adiabaticity between the electronic states involved contributing to the activation of non-radiative relaxation channels of the entire charge transfer complex from the excited to the ground state. This PhD project involved a period of study and training abroad: at the University of Minnesota under the guidance of Prof. R.R. Frontiera we investigated a recently synthetized electron acceptor, which presumably undergoes to Singlet Fission when photoexcited. We used Transient Absorption and Femtosecond Stimulated Raman microscopy to unveil the structural dynamics and identify the vibrational fingerprint probes of the formation and separation of triplets in crystalline phase. Along with this experimental study, a suitable model and computational method is being developed that can provide an atomistic description and further insights. In conclusion, the study of transient and far from equilibrium processes, which represent an emerging challenge in the field of physical chemistry, is the cornerstone for a deep knowledge of excited state chemistry which can be monitored and handled to pave the way for a rational design of more performing photovoltaic, efficient light harvesting and sensor devices.

Downloads

Actions (login required)

View Item