Research Highlight4

Boltzmann distribution is the distribution of particles among various energy levels at thermal equilibrium, to which all systems attempts to reach when left to evolve. For example, when a molecule transitions to an electronic excited state by absorbing light, it can relax back to the ground state apparently without any agent causing the de-excitation. However, as each state is generally a stationary state, transition from an excited state to the ground state requires a suitable perturbation that is as important as the perturbation causing the excitation process. Any non-Boltzmann distribution while evolving towards Boltzmann distribution experiences perturbations that drive the evolution. Our research on Spin Dynamics attempts to understand mechanistic details that govern the generation electron spin systems in non-Boltzmann distribution and their evolution to Boltzmann distribution governed by the perturbing electron spin-lattice relaxation processes.

Very often, the primary photochemical event results in the formation of paramagnetic species, such as triplet states or free radicals with their spin distribution deviating from Boltzmann distribution. Their spins are said to be polarised. There are very small but important magnetic interactions that play crucial roles in controlling the dynamics of such species.

An example in photochemistry (spectra on left in figure): When acetone is photolyzed in 2-propanol by a steady-state UV light, the EPR spectrum of the photogenerated radical (CH3)2COH shows the usual absorptive (A) signal with characteristic hyperfine lines, whose relative intensities are governed by the degeneracy of the nuclear spin arrangements. But if a pulsed laser is used as the photolyzing source, and the EPR spectrum of the radical is recorded immediately after the laser pulse in a time-resolved mode, the EPR spectrum of the radical shows anomalous intensities in the hyperfine lines. Some lines appear in emission (E), some in enhanced absorption.

An example in photophysics (spectra on right in figure): Excited states are often quenched by free radicals through energy transfer mechanism without involvement of any chemical reaction. When the stable radical TEMPO quenches an excited naphthalene triplet state, the EPR spectrum of the TEMPO radical, which normally gives the usual absorptive (A) steady-state EPR signal, gives emissive (E) EPR signal post quenching due to spin selective interactions.

Such remarkable observation of anomalous EPR signals indicates that there are very specific interactions, coming into play in the presence of light, that drive the spins from the usual Boltzmann distribution to a non-Boltzmann one, and can even cause an inversion of spin population. Occurrence of such non-Boltzmann distributions carries rich information on the ultra-fast photophysical and photochemical processes. Magnetic resonance spectroscopy, such as EPR and NMR, is an aptly suitable technique to study spin dynamics in photochemical systems, and gain important insights into the unique intra- and intermolecular interactions which are not observable by any other spectroscopic techniques.

Within the Chemical Physics and Dynamics group we use time-resolved EPR spectroscopy, nano- and pico-second laser flash photolysis techniques and time-resolved fluorescence to study primary events of a photochemical reaction and the associated spin dynamics, in homogeneous solutions and in confined systems, where molecular motions are restricted. We aim to understand the interactions giving rise to spin-selective pathways, and how spatial restrictions govern the evolution. More information about our methods and activities in this area can be found here.