Elucidating Intricate Excited-state Dynamics in Molecular Systems Using Time-resolved Transient Absorption and Ultrafast Raman Spectroscopy
Abstract
Abstract
The excited-state dynamics of molecular materials is of great importance to understand early time photodynamics. We have used Femtosecond Transient Absorption (fs-TA) and Ultrafast Raman Loss Spectroscopy (URLS) spectroscopy to understand early time photophysical dynamics in the ultrafast timescale. fs-TA measures the electronic absorption of photoexcited molecule revealing the population dynamics on the complex potential manifold. Whereas, URLS measures the vibrational signature of excited species that depicting the structural dynamics during the evolution. Such ultrafast spectroscopic methods are very essential to unravel the early time events of photophysical and photochemical processes. We performed time-resolved URLS measurements (stimulated Raman scattering based on third-order nonlinear spectroscopic principles) by choosing the actinic pump in the UV-Vis region, Raman pump in the visible region and Raman probe as a broadband white light continuum to understand intricacies related to the excited state dynamics in few interesting systems
Singlet Fission (SF), a spin allowed fast internal conversion process of multiexciton generation from photoexcited singlet 〖(S〗_0) exciton in which a pair of correlated triplet exciton 〖(TT)〗^1 is formed. SF can overcome Shockley-queisser limit and vastly improve photo-energy conversion efficiency. We have studied the SF process in the solution phase for 9,10-Bis(phenylethynyl) anthracene (BPEA) to understand the electronic and structural dynamics in ultrafast time-scale. The extent of electronic coupling between singlet exciton and triplet pair state governs the efficiency of SF process. SF is very efficient for crystalline forms, as a consequence tracking the initial events related to evolution from the initial singlet state to two triplet states is non-trivial due to ultrafast in nature. However, in solution, SF is essentially limited by the diffusion. Here, the dynamics of the BPEA (in solution) in the entangled singlet and multi-excitonic states is elucidated in terms of the intricate structural dynamics, which is otherwise non-trivial from fs-TA measurements alone.
The study of the effect of coupled electronic states for the SF process in BPEA has been studied next. In BPEA, a strong one-photon allowed electronic transition (B_1u) with long axis polarization is overlapped with moderately allowed short-axis polarized electronic transition (B_2u). These two electronic states are coupled via Pseudo Jahn-Teller coupling along with modes having b_3g symmetry. The TA of the excited species is a result of overlapping spectral signatures from both hot-multiexcitionic and moderately allowed states. The photoexcitation of BPEA in n-hexane (HX) at different wavelengths across the steady-state absorption spectrum exhibits a distinct intensity ratio of these two transient absorption bands although their normalized intensity kinetics show similar behaviour. However, with URLS, it is shown that the modes that are involved in the Pseudo-Jahn-Teller coupling exhibit distinct response as a consequence of quantum interference of the Raman polarizabilities related to such states. We have performed an extensive quantum mechanical calculation by utilizing Gaussian 09 software to understand the experimentally observed absorption, vibrational spectra by means of orbital calculation, orbital symmetry, oscillator strength calculation, vibrational frequency calculation, energy level diagram along specific coordinates. The corroboration of quantum calculation with experimentally observed data provides a deeper understanding of the photodynamics particularly in the excited state analysis.
Next, we have investigated the excess energy dissipation and isomerization process in excited t-stilbene in the condensed phase. Upon photoexcitation, the excess energy is deposited in the initially prepared Franck-Condon (FC) excited state. However, such energy is redistributed among intramolecular modes and subsequently dissipated to the solvent molecules through anharmonic coupling and solute-solvent coupling respectively. The energy dissipation process can be described by two exchange parameters i.e., backward exchange rate (W_1) and forward exchange rate (W_2) that are related to the thermally generated exciton in the bath and the low-frequency solute modes respectively. Here, the influence of alkane chain length of solvents on the exchange rates and associated mode coupling is elucidated in terms of the W_2/W_1 ratio.
Finally, a detailed analytical calculation has been performed to compute the third- order nonlinear susceptibility that contributes to the URLS signatures. It also depicts the method of identifying the relevant quantum processes using closed path time loop (CPTL) Feynman diagrams. Here, it is clearly demonstrated the underlying reasons for asymmetry in the intensity between gain (Stokes) and loss (anti-Stokes) signatures of URLS process. A model simulation has been carried out with different molecular parameters.