Research Projects

Photoinduced excited-state processes such as intersystem crossing (ISC) and electron transfer (ET) are central to solar energy capture, optoelectronics, photosensitization, and quantum information technologies. The traditional theories cannot satisfactorily explain the ultrafast (sub-100 fs) dynamics, highlighting the requirement for alternative mechanistic frameworks. For example, while spin–orbit coupling (SOC) is considered to drive the ISC, it can not explain the driving force for ISC in many systems where SOC value is low. Further, ballistic ET processes cannot be explained by conventional ET theories and are suggested to occur beyond the Born–Oppenheimer approximation. Excited-state reactions have been viewed as progressing along a single reaction coordinate formed by the collective motion of many nuclear degrees of freedom. However, recent evidence shows that specific intramolecular vibrational modes can act as additional reaction coordinates to control the excited-state reactivity. However, experimental validation of vibration-assisted mechanisms remains limited and underexplored. This proposal aims to bridge this gap by integrating ultrafast spectroscopy with computational simulations to directly identify the vibrational modes governing excited-state reactivity. The primary objectives are: (i) investigating vibration-assisted ISC in heavy-atom-substituted positional isomers, (ii) exploring ISC driven by vibrations in heavy-atom-free molecules, and (iii) understanding vibrational influence on photoinduced electron transfer. For heavy-atom-substituted molecules, the project will analyze how positional isomerism affects ISC efficiency beyond SOC contributions, using synthesized halocoumarin derivatives as models. For heavy-atom-free systems, the focus is on porphyrins where ISC is formally forbidden yet occurs ultrafast, possibly driven by out-of-plane distortions. Temperature-dependent ISC measurements, UV-Vis/IR co-excitation experiments, and structural rigidification studies are planned to validate this mechanism. For ET processes, ultrafast electron transfer in ferrocenium–bridge–acceptor complexes will be examined by modifying bridging bonds to reveal vibrational contributions to ballistic ET on femtosecond timescales. The methodology combines steady-state absorption and emission spectroscopy, femtosecond transient absorption, fluorescence up-conversion, UV-vis/IR co-excitation experiments, TDDFT calculations of excited-state vibrations, and potential energy surface analysis. The innovative UV-vis/IR co-excitation setup will allow direct probing of vibration-assisted ISC by comparing fluorescence yield with and without IR excitation. Such approaches will provide unambiguous experimental evidence for spin-vibronic coupling. The experimental identification of active vibrations that drive the ISC/ET process in the excited state manifold makes this project unique and significant. The outcomes of this project are expected to provide essential knowledge for the predictive design of triplet photosensitizers, efficient TADF emitters, and materials for photodynamic therapy/OLEDs/solar energy harvesting. This work will establish a generalized framework for vibration-driven excited-state reactivity. The results of this research will provide predictive design strategies for developing highly efficient triplet photosensitizers, TADF emitters, and photostable materials. The project will be useful to train young researchers in advanced ultrafast techniques, contributing to India's scientific workforce. Dissemination is planned through high-impact publications and conferences. Overall, the project promises fundamental advances with far-reaching technological and societal benefits.