Special Seminar

Name: Dr. Avinash Chettri

Title: Investigating the excited-state dynamics of a ruthenium-based photosensitizer, TLD1433, in biologically relevant environments.

Date & Time: Tuesday, 02nd April at 11.00 a.m.

Venue: Rajarshi Bhattacharya Memorial Lecture Hall, Chemical Sciences Building


The ability of light to initiate photo-chemical reactions is not unknown. The scientific community has embraced this knowledge by designing photo-responsive systems which show promise in the field of energy-storage, therapeutics and controlling bio-chemical reactions, to name a few. In general, the photochemistry of such a system depends on the photo-physical landscape which it adopts upon excitation. These excited-state landscapes can be tuned rationally by changing the chemical entity and / or the local environment to steer the system towards a direction which demands its application thereby highlighting the need tounderstand the electronic properties of the excited-states and thephoto-physical pathways responsible for their dynamics. However these pathwayscould be“ultrafast” i.e., typically occurring within a picosecond (ps) to “ultraslow” i.e., taking several microseconds (µs) for the process to complete. Therefore spectroscopic techniques which utilizes pulsed-laser source such as time-resolved emission and absorption (also known as pump-probe spectroscopy) spectroscopyare often used for investigating these processes. While the former reports on the dynamics of only the emissive states, the later technique is more robust and reports on the dynamics of both the emissive and non-emissive states by recording a difference absorption spectrum between the excited and the ground-state population at different time-delays under “pump-on” and “pump-off” conditions, respectively. Primarily aided by the aforementioned spectroscopic techniques, the talk focusses on unravelling the excited-state dynamics of a molecular photosensitizer (PS)which shows promise in the field of photodynamic therapy (PDT).

PDT is a cancer treatment regimen in which light of a suitable wavelength is used to excite a PS which in turn populates long-lived triplet states. These triplet states have the ability to collide with ground state oxygen and generate reactive oxygen species (ROS) such as singlet oxygen (1O2) which selectively destroys cancerous over non-cancerous cells, making PDT a non-invasive mode of cancer treatment. However due to radiative losses by fluorescence the triplet quantum yield (ФT) is often reduced in organic systems which in turn decreases the quantum yield of 1O2(ФΔ), a parameter which directly effects the PDT efficacy of a PS. To circumvent this issue heavy metal-based transition metal complexes are often used due to ultrafast intersystem crossing from singlet to triplet states with near-unity efficiency. While several studies are devoted towardsunderstanding the excited-state dynamics of PSsin solutionnot much effort is invested indoing the same in cells, which is the environment of interest. Therefore the talk focusses on unravelling the excited-state dynamics of TLD1433, a ruthenium-based transition metal complex appended with a ter-thiophene chain, which is currently under Phase II clinical trial for the treatment of non-muscle invasive bladder cancer. To unravel the dynamics of the PS in cells, a systematic approach is adopted in which the PS’s dynamics is investigated bygradually increasing the complexity of the environment i.e., from water to ct DNA to MCF7 cells.In general, the study highlights the importance of understanding the dynamics of PSs relevant in PDT in heterogeneous systems rather than in homogeneous ones where additional factors which are previously unobserved in the lattersuch as PS aggregation and PS-biomolecule interaction are likely to play a key role in altering the photo-physics of the PS in the former.