Probing the effect of solvation on photoexcited 2-(2′-hydroxyphenyl)benzothiazole via ultrafast Raman loss spectroscopic studies Surajit Kayal, Khokan Roy, Y. Adithya Lakshmanna, Siva Umapathy Journal of Chemical Physics, 2018 2-(2'-Hydroxyphenyl)benzothiazole (HBT) molecule is known to exhibit efficient excited state intramolecular proton transfer. As a consequence, it shows fluorescence with a large Stokes shift (∼10 000 cm-1) in non-polar solvents. However, fluorescence in polar solvents has a dual-band which corresponds to the emission from both the enol* and the keto* forms. Also, the excited state lifetime significantly varies with the solvent polarity. Recently, Mohammed et al. [J. Phys. Chem. A 115, 7550 (2011)] have shown that the excited state of HBT in acetonitrile (ACN) relaxes back to its ground electronic state through two competitive decay pathways, i.e., intramolecular proton transfer and intramolecular twisting between hydroxyphenyl and benzothiazole units in contrast to its behavior when it is in tetrachloroethene, a non-polar solvent. Here, by following the time-evolution of vibrational features of excited state HBT in ACN through ultrafast Raman loss spectroscopy, we demonstrate a direct evidence for the involvement of torsional motion leading to an ultrashort lifetime of HBT. The time evolution of the C7-N vibrational frequency exhibited a red-shift in its peak position, clearly indicating the evolution of the initially planar cis-keto* form to the more twisted keto* form. Density functional theory calculations also well corroborate the experimental findings. Furthermore, wavepacket analysis of this mode reveals a strong correlation with the torsional motion in ACN.
Ultrafast Raman Loss Spectroscopy Unravels the Dynamics in Entangled Singlet and Triplet States in Thioxanthone Surajit Kayal, Khokan Roy, Yapamanu Adithya Lakshmanna, Siva Umapathy Journal of Physical Chemistry A, 2018 Thioxanthone (TX), an aromatic ketone, exhibits significant solvent-dependent photophysical properties. Herein, we employed time-resolved ultrafast Raman loss spectroscopy (URLS) to decipher the solvent-dependent structural dynamics in entangled singlet and triplet states of photoexcited TX. The evolution of the vibrational spectrum reveals structural changes that occur during the intersystem-crossing (ISC) process and the subsequent energy dissipation to the surrounding solvent. The C═O stretch (∼1320 cm-1) of TX in the excited state acts as the marker band as it undergoes a red shift with time constants of ∼45 and ∼5 ps in acetonitrile and methanol, respectively. Such a red shift is an indicator of the softening of the bond due to the change in the electronic spin states. We also observed a blue shift in Raman frequencies corresponding to the C═C stretch and the C═O stretching modes of TX in acetonitrile and methanol, indicating vibrational cooling in the excited singlet and triplet states. In the case of TX in cyclohexane, vibrational modes at 190 and 415 cm-1 exhibit a blue shift with a time constant of ∼700 fs, which represents the structural distortion during internal conversion (S2 → S1) process. The kinetics of amplitudes of these modes follows biexponential growth with time constants of ∼3 and ∼14 ps representing the time scales for the ISC process and the planarization process in the triplet state, respectively. The URLS study therefore provides a direct measure of the various stages of the solvent-dependent structural dynamics in the excited state of TX.
Femtosecond coherent nuclear dynamics of excited tetraphenylethylene: Ultrafast transient absorption and ultrafast Raman loss spectroscopic studies Surajit Kayal, Khokan Roy, Siva Umapathy Journal of Chemical Physics, 2018 Ultrafast torsional dynamics plays an important role in the photoinduced excited state dynamics. Tetraphenylethylene (TPE), a model system for the molecular motor, executes interesting torsional dynamics upon photoexcitation. The photoreaction of TPE involves ultrafast internal conversion via a nearly planar intermediate state (relaxed state) that further leads to a twisted zwitterionic state. Here, we report the photoinduced structural dynamics of excited TPE during the course of photoisomerization in the condensed phase by ultrafast Raman loss (URLS) and femtosecond transient absorption (TA) spectroscopy. TA measurements on the S1 state reveal step-wise population relaxation from the Franck-Condon (FC) state → relaxed state → twisted state, while the URLS study provides insights on the vibrational dynamics during the course of the reaction. The TA spectral dynamics and vibrational Raman amplitudes within 1 ps reveal vibrational wave packet propagating from the FC state to the relaxed state. Fourier transformation of this oscillation leads to a ∼130 cm-1 low-frequency phenyl torsional mode. Two vibrational marker bands, Cet=Cet stretching (∼1512 cm-1) and Cph=Cph stretching (∼1584 cm-1) modes, appear immediately after photoexcitation in the URLS spectra. The initial red-shift of the Cph=Cph stretching mode with a time constant of ∼400 fs (in butyronitrile) is assigned to the rate of planarization of excited TPE. In addition, the Cet=Cet stretching mode shows initial blue-shift within 1 ps followed by frequency red-shift, suggesting that on the sub-picosecond time scale, structural relaxation is dominated by phenyl torsion rather than the central Cet=Cet twist. Furthermore, the effect of the solvent on the structural dynamics is discussed in the context of ultrafast nuclear dynamics and solute-solvent coupling.
Understanding Ultrafast Dynamics of Conformation Specific Photo-Excitation: A Femtosecond Transient Absorption and Ultrafast Raman Loss Study Khokan Roy, Surajit Kayal, Venkatraman Ravi Kumar, Andrew Beeby, Freek Ariese, Siva Umapathy Journal of Physical Chemistry A, 2017 Excited state ultrafast conformational reorganization is recognized as an important phenomenon that facilitates light-induced functions of many molecular systems. This report describes the femtosecond and picosecond conformational relaxation dynamics of middle-ring and terminal ring twisted conformers of the acetylene π-conjugated system bis(phenylethynyl)benzene, a model system for molecular wires. Through excitation wavelength dependent, femtosecond-transient absorption measurements, we found that the middle-ring and terminal ring twisted conformers relax at femtosecond (400-600 fs) and picosecond (20-24 ps) time scales, respectively. Actinic pumping into the red flank of the absorption spectrum leads to excitation of primarily planar conformers, and results in very different excited state dynamics. In addition, ultrafast Raman loss spectroscopic studies revealed the vibrational mode dependent relaxation dynamics for different excitation wavelengths. To corroborate our experimental findings, DFT and time-dependent DFT calculations were carried out. The Franck-Condon simulation indicated that the vibronic structure observed in the electronic absorption and the fluorescence spectra are due to progressions and combinations of several vibrational modes corresponding to the phenyl ring and the acetylenic groups. Furthermore, the middle ring torsional rotation matches the room-temperature electronic absorption, in stark contrast to the terminal ring torsional rotation. Finally, we show that the middle-ring twisted conformer undergoes femtosecond torsional planarization dynamic, whereas the terminal rings relax on a few tens of picosecond time scale.
Mode specific excited state dynamics study of bis(phenylethynyl)benzene from ultrafast Raman loss spectroscopy Khokan Roy, Surajit Kayal, Freek Ariese, Andrew Beeby, Siva Umapathy Journal of Chemical Physics, 2017 Femtosecond transient absorption (fs-TA) and Ultrafast Raman Loss Spectroscopy (URLS) have been applied to reveal the excited state dynamics of bis(phenylethynyl)benzene (BPEB), a model system for one-dimensional molecular wires that have numerous applications in opto-electronics. It is known from the literature that in the ground state BPEB has a low torsional barrier, resulting in a mixed population of rotamers in solution at room temperature. For the excited state this torsional barrier had been calculated to be much higher. Our femtosecond TA measurements show a multi-exponential behaviour, related to the complex structural dynamics in the excited electronic state. Time-resolved, excited state URLS studies in different solvents reveal mode-dependent kinetics and picosecond vibrational relaxation dynamics of high frequency vibrations. After excitation, a gradual increase in intensity is observed for all Raman bands, which reflects the structural reorganization of Franck-Condon excited, non-planar rotamers to a planar conformation. It is argued that this excited state planarization is also responsible for its high fluorescence quantum yield. The time dependent peak positions of high frequency vibrations provide additional information: a rapid, sub-picosecond decrease in peak frequency, followed by a slower increase, indicates the extent of conjugation during different phases of excited state relaxation. The CC triple (-C≡C-) bond responds somewhat faster to structural reorganization than the CC double (>C=C<) bonds. This study deepens our understanding of the excited state of BPEB and analogous linear pi-conjugated systems and may thus contribute to the advancement of polymeric "molecular wires."
Probing Ultrafast Photoinduced Electron Transfer to TiO2 from CdS Nanocrystals of Varying Crystallographic Phase Content Sayantan Mazumdar, Khokan Roy, Velaga Srihari, Siva Umapathy, Aninda J. Bhattacharyya Journal of Physical Chemistry C, 2015 The photoinduced electron transfer processes in a nanoheterostructured semiconductor assembly are complex and depend on various parameters of the constituents of the assembly. We present here the ultrafast electron transfer characteristics of an assembly comprised of a wide band semiconductor, titanium dioxide (TiO2), attached to light-harvesting cadmium sulfide (CdS) nanocrystals of varying crystallographic phase content. Quantitative analysis of synchrotron high-resolution X-ray diffraction data of CdS nanocrystals precisely reveals the presence of both wurtzite and zinc blende phases in varying amounts. The estimated content of crystal phases is observed to be strongly dependent on an important synthesis parameter, viz., the ratio of the two solvents. The biphasic nature of CdS influences directly the shape of the nanocrystal at long reaction times as well as the transfer of the photoexcited electrons from the CdS to TiO2 as obtained from transient absorption spectroscopy. A higher amount of zinc blend...
RECENT SCHOLAR PUBLICATIONS
Spectroscopic analysis of the sum-frequency response of the carbon–hydrogen stretching modes in collagen type I YYL Palacios, S Khandani, EP Garcia, A Chen, S Wang, K Roy, D Knez, ... J. Chem. Phys 160, 185101 , 2024 2024 Citations: 14
Enhancement of molecular coherent anti-Stokes Raman scattering with silicon nanoantennas S Abedin, Y Li, AA Sifat, K Roy, EO Potma Nano letters 22 (16), 6685-6691 , 2022 2022 Citations: 6
Surface-Enhanced Coherent Anti-Stokes Raman Scattering of Molecules near Metal–Dielectric Nanojunctions Shamsul Abedin, Khokan Roy, Xin Jin, Hui Xia, S. R. J. Brueck, Eric O. Potma J. Phys. Chem. C , 2022 2022 Citations: 9
Laser Raman Spectroscopy: Fundamentals to Applications S Umapathy, DR Nayak, K Roy, S Sil Handbook of Laser Technology and Applications, 171-195 , 2021 2021
Understanding phase transition and vibrational mode coupling in ammonium nitrate using 2D correlation Raman spectroscopy S Sil, N Kuhar, K Roy, D Chaturvedi, S Morita, Y Ozaki, S Umapathy Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 254, 119581 , 2021 2021 Citations: 18
TERS-relayed molecular force microscopy using CO-terminated tips K Roy, J Lee, B Taber, V Apkarian ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 258 , 2019 2019
Probing the effect of solvation on photoexcited 2-(2′-hydroxyphenyl) benzothiazole via ultrafast Raman loss spectroscopic studies S Kayal, K Roy, YA Lakshmanna, S Umapathy The Journal of Chemical Physics 149 (4) , 2018 2018 Citations: 12
Ultrafast Raman Loss Spectroscopy Unravels the Dynamics in Entangled Singlet and Triplet States in Thioxanthone S Kayal, K Roy, Y Adithya Lakshmanna, S Umapathy The Journal of Physical Chemistry A 122 (29), 6048-6054 , 2018 2018 Citations: 10
Ultrafast Raman Loss Spectroscopic Investigations of Excited State Structural Dynamics of Bis (phenylethynyl) benzene and trans-Stilbene KR ,Indian Institute of Science Bangalore, 2018 2018
Femtosecond coherent nuclear dynamics of excited tetraphenylethylene: ultrafast transient absorption and ultrafast Raman loss spectroscopic studies S Kayal, K Roy, S Umapathy The Journal of Chemical Physics 148 (2) , 2018 2018 Citations: 38
Time‐Resolved Spectroscopy: Instrumentation and Applications F Ariese, K Roy, V Ravi Kumar, HC Sudeeksha, S Kayal, S Umapathy Encyclopedia of Analytical Chemistry: Applications, Theory and … , 2017 2017 Citations: 21
Understanding ultrafast dynamics of conformation specific photo-excitation: a femtosecond transient absorption and ultrafast Raman loss study K Roy, S Kayal, V Ravi Kumar, A Beeby, F Ariese, S Umapathy The Journal of Physical Chemistry A 121 (35), 6538-6546 , 2017 2017 Citations: 39
Mode specific excited state dynamics study of bis (phenylethynyl) benzene from ultrafast Raman loss spectroscopy K Roy, S Kayal, F Ariese, A Beeby, S Umapathy The journal of chemical physics 146 (6) , 2017 2017 Citations: 39
Probing Ultrafast Photoinduced Electron Transfer to TiO 2 from CdS Nanocrystals of Varying Crystallographic Phase Content S Mazumdar, K Roy, V Srihari, S Umapathy, AJ Bhattacharyya The Journal of Physical Chemistry C 119 (30), 17466-17473 , 2015 2015 Citations: 14
Structure and dynamics from time resolved absorption and Raman spectroscopy S Umapathy, K Roy, S Kayal, N Rai, RK Venkatraman The Future of Dynamic Structural Science, 25-41 , 2013 2013 Citations: 9
Femtosecond Stimulated Raman Scattering: Theory and Experiments K Roy, K Kayal, N Rai, S Umapathy Kiran. A Bulletin of the Indian Laser Association 24, 8-11 , 2013 2013 Citations: 2
MOST CITED SCHOLAR PUBLICATIONS
Understanding ultrafast dynamics of conformation specific photo-excitation: a femtosecond transient absorption and ultrafast Raman loss study K Roy, S Kayal, V Ravi Kumar, A Beeby, F Ariese, S Umapathy The Journal of Physical Chemistry A 121 (35), 6538-6546 , 2017 2017 Citations: 39
Mode specific excited state dynamics study of bis (phenylethynyl) benzene from ultrafast Raman loss spectroscopy K Roy, S Kayal, F Ariese, A Beeby, S Umapathy The journal of chemical physics 146 (6) , 2017 2017 Citations: 39
Femtosecond coherent nuclear dynamics of excited tetraphenylethylene: ultrafast transient absorption and ultrafast Raman loss spectroscopic studies S Kayal, K Roy, S Umapathy The Journal of Chemical Physics 148 (2) , 2018 2018 Citations: 38
Time‐Resolved Spectroscopy: Instrumentation and Applications F Ariese, K Roy, V Ravi Kumar, HC Sudeeksha, S Kayal, S Umapathy Encyclopedia of Analytical Chemistry: Applications, Theory and … , 2017 2017 Citations: 21
Understanding phase transition and vibrational mode coupling in ammonium nitrate using 2D correlation Raman spectroscopy S Sil, N Kuhar, K Roy, D Chaturvedi, S Morita, Y Ozaki, S Umapathy Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 254, 119581 , 2021 2021 Citations: 18
Spectroscopic analysis of the sum-frequency response of the carbon–hydrogen stretching modes in collagen type I YYL Palacios, S Khandani, EP Garcia, A Chen, S Wang, K Roy, D Knez, ... J. Chem. Phys 160, 185101 , 2024 2024 Citations: 14
Probing Ultrafast Photoinduced Electron Transfer to TiO 2 from CdS Nanocrystals of Varying Crystallographic Phase Content S Mazumdar, K Roy, V Srihari, S Umapathy, AJ Bhattacharyya The Journal of Physical Chemistry C 119 (30), 17466-17473 , 2015 2015 Citations: 14
Probing the effect of solvation on photoexcited 2-(2′-hydroxyphenyl) benzothiazole via ultrafast Raman loss spectroscopic studies S Kayal, K Roy, YA Lakshmanna, S Umapathy The Journal of Chemical Physics 149 (4) , 2018 2018 Citations: 12
Ultrafast Raman Loss Spectroscopy Unravels the Dynamics in Entangled Singlet and Triplet States in Thioxanthone S Kayal, K Roy, Y Adithya Lakshmanna, S Umapathy The Journal of Physical Chemistry A 122 (29), 6048-6054 , 2018 2018 Citations: 10
Surface-Enhanced Coherent Anti-Stokes Raman Scattering of Molecules near Metal–Dielectric Nanojunctions Shamsul Abedin, Khokan Roy, Xin Jin, Hui Xia, S. R. J. Brueck, Eric O. Potma J. Phys. Chem. C , 2022 2022 Citations: 9
Structure and dynamics from time resolved absorption and Raman spectroscopy S Umapathy, K Roy, S Kayal, N Rai, RK Venkatraman The Future of Dynamic Structural Science, 25-41 , 2013 2013 Citations: 9
Enhancement of molecular coherent anti-Stokes Raman scattering with silicon nanoantennas S Abedin, Y Li, AA Sifat, K Roy, EO Potma Nano letters 22 (16), 6685-6691 , 2022 2022 Citations: 6
Femtosecond Stimulated Raman Scattering: Theory and Experiments K Roy, K Kayal, N Rai, S Umapathy Kiran. A Bulletin of the Indian Laser Association 24, 8-11 , 2013 2013 Citations: 2
Laser Raman Spectroscopy: Fundamentals to Applications S Umapathy, DR Nayak, K Roy, S Sil Handbook of Laser Technology and Applications, 171-195 , 2021 2021
TERS-relayed molecular force microscopy using CO-terminated tips K Roy, J Lee, B Taber, V Apkarian ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 258 , 2019 2019
Ultrafast Raman Loss Spectroscopic Investigations of Excited State Structural Dynamics of Bis (phenylethynyl) benzene and trans-Stilbene KR ,Indian Institute of Science Bangalore, 2018 2018
