We studied the origin of the vibrational signatures in the sum-frequency generation (SFG) spectrum of fibrillar collagen type I in the carbon-hydrogen stretching regime. For this purpose, we developed an all-reflective, laser-scanning SFG microscope with minimum chromatic aberrations and excellent retention of the polarization state of the incident beams. We performed detailed SFG measurements of aligned collagen fibers obtained from rat tail tendon, enabling the characterization of the magnitude and polarization-orientation dependence of individual tensor elements Xijk2 of collagen's nonlinear susceptibility. Using the three-dimensional atomic positions derived from published crystallographic data of collagen type I, we simulated its Xijk2 elements for the methylene stretching vibration and compared the predicted response with the experimental results. Our analysis revealed that the carbon-hydrogen stretching range of the SFG spectrum is dominated by symmetric stretching modes of methylene bridge groups on the pyrrolidine rings of the proline and hydroxyproline residues, giving rise to a dominant peak near 2942 cm-1 and a shoulder at 2917 cm-1. Weak asymmetric stretches of the methylene bridge group of glycine are observed in the region near 2870 cm-1, whereas asymmetric CH2-stretching modes on the pyrrolidine rings are found in the 2980 to 3030 cm-1 range. These findings help predict the protein's nonlinear optical properties from its crystal structure, thus establishing a connection between the protein structure and SFG spectroscopic measurements.
Carbon , Collagen Type I , Hydrogen , Hydrogen/chemistry , Carbon/chemistry , Collagen Type I/chemistry , Rats , Animals , Spectrum Analysis/methods
Surface-enhanced coherent anti-Stokes Raman scattering (SE-CARS) takes advantage of surface plasmon resonances supported on metallic nanostructures to amplify the coherent Raman response of target molecules. While these metallic antennas have found significant success in SE-CARS studies, photoinduced morphological changes to the nanoantenna under ultrafast excitation introduce significant hurdles in terms of stability and reproducilibty. These hurdles need to be overcome in order to establish SE-CARS as a reliable tool for rapid biomolecular sensing. Here, we address this challenge by performing molecular CARS measurements enhanced by nanoantennas made from high-index dielectric particles with more favorable thermal properties. We present the first experimental demonstration of enhanced molecular CARS signals observed at Si nanoantennas, which offer much improved thermal stability compared to their metallic counterparts.
Nanostructures , Spectrum Analysis, Raman , Silicon , Surface Plasmon Resonance
Ammonium nitrate (AN) is an important component of the chemical industry such as an active ingredient in fertilizers, as an oxidizer in explosive compositions and propellants, and as a blasting agent in civil explosives. Numerous accidents have been reported in the past which concerns its thermal instability and poses a big threat to its processing, transportation, and storage. Despite much literature being reported to understand its thermal instability, a mechanistic view remains unclear. In the present work, we have studied the behavior of AN to temperature change using a mathematical approach called 2D correlation (2D Cos) Raman spectroscopy to provide complete insight into the detailed dynamical nature of the interactions between the species (ionic or molecular) occurring with an increase in temperature. We have analyzed various libration and translational modes of nitrate in the low-frequency region using this mathematical tool. It is observed from 2D maps that the phase transition of AN starts with changes in libration modes followed by various nitrate modes and ammonium modes which further precedes low-frequency translational modes. Further, the 2D correlation could differentiate between modes splitting and shifting based on specific 2D Cos pattern. The changes occurring in the N-O deformation modes, symmetric stretching modes as well as anti-symmetric stretching modes which have been attributed to the weakening of the hetero-ionic coupling between the NH4+ and the NO3- ions could be clearly distinguished in the 2D synchronous and asynchronous plots. Besides, moving window analysis was performed to visualize the transition temperature at which phase change of AN takes place.
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 (â¼10 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.
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 CâO stretch (â¼1320 cm-1) of TX in the excited state acts as the marker band as it undergoes a red shift with time constants of â¼45 and â¼5 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 CâC stretch and the CâO 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 â¼700 fs, which represents the structural distortion during internal conversion (S2 â S1) process. The kinetics of amplitudes of these modes follows biexponential growth with time constants of â¼3 and â¼14 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.
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 â relaxed state â 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 â¼130 cm-1 low-frequency phenyl torsional mode. Two vibrational marker bands, Cet=Cet stretching (â¼1512 cm-1) and Cph=Cph stretching (â¼1584 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 â¼400 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.
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.
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."
