Noncovalent interactions, in particular the hydrogen bonds and nonspecific long-range electrostatic interactions are fundamental to biomolecular functions. A molecular understanding of the local electrostatic environment, consistently for both specific (hydrogen-bonding) and nonspecific electrostatic (local polarity) interactions, is essential for a detailed understanding of these processes. Vibrational Stark Effect (VSE) has proven to be an extremely useful method to measure the local electric field using infrared spectroscopy of carbonyl and nitrile based probes. The nitrile chemical group would be an ideal choice because of its absorption in an infrared spectral window transparent to biomolecules, ease of site-specific incorporation into proteins, and common occurrence as a substituent in various drug molecules. However, the inability of VSE to describe the dependence of IR frequency on electric field for hydrogen-bonded nitriles to date has severely limited nitrile's utility to probe the noncovalent interactions. In this work, using infrared spectroscopy and atomistic molecular dynamics simulations, we have reported for the first time a linear correlation between nitrile frequencies and electric fields in a wide range of hydrogen-bonding environments that may bridge the existing gap between VSE and H-bonding interactions. We have demonstrated the robustness of this field-frequency correlation for both aromatic nitriles and sulfur-based nitriles in a wide range of molecules of varying size and compactness, including small molecules in complex solvation environments, an amino acid, disordered peptides, and structured proteins. This correlation, when coupled to VSE, can be used to quantify noncovalent interactions, specific or nonspecific, in a consistent manner.

Deep eutectic solvents (DESs) have gained popularity in recent years as an environmentally benign, inexpensive alternative to organic solvents for diverse applications in chemistry and biology. Among them, alcohol-based DESs serve as useful media in various applications due to their significantly low viscosity as compared to other DESs. Despite their importance as media, little is known how their solvation dynamics change as a function of the hydrocarbon chain length of the alcohol constituent. In order to obtain insights into the chain-length dependence of the solvation dynamics, we have performed two-dimensional infrared spectroscopy on three alcohol-based DESs by systematically varying the hydrocarbon chain length. The results reveal that the solvent dynamics slows down monotonically with an increase in the chain length. This increase in the dynamic timescales also shows a strong correlation with the concomitant increase in the viscosity of DESs. In addition, we have performed molecular dynamics simulations to compare with the experimental results, thereby testing the capacity of simulations to determine the amplitudes and timescales of the structural fluctuations on fast timescales under thermal equilibrium conditions.

A set of triazole-based chromogenic and fluorescent chemosensors with amino acid/carbohydratefluorophore conjugates have been designed and synthesized. The metal cation-sensing properties of glycine-anthracene, C 24 H 24 O 4 N 4 (3), glycine-pyrene C 26 H 24 O 4 N 4 (4), glucose-anthracene, C 32 H 33 O 10 N 3 (5) and glucose-pyrene, C 34 H 33 O 10 N 3 (6) bio-conjugates have been studied systematically.The significant changes in their absorption spectra are accompanied by a strong color change from light yellow to brown for 3 and 4 and colorless to greenish blue for 5 and 6. Receptors 3 and 4 have potential in the "naked eye" detection of Cu 2+ and 5 and 6 for Pb 2+ /Hg 2+ ion. The receptors 3 and 4 show fluorescence diminution following Cu 2+ coordination within the limit of detection at 0.89 parts per billion (ppb) and this is unprecedented, whereas the receptors 5 and 6 present drastic fluorescence quenching upon addition of Hg 2+ and Pb 2+ within the limit of detection at 4 and 2 ppb respectively. Interestingly, their fluorescence and colorimetric responses are preserved in the presence of water that can be used for the selective colorimetric detection of these ions in aqueous environments. Along with the spectroscopic data, combined 1 H NMR titration of the complexes and the DFT studies suggest the proposed coordination modes.

Molecular structure and function depend on myriad noncovalent interactions. However, the weak and transient nature of noncovalent interactions in solution makes them challenging to study. Information on weak interactions is typically derived from theory and indirect structural data. Solvent fluctuations, not revealed by structure analysis, further complicate the study of these interactions. Using 2D infrared spectroscopy, we show that the strong hydrogen bond and the weak n → π* interaction coexist and interconvert in aqueous solution. We found that the kinetics of these interconverting interactions becomes faster with increasing water content. This experimental observation provides a new perspective on the existence of weak noncovalent interactions in aqueous solution.

Electrostatic interactions in proteins play a crucial role in determining the structure-function relation in biomolecules. In recent years, fluorescent probes have been extensively employed to interrogate the polarity in biological cavities through dielectric constants or semiempirical polarity scales. A choice of multiple spectroscopic methods, not limited by fluorophores, along with a molecular level description of electrostatics involving solute-solvent interactions, would allow more flexibility to pick and choose the experimental technique to determine the local electrostatics within protein interiors. In this work we report that ultraviolet/visible-absorption, infrared-absorption, or (13)C NMR can be used to calibrate the local electric field in both hydrogen bonded and non-hydrogen bonded protein environments. The local electric field at the binding site of a serum protein has been determined using the absorption wavelength as well as the carbonyl stretching frequency of its natural steroid substrate, testosterone. Excellent agreement is observed in the results obtained from two independent spectroscopic techniques.

Charge transport and collection in organic solar cells are heavily influenced by traps which ultimately limit the ability to harvest all photogenerated carriers. We investigate photocurrent responses of organic solar cells subjected to varying degrees of aging from time-and frequency-domain perspectives. Intensitymodulated photocurrent spectroscopy (IMPS) is primarily used here to resolve the effect of trap-assisted nongeminate charge recombination over a broad frequency range (e.g., ∼1 mHz−1 MHz). We use a combination of IMPS and time-dependent photocurrent transients to understand characteristic degradation signatures (i.e., positive, low-frequency imaginary component and "gain peak" where the real photocurrent exhibits a characteristic maximum, I max , at high frequencies) unique to organic solar cells. As trap densities and occupation increase with aging and light intensity, the photocurrent contrast (i.e., maximum/steady-state photocurrent, I max /I DC ) and the size of the low-frequency imaginary contribution increase. Substantial harmonic content underlies this trend which becomes more prominent as modulation frequencies and trap levels increase. We then use drift-diffusion simulations to describe IMPS responses and photocurrent transient signals over the entire frequency sampling window for aged devices that show excellent agreement with experiment. The results provide deeper insights into trap-related phenomena over a larger frequency bandwidth and further demonstrate the effectiveness of IMPS in its ability to identify mechanistic and kinetic details of degradation.

Synthesis and sensing properties of 1,19-disubstituted unsymmetrical ferrocene-triazole derivatives: a multichannel probe for Hg(II) ion3

We investigate the degradation phenomena of organic solar cells based on nonfullerene electron acceptors (NFA) using intensity-modulated photocurrent spectroscopy (IMPS). Devices composed of NIR absorbing blends of a polymer (PTB7) and NFA molecules (COi8DFIC) were operated in air for varying periods of time that display unusual degradation trends. Light aging (e.g., ∼3 days) results in a characteristic first quadrant (positive phase shifts) degradation feature in IMPS Nyquist (Bode) plots that grow in amplitude and frequency with increasing excitation intensity and then subsequently turns over and vanishes. By contrast, devices aged and operated in air for longer times (>5 days) display poor photovoltaic performance and have a dominant first quadrant IMPS component that grows nonlinearly with excitation intensity. We analyze these degradation trends using a simple model with descriptors underlying the first quadrant feature (i.e., trap lifetime and occupancy). The results indicate that the quasi first-order recombination rate constant, k rec, is significantly slower in addition to lower trap densities in devices exhibiting light aging effects that are overcome by increasing carrier densities (viz. excitation intensity). By contrast, larger trap densities and distributions coupled with larger k rec values are found to be responsible for the continuous growth of the first quadrant with light intensity. We believe that defect formation and charge recombination at device contact interfaces is chiefly responsible for performance degradation, which offers several directions for materials and device optimization strategies to minimize long-term detrimental factors.