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Fluorescence-encoded infrared (FEIR) spectroscopy is a recently developed technique for solution-phase vibrational spectroscopy with detection sensitivity at the single-molecule level. While its spectroscopic information content and important criteria for its practical experimental optimization have been identified, a general understanding of the electronic and nuclear properties required for highly sensitive detection, i.e., what makes a molecule a "good FEIR chromophore," is lacking. This work explores the molecular factors that determine FEIR vibrational activity and assesses computational approaches for its prediction. We employ density functional theory (DFT) and its time-dependent version (TD-DFT) to compute vibrational and electronic transition dipole moments, their relative orientation, and the Franck–Condon factors involved in FEIR activity. We apply these methods to compute the FEIR activities of normal modes of chromophores from the coumarin family and compare these predictions with experimental FEIR cross sections. We discuss the extent to which we can use computational models to predict the FEIR activity of individual vibrations in a candidate molecule. The results discussed in this work provide the groundwork for computational strategies for choosing FEIR vibrational probes or informing the structure of designer chromophores for single-molecule spectroscopic applications. [ABSTRACT FROM AUTHOR]

Developing fluorescence-encoded infrared (FEIR) vibrational spectroscopy for single-molecule applications requires a detailed understanding of how the molecular response and external experimental parameters manifest in the detected signals. In Paper I [L. Whaley-Mayda, A. Guha, and A. Tokmakoff, J. Chem. Phys. 159, 194201 (2023)] we introduced a nonlinear response function theory to describe vibrational dynamics, vibronic coupling, and transition dipole orientation in FEIR experiments with ultrashort pulses. In this second paper, we apply the theory to investigate the role of intermode vibrational coherence, the orientation of vibrational and electronic transition dipoles, and the effects of finite pulse durations in experimental measurements. We focus on measurements at early encoding delays—where signal sizes are largest and therefore of most value for single-molecule experiments, but where many of these phenomena are most pronounced and can complicate the appearance of data. We compare experiments on coumarin dyes with finite-pulse response function simulations to explain the time-dependent behavior of FEIR spectra. The role of the orientational response is explored by analyzing polarization-dependent experiments and their ability to resolve relative dipole angles in the molecular frame. This work serves to demonstrate the molecular information content of FEIR experiments, and develop insight and guidelines for their interpretation. [ABSTRACT FROM AUTHOR]

Fluorescence-encoded infrared (FEIR) spectroscopy is an emerging technique for performing vibrational spectroscopy in solution with detection sensitivity down to single molecules. FEIR experiments use ultrashort pulses to excite a fluorescent molecule's vibrational and electronic transitions in a sequential, time-resolved manner, and are therefore sensitive to intervening vibrational dynamics on the ground state, vibronic coupling, and the relative orientation of vibrational and electronic transition dipole moments. This series of papers presents a theoretical treatment of FEIR spectroscopy that describes these phenomena and examines their manifestation in experimental data. This first paper develops a nonlinear response function description of Fourier-transform FEIR experiments for a two-level electronic system coupled to multiple vibrations, which is then applied to interpret experimental measurements in the second paper [L. Whaley-Mayda et al., J. Chem. Phys. 159, 194202 (2023)]. Vibrational coherence between pairs of modes produce oscillatory features that interfere with the vibrations' population response in a manner dependent on the relative signs of their respective Franck–Condon wavefunction overlaps, leading to time-dependent distortions in FEIR spectra. The orientational response of population and coherence contributions are analyzed and the ability of polarization-dependent experiments to extract relative transition dipole angles is discussed. Overall, this work presents a framework for understanding the full spectroscopic information content of FEIR measurements to aid data interpretation and inform optimal experimental design. [ABSTRACT FROM AUTHOR]

Fluorescence-encoded infrared (FEIR) spectroscopy is a vibrational spectroscopy technique that has recently demonstrated the capability of single-molecule sensitivity in solution without near-field enhancement. This work explores the practical experimental factors that are required for successful FEIR measurements in both the single-molecule and bulk regimes. We investigate the role of resonance conditions by performing measurements on a series of coumarin fluorophores of varying electronic transition frequencies. To analyze variations in signal strength and signal to background between molecules, we introduce an FEIR brightness metric that normalizes out measurement-specific parameters. We find that the effect of the resonance condition on FEIR brightness can be reasonably well described by the electronic absorption spectrum. We discuss strategies for optimizing detection quality and sensitivity in bulk and single-molecule experiments. [ABSTRACT FROM AUTHOR]

Ionic liquids have been regarded as promising alternatives to conventional volatile organic solvents. For effective employment of ionic liquids as solvents in various industrial set-ups, the knowledge of their common physical properties is essential. Densities and dynamic viscosities of seven common and popular ionic liquids in the temperature range (283.15 to 363.15) K at 10 K interval are presented. In order to assess the role of the structure of the ionic liquid cation and anion, the selected ionic liquids are comprised of two groups. First group of four ionic liquids is constituted of same 1-butyl-3-methylimidazolium ([bmim + ]) cation and hexafluorophosphate [PF 6 − ], ethylsulfate [CH 3 CH 2 OSO 3 − ], trifluoromethysulfonate [CF 3 SO 3 − ], and bis(trifluoromethylsulfonyl)imide [(CF 3 SO 2 ) 2 N − ] anions, and the second group of four ionic liquids is constituted of same [(CF 3 SO 2 ) 2 N − ] anion and 1-methyl-1-propylpiperidinium [pmpip + ], N , N , N -trimethylethanolammonium [choline + ], 1-ethyl-3-methylimidazolium [emim + ], and [bmim + ] cations. Decrease in density with increasing temperature is found to follow a simple linear expression for all ionic liquids. Anion of the ionic liquid appears to control the density more than the cation. Density and its temperature dependence depend on the number of fluorine atoms present in the anion of the ionic liquid followed by the molar mass. The temperature dependence of the dynamic viscosity of the ionic liquids is found to be better described by a Vogel–Fulcher–Tamman (VFT) model as opposed to an Arrhenius expression. The two ionic liquids with aromatic imidazolium cations have relatively lower dynamic viscosities as compared to the two with non-aromatic cations. [ABSTRACT FROM AUTHOR]