Single-molecule spectroscopy has had a revolutionary impact on how we describe molecular behavior in chemistry, biology, and materials science. While the capabilities of fluorescence-based techniques continue to advance rapidly, they are often difficult to apply to chemical problems due to their limited structural specificity. Single-molecule vibrational techniques address this shortcoming by offering a sensitivity to chemical bonding and specific molecular contacts through the frequencies and lineshapes of vibrational bands. However, well-established single-molecule vibrational methods like surface- and tip-enhanced Raman spectroscopy require direct contact with a probe or nanostructure, which is usually too perturbative for many solution- or condensed-phase systems. Here, we describe a new strategy—fluorescence-encoded infrared (FEIR) spectroscopy—that achieves single-molecule sensitivity by coupling IR-vibrational absorption to a fluorescent electronic transition, thereby benefitting from the experimental advantages and solution-phase compatibility of far-field fluorescence detection. Employing femtosecond IR and visible pulses for vibrational excitation and up-conversion to the electronic excited state enables broadband Fourier transform acquisition of ground-state vibrational spectra with ultrafast time-resolution. We demonstrate single-molecule sensitivity by introducing FEIR correlation spectroscopy, an IR-vibrational analogue of fluorescence correlation spectroscopy, to count the average number of molecules contributing to the signal. We discuss the possibilities and potential applications that single-molecule and fluctuation-correlation FEIR experiments offer for studying chemical dynamics in solution.

Ultrafast fluorescence-encoded infrared vibrational spectroscopy with single-molecule sensitivity

ACS Spring Meeting 2021

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