Long Vibrational Lifetime R-Selenocyanate Probes for Ultrafast Infrared Spectroscopy: Properties and Synthesis

Abstract

Ultrafast infrared vibrational spectroscopy is widely used for the investigation of dynamics in systems from water to model membranes. Because the experimental observation window is limited to a few times the probe's vibrational lifetime, a frequent obstacle for the measurement of a broad time range is short molecular vibrational lifetimes (typically a few to tens of picoseconds). Five new longlifetime aromatic selenocyanate vibrational probes have been synthesized and their vibrational properties characterized. These probes are compared to commercial phenyl selenocyanate. The vibrational lifetimes range between ∼400 and 500 ps in complex solvents, which are some of the longest room-temperature vibrational lifetimes reported to date. In contrast to vibrations that are long-lived in simple solvents such as CCl 4 , but become much shorter in complex solvents, the probes discussed here have ∼400 ps lifetimes in complex solvents and even longer in simple solvents. One of them has a remarkable lifetime of 1235 ps in CCl 4 . These probes have a range of molecular sizes and geometries that can make them useful for placement into different complex materials due to steric reasons, and some of them have functionalities that enable their synthetic incorporation into larger molecules, such as industrial polymers. We investigated the effect of a range of electron-donating and electron-withdrawing para-substituents on the vibrational properties of the CN stretch. The probes have a solvent-independent linear relationship to the Hammett substituent parameter when evaluated with respect to the CN vibrational frequency and the ipso 13 C NMR chemical shift.

Rethinking Vibrational Stark Spectroscopy: Peak Shifts, Line Widths, and the Role of Non-Stark Solvent Coupling

“…The slope for these data is 3.57 cm –1 /(GV/m), which is 49% lower than the 7.25 cm –1 /(GV/m) obtained for PhSeCN (Table ). This difference is in agreement with previous measurements of the Stark tuning rate of the CN mode of these two molecules. , Since the line width data obtained from BZN do not display a field dependence (Figure ), it is not possible to obtain a linear equation that describes a field-dependent spectral width. Instead, this value was approximated by taking the slope obtained for the field-dependent standard deviations in PhSeCN and reducing it by 49% (the same difference between the slopes of the two molecules’ field-dependent frequencies).…”

Characterization of Acetonitrile Isotopologues as Vibrational Probes of Electrolytes

“…The second requirement of the nonperturbative nature of the vibrational reporter represents a challenge and is usually implicitly assumed to be fulfilled. It might be a sensible assumption in some cases, − such as in dilute solutions when the introduction of extrinsic reporters dramatically extends the available temporal range, but extrinsic probes cannot generally be considered innocent. The problem is augmented in unconventional systems, such as superconcentrated (“solvent-in-salt”) electrolytes, where there are as many, or even fewer, solvent molecules as ions, no free bulk-like solvent remains, and the structure is highly coordinated and tightly packed.…”

“…The third requirement imposed by time-resolved IR is one of the most challenging as typical lifetimes of vibrational chromophores are on the order of several picoseconds, thus limiting the accessible range of time scales up to 10–20 ps and making the selection of usable probes rather limited. It largely motivated the incorporation of extrinsic long-lived species that take advantage of vibrational decoupling by introducing insulating heavy atoms between the vibrational reporter and the rest of the molecule (e.g., thio- ,, and selenocyanates ,− ) that dramatically slows down vibrational relaxation.…”

Cationic Effects on the Structural Dynamics of the Metal Ion–Crown Ether Complexes Investigated by Ultrafast Infrared Spectroscopy

“…The application of hydroxyl stretch as the vibrational probe is usually limited by its short vibrational lifetime which is at the time scale of 2 ps. The anionic vibrational probes (SCN – , SeCN – , N 3 – ) with longer vibrational lifetime have been also developed for the aforementioned spectroscopic studies. , In the case of metal ion–crown ether complex, SCN – happens to be the most widely studied counteranion since the discovery of crown ether in 1967 . The crystal structures of the complexes of alkali metal thiocyanates with crown ether have also been determined thoroughly. − Therefore, it is suitable to apply the ultrafast 2D spectroscopy to study the host–guest interaction in the complexes of crown ether with metal cations in the solution, where the SCN – ion can be employed as the local vibrational probe.…”

“…The anionic vibrational probes (SCN − , SeCN − , N 3 − ) with longer vibrational lifetime have been also developed for the aforementioned spectroscopic studies. 55,56 In the case of metal ion−crown ether complex, SCN − happens to be the most widely studied counteranion since the discovery of crown ether in 1967. 2 The crystal structures of the complexes of alkali metal thiocyanates with crown ether have also been determined thoroughly.…”