Prof. Christopher Elles named October 2023 Sutton Family Research Impact Award recipient

The Department of Chemistry congratulates Professor Christopher Elles on receiving the October 2023 Sutton Family Research Impact Award!

The Sutton Award is a monthly competition among chemistry faculty. Every month, the Chemistry Department Chair and Associate Chairs review the peer-reviewed papers published by chemistry faculty from the three previous months to select a winner. The recipient receives a $500 cash prize and is featured on the departmental website.

For a full list of winners, visit our Sutton Family Research Impact Award webpage.

Broadband Two-Photon Absorption Spectroscopy with Stimulated Raman Scattering as an Internal Standard

By Prasenjit Srivastava, David A. Stierwalt, and Christopher G. Elles

Published in Analytical Chemistry, 95, 13227−13234 (2023)

The fields of chemical and biological imaging have flourished in recent years based on the development of new chemical probes that can be activated with microscopic spatial resolution. For example, fluorescence imaging studies at the sub-cellular level are becoming almost routine, with new advances being reported at a rapid pace. This development has been made possible through the design and implementation of novel molecular probes that are selectively activated using a process called two-photon absorption (2PA). The 2PA process involves the simultaneous absorption of two photons from a laser. Because neither photon can be absorbed individually, this process only occurs at the focal point of the excitation laser, where the light is sufficiently intense, and therefore results in extremely high spatial resolution. The two-photon absorption process also has the added benefit of using longer-wavelength light to achieve the same total excitation energy compared with linear absorption, which typically results in deeper penetration of the sample (i.e., for deep tissue imaging) and eliminates the need for damaging UV light.

The development of new dyes is often limited by the difficulty in obtaining accurate two-photon absorption cross sections using traditional techniques, which makes it difficult to measure the nonlinear optical properties of molecules that are being developed by synthetic chemistry groups. However, the Elles Group has been working to develop new approaches to measure the two-photon absorption spectra of target molecules using a relatively common pump-probe spectroscopy technique. This approach is relatively simple and easy to implement, provides broadband 2PA spectra covering a wide range of excitation energies in a fraction of the time required for traditional methods, and gives absolute 2PA cross sections with very high accuracy.

The paper featured here, which was published in the journal Analytical Chemistry, shows how stimulated Raman scattering from the solvent can be used as a convenient and robust internal standard for obtaining accurate absolute 2PA cross sections using the broadband approach. Stimulated Raman scattering has the same pump-probe overlap dependence as 2PA, thus eliminating the need to measure the intensity-dependent overlap of the pump and probe directly. Eliminating the overlap represents an important improvement because intensity profiles are typically the largest source of uncertainty in the measurement of absolute 2PA cross sections using any method. Raman scattering cross sections are a fundamental property of the solvent and therefore provide a universal standard that can be applied any time the 2PA and Raman signals are present within the same probe wavelength range. Furthermore, this approach is very general and can be applied to almost any 2PA chromophore. Considering the growing importance of two-photon excited processes, this work is likely to have a significant impact and will be of widespread interest to the analytical chemistry community and beyond.

Figure 1: Graduate student P.J. Srivastava helped develop a new approach for measuring accurate two-photon absorption cross sections of molecules in solution using broadband ultrafast laser spectroscopy.

Figure 2: The table-of-contents image from the paper recently published in Analytical Chemistry illustrates the broadband pump-probe approach using stimulated Raman scattering (SRS) from the solvent as an internal standard for the two-photon absorption (2PA) spectrum of a dye molecule, coumarin 153, in methanol.