Raman Snapshots of Side-Chain Dependent Polaron Dynamics in PolyThiophene Films

Mastering Femtosecond Stimulated Raman Spectroscopy: A Practical Guide

Femtosecond stimulated Raman spectroscopy (FSRS) is a powerful nonlinear spectroscopic technique that probes changes in molecular and material structure with high temporal and spectral resolution. With proper spectral interpretation, this is equivalent to mapping out reactive pathways on highly anharmonic excited-state potential energy surfaces with femtosecond to picosecond time resolution. FSRS has been used to examine structural dynamics in a wide range of samples, including photoactive proteins, photovoltaic materials, plasmonic nanostructures, polymers, and a range of others, with experiments performed in multiple groups around the world. As the FSRS technique grows in popularity and is increasingly implemented in user facilities, there is a need for a widespread understanding of the methodology and best practices. In this review, we present a practical guide to FSRS, including discussions of instrumentation, as well as data acquisition and analysis. First, we describe common methods of generating the three pulses required for FSRS: the probe, Raman pump, and actinic pump, including a discussion of the parameters to consider when selecting a beam generation method. We then outline approaches for effective and efficient FSRS data acquisition. We discuss common data analysis techniques for FSRS, as well as more advanced analyses aimed at extracting small signals on a large background. We conclude with a discussion of some of the new directions for FSRS research, including spectromicroscopy. Overall, this review provides researchers with a practical handbook for FSRS as a technique with the aim of encouraging many scientists and engineers to use it in their research.

Unraveling Excited State Dynamics of a Single-Stranded DNA-Assembled Conjugated Polyelectrolyte

Conformational templating of conjugated polyelectrolytes with single-stranded DNAs (ssDNAs) has the prospect of tailoring excited state dynamics for specific optoelectronic applications. We use ultrafast time-resolved infrared spectroscopy to study the photophysics of a cationic polythiophene assembled with different ssDNAs, inducing distinct conformations (flexible disordered structures vs more rigid complexes with increased backbone planarity). Intrachain polarons are always produced upon selective excitation of the polymer, the extent being dependent on backbone torsional order. Polaron formation and decay were monitored through evolution of IR-active vibrational modes that interfere with mid-IR polaron electronic absorption giving rise to Fano-antiresonances. Selective UV excitation of ssDNAs revealed that stacking interactions between thiophene rings and nucleic acid bases can promote the formation of an intermolecular charge transfer complex. The findings inform designers of functional conjugated polymers by identifying that involvement of the scaffold in the photophysics needs to be considered when developing such structures for optoelectronic applications.

Molecular-Enhanced Raman Spectroscopy Driven by Phosphoester Electron-Transfer Bridge

Although both electromagnetic and charge transfer (CT) mechanisms play a role in surface-enhanced Raman scattering (SERS), the contribution of the latter is limited by poor CT efficiency. Herein, we propose molecular-enhanced Raman spectroscopy (MERS) for the first time and develop a simple strategy to induce strong CT-enhanced Raman signals using a phosphoester (POE) electron-transfer bridge. Consequently, an excellent POE-enhanced Raman effect was found when various mono-, bis-, and trisaminobenzene compounds were used as probe analytes. Quantification analysis of this MERS effect revealed that the enhancement ratio and factor of the POE molecules can be up to 87% and ∼109, respectively. Spectroscopic analysis and density functional theory calculation confirmed that this effect was because of the formation of intermolecular hydrogen bonds, which promotes CT via electronic reorganization and enhances the Raman signals of target analytes. These results demonstrate the feasibility of MERS for highly CT-enhanced Raman signals.