Orientation and Polarization Dependence of Ground- and Excited-State FSRS in Crystalline Betaine-30

Abstract

Charge transfer reactions are frequently accompanied by large changes in dipole and are of instrumental importance in many biological and chemical processes. Understanding the mechanism and dynamics associated with charge transfer reactions has been the focus of decades of investigations; however, most studies have been done with solvated analytes in which the measurements can be significantly impacted by environmental broadening or solvent-mediated effects. Here, we study photoinduced intramolecular charge transfer in crystalline betaine-30, thus dramatically reducing environmental heterogeneities and solvent contributions to the charge transfer process. We use femtosecond stimulated Raman spectroscopy in a reflective microscope configuration to probe the ground and excited state structures in a well-characterized betaine-30 crystal, with a unit cell consisting of four nearly orthogonal betaine-30 molecules and six water molecules. This study highlights how changing the crystal orientation and laser polarization can impact the excited- and ground-state Raman signals due to differences in the unit cell net dipole moment along different crystal axes. We find that the forward charge transfer process (τ ∼ 500 fs) is unaffected by the crystalline environment, while the back electron transfer process (τ ∼ 20 ps) is delayed in the crystalline environment as compared to lifetimes obtained in a range of solvents. This work highlights the role of the net transition dipole orientation on signals resulting from charge transfer in an anisotropic system and should aid in guiding the rational design of solid-state photoactivated devices.

Facile Background Discrimination in Femtosecond Stimulated Raman Spectroscopy Using a Dual-Frequency Raman Pump Technique

“…To further test the broad applicability of this experimental technique, we then applied our dual-frequency Raman pump FSRS on crystalline betaine-30, a more challenging sample due to its interesting polarization and orientation-dependent excited-state dynamics and the narrowband excited-state features observed in its FSR spectra. 48 The crystal was oriented such that the long axis of the crystal was approximately at an angle of 60°with respect to the probe polarization (Figure S8). We measured the time-resolved FSR spectra and performed a one-to-one subtraction of the groundstate betaine-30 spectrum from each excited-state spectrum at different time intervals.…”

Mastering Femtosecond Stimulated Raman Spectroscopy: A Practical Guide

“…In some cases, when samples have high optical densities or when investigating nontransmissive samples, FSRS can be measured in a reflective geometry rather than transmissive. 23 Sample damage can be an issue in ultrafast spectroscopy, particularly for biological samples, because of the high peak intensity of the short laser pulses used. For example, for a beam that has a 1 kHz repetition rate, 200 μW average power, 100 fs pulse width, and is focused to 10 μm diameter, the peak power is 2 MW, and the peak flux is 25 TW/cm 2 , which is more than capable of damaging most samples.…”

“…While the Raman pump and probe beams are parallel to each other, their orientation to the axes of a crystal's unit cell can change the relative intensities of Raman modes measured. 23 FSRS is typically used to measure early time dynamics where rotational anisotropy is generally not a factor, and so, the actinic pump is usually parallel to the Raman pump and probe to maximize the excited-state signal. The intensity of Raman signal in solids is highly dependent on crystalline orientation and the polarization of the incoming laser, and different transition dipole moments can be accessed using different polarization conditions.…”

Femtosecond stimulated Raman spectroscopy – guided library mining leads to efficient singlet fission in rubrene derivatives

“…We measured time-resolved FSRS experiments on our homebuilt optical setup, described elsewhere. 48,[58][59][60][61] In short, we used the fundamental output of 4.6 W at 800 nm from a 1 kHz repetition rate Ti:sapphire regenerative amplier (Coherent model Libra-F-1K-HE-110) to generate the Raman pump, Raman probe and actinic pulses for the FSRS experiments. We focused 2.5 mW of the 800 nm fundamental output through a 2 mm sapphire crystal to generate a white light continuum and then compressed it with a fused silica prism pair to generate the femtosecond broadband Raman probe.…”