Introduction to Time-Resolved Spectroscopy: Nanosecond Transient Absorption and Time-Resolved Fluorescence of Eosin B

Abstract

Here we present a new undergraduate laboratory that will introduce the concepts of time-resolved spectroscopy and provide insight into the natural time scales on which chemical dynamics occur through direct measurement. A quantitative treatment of the acquired data will provide a deeper understanding of the role of quantum mechanics and various phenomenological expressions in predicting kinetic rates for fluorescence, phosphorescence, and nonradiative decay mechanisms. This laboratory framework focuses specifically on spectroscopy in the nanosecond regimeassisted by various steady-state spectroscopic techniquesin order to fully characterize the electronic structure and the picosecond-to-microsecond dynamics of the dye eosin B. There is great flexibility in both the recommended lab duration (1 week to several months) and course level (upper-division to graduate) due to the numerous additional experiments that may be performed at varying levels of difficulty. The necessary components include pump and probe light sources, photodiode detectors, a programmable signal delay generator, and an oscilloscope for measurements with requisite resolution. The cost of building this experiment from scratch is less than $20,000 at the time of publication, but costs are expected to decrease over time and alternate excitation sources are available. Although this lab requires some expertise with optical spectroscopy to initially build and troubleshoot, use by students has been a straightforward and valuable experience.

Mechanistic Studies in Photocatalysis

“…[75] Thed etection of highly elusive intermediates,i ncluding non-radical ones,r equires more sensitive time-resolved spectroscopic techniques, [76] such as laser flash photolysis (LFP). [77] These methods enable the observation of species with lifespans in the order of pico/femto-seconds,i ncluding molecules in their triplet/singlet excited states and transient ionic radicals.I nL FP,a na nalyte mixture is exposed to intermitting irradiation by an ultrafast pulsing laser. The difference in absorbance or emission before and after light collision (the optical deviation, DOD) is recorded.…”

Molecular Dynamics—From Small Molecules to Macromolecules

“… Overview of time scales of molecular motions. The contents of this figure were excerpted from the contents of the study mentioned in [ 8 , 9 , 10 ]. …”

“…In order to observe the dynamics of a molecule, it is important to understand the time scale of the motion of the target molecule ( Figure 1 ). The time scales of molecular motion such as electronic motion, photodissociation, photoionization, bond vibrations, molecular collisions, vibration relaxation, solvation, proton transfer, temperature jump Raman, fluorescence, molecular rotations, large molecule relaxation, molecular diffusion, and phosphorescence motion varies between a few seconds to femtoseconds [ 8 , 9 , 10 ]. When observing the dynamics of a molecule, the movement time scale of the molecule must be considered and an appropriate experimental technique must be applied to observe it.…”

Shining Light on the Light-Bearing Element: A Brief Review of Photomediated C–H Phosphorylation Reactions

“…The high prices and toxicity of organometallic complexes has led researchers to explore more environmentally friendly and inexpensive options such as organic dyes, which are organic molecules that absorb in the visible light spectrum and also display long-lived triplet states with redox properties ( Figure 6). [96][97][98][99][100][101] In 2016, Luo et al reported the first example of the direct C-H phosphorylation of thiazoles photocatalyzed by an organic dye, Eosin B (Scheme 24). 102 Radical-scavenger experiments suggest the mechanism to be radical, and the tentative mechanism proposed by the authors follows the direct oxidation of the trivalent tautomer form of diphenylphosphine oxide by the excited Eosin B to give a P radical.…”