Photochemistry and Photophysics Measurements

Photophysics and photochemistry of solutions can typically be investigated by traditional "steady-state" absorption and fluorescence techniques commonly available in the lab. In this case, careful measurements of absorption spectra to determine a molecules absorptivity, or molar extinction coefficient, enables the determination of the strength of each transition in the spectrum. This property is critical in the light harvesting ability of a chromophore. Fluorescence spectroscopy can then be used to determine a substance's fluorescence quantum yield, or how often a molecule emits a photon following absorption. This measurement indirectly probes other pathways that deactivate a molecule's excited state, and encodes the timescales for such pathways (in tandem with absorption spectra) down to sub-picosecond timescales in some cases. With the information taken from absorption and fluorescence spectra then, one can infer the existence of deactivation pathways in addition to a molecule's absorption properties. However, how many non-radiative pathways and their individual timescales cannot be determined from a single absorption and fluorescence measurement. Transient absorption spectroscopy using pulsed lasers however can deconvolute the excited state decay mechanisms by resolving each step in time. This is done by using a "pump" and "probe" laser pulse which are scanned in time to directly measure the time evolution. Our lab does not have transient absorption capabilities, but collaborations around the US provide us the privilege to occasionally perform these measurements where needed.

Photochemistry on the other hand can take place on longer timescales such as minutes to hours. Steady-state absorption and fluorescence is then sufficient to measure the evolution of signals pertaining to "reactants" and "products" in that case. By taking a time sequence of those measurements, the kinetics and therefore reaction mechanism can be explored. Usually, conditions are varied to elucidate details of the mechanism such as solvent choice, light intensity, de-aerated solution, etc.