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Time-Correlated Single Photon Counting

Fluorescence population, anisotropy, and solvation dynamics experiments are conducted utilizing time correlated single photon counting (TCSPC). A Ti:Sapphire oscillator producing approximately 100 fs laser pulses at a tunable excitation wavelength range of 710 nm to 990 nm. This wavelength was frequency doubled in a barium borate crystal producing light ranging from 355 nm to 495 nm for excitation of fluorescent probes. The bandwidth of the frequency doubled excitation pulse was found to be 5.8 nm. The laser repetition rate was lowered from the 80 MHz outputted by the oscillator to 5 MHz by means of an acousto-optic modulator functioning as a single pulse selector. A computer controlled half wave plate rotated the excitation beam such that measurements could be collected parallel, perpendicular, and at magic angle relative to a fixed polarizer secured at the entrance slit of a monochromator. The sample was excited from the front surface in a near-normal geometry through a hole in the lens that collected the fluorescence. A second lens imaged the fluorescence onto the monochromator entrance slit. The fluorescence was frequency resolved by the monochromator and single photons were detected with a multichannel plate detector (MCP) at desired emission wavelengths. This robust experimental setup allows for a variety of diverse experiments to be performed including solvation dynamics, proton transfer, and fluorescence anisotropy. Below are typical procedures for the aforementioned experiments, more detailed descriptions can be found in the listed publications:

Solvation Dynamics

Solvation dynamics were calculated by setting the excitation polarization to the magic angle. Population decay curves were then collected from 460 nm to 580 nm in 2 nm increments. Each emission wavelength was collected in series for 60 seconds. The wavelength scan was repeated until it was determined that an acceptable signal-to-noise level had been reached. All data were taken with the same entrance slit width, and all other experimental conditions were identical so that the relative amplitude at each emission wavelength was maintained.

Excited State Proton Transfer

Population decays of the fluorescent samples used 380 nm excitation with detection at 440, 450, 460, and 515 nm. In this case, a Glan−Thompson polarizer in the excitation beam just prior to the sample was fixed at the magic angle relative the fixed polarizer in front of the entrance slit of the monochromator. To prevent possible photodecomposition of the chromophores from repeated excitation, the sample was slowly moved. The sample was mounted on a motorized computer controlled translation stage. As the data were collected, the sample was slowly translated across its width, back and forth. No appreciable reduction in total fluorescence intensity was detected in the course of the experiments. As a test, data taken at the beginning of an experiment and at the end of an experiment were compared, and they were identical within the noise of the measurement.

Fluorescence Anisotropy

For fluorescence anisotropy measurements, samples were excited at the 395 nm and emission was collected at 440 nm. A computer controlled half wave plate rotated the excitation beam such that measurements could be collected parallel, perpendicular, and at magic angle relative to a fixed polarizer secured at the entrance slit of a monochromator. The sample was excited from the front surface in a near-normal geometry through a hole in the lens that collected the fluorescence. Fluorescent impurities in the samples were excitable at 395 nm and emitted at 440 nm and therefore contributed to the signal. To address this, a moving sample stage was used that interchanges the two samples under computer control so that data were collected on both probe-containing samples, and those without as a reference, keeping all experimental conditions identical over the course of the experiment. The resulting decay curves attained after subtraction of the reference sample extracted the desired rotational and excited state population information.

Relevant Publications

455. "The Influence of Water on the Alkyl Region Structure in Variable Chain Length Imidazolium-Based Ionic Liquid/Water Mixtures," Joseph E. Thomaz, Christian M. Lawler, and Michael D. Fayer J. Phys. Chem. B 120, 10350-10357 (2016).

463. "Proton Transfer in Perfluorosulfonic Acid Fuel Cell Membranes with Differing Pendant Chains and Equivalent Weights," Joseph E. Thomaz, Christian M. Lawler, and Michael D. Fayer J. Phys. Chem. B 121, 4544-4553 (2017).