Experimental Methods 1 Materials and Preparation

7-hydroxy-4-(trifluoromethyl)-1-coumarin (hydroxycoumarin) (98%), 1-methylimidazole (99%), and toluene (Chromasolv Plus for HPLC, > 99.9%) were all

purchased from Sigma-Aldrich and used as received. Solutions were prepared by first creating stock solutions of the hydroxycoumarin and 1-methylimidazole in toluene. The stock solutions were then used to make final solutions of the appropriate concentrations. Prior to time-resolved emission measurements the samples were deaerated by bubbling Argon gas through the sample for ~30 minutes.

5.2.2 UV-Vis Absorption

UV-Vis absorption measurements were done using an Agilent Technologies Model 8453 diode-array spectrophotometer. Initial absorption measurements were done in a 1 cm cuvette and then repeated in a 2 mm cuvette to recreate the path length and concentration conditions used in the femtosecond transient absorption measurements.

5.2.3 Steady State Emission

Steady state emission measurements were taken using PTI QuantaMaster Emission Spectrometer. Samples were excited with 355 nm light and the sample emission was scanned from 360 nm to 700 nm using background correction. Emission collected using the 2 mm cuvette required it be placed at a 45 degree angle relative to the excitation and emission collection slits in the spectrometer. Slit widths of 0.35 mm were used.

5.2.4 Femtosecond Transient Absorption

Femtosecond transient absorption measurements were done using a pump probe technique which has been described in detail in Chapter 3 and previously. 16 _{Briefly, the}

excitation source is a chirped pulse Ti:Sapphire regenerative amplification laser system (Clark CPA 2001) which outputs a 800 mW, 775 nm pulse at a 1 kHz repetition rate, with an autocorrelation full width half max of 250 fs. The probe pulse was generated by focusing a small portion of the beam into a CaF2 window to generate a white light continuum from

380-700 nm. The spot size at the sample was ~ 280 m. The 355 nm pump pulse was created with a tunable Clark Optical Parametric Amplifier (OPA) (1420 nm) followed by second harmonic generation (710 nm) and fourth (355 nm) harmonic generation by focusing

the respective beams through beta barium borate (BBO) crystals. The data was collected at magic angle polarization (54.7 degrees) with pump beam focused to ~ 1400 m spot size and power of 0.60 mW. Samples with concentrations of 0.34 mM hydroxycoumarin with 2 mM 1-methylimidazole low base and 500 mM 1-methylimidazole high base were prepared. Samples were placed in a 2 mm quartz cuvette and degassed with Ar gas for 30 minutes prior to data collection. The chirp in the white light was accounted for using an optical gating technique.16 Femtosecond experiments showed a strong nuclear response in the toluene

solvent. Additional experiments were done to determine the increased instrument response at early times increased to 800 fs.

5.2.5 Time-Correlated Single-Photon Counting (TCSPC)

This experiment has been described in detail elsewhere17, 18 briefly the apparatus

consists of a mode-locked Ti:Sapphire oscillator (Spectra Physics Tsunami) tuned to output a 720 nm pump pulse. This pulse is frequency doubled to 360 nm using a BBO crystal. The repetition rate of the pulse is adjusted by an acousto-optic modulator (AOM) used in a single pass configuration. The femtosecond pulses selected by the AOM excite the sample and the emitted light is collected at 90° relative to excitation, focused onto the slit of a 240 mm focal length single grating monochromator, and delivered to a cooled, multichannel plate- photomultiplier tube (MCP, Hamamatsu R3809U-51). The signal from the MCP is amplified, sent into a 200 MHz constant fraction discriminator (CFD, Tennelec 454) and then used as the start pulse for a time-to-amplitude converter (TAC, Tennelec 864). The stop pulse is obtained by focusing 10% of the excitation beam onto a Si:PIN photodiode, whose output is sent into a variable delay box, then to a CFD, and finally to the TAC. The TAC’s

output is sent to a multi-channel analyzer that is interfaced to a PC. The instrument response of the apparatus is 80 ps at the FWHM.

5.2.6 Coherent Raman

Coherent Raman experiments were done by Stephen Miller from the Moran Lab. The experiments were done on an interferometer described previously.19 The data shown in this

thesis represent the solution signal minus the pure solvent spectra, or difference spectra. The light source for these experiments is a Ti:Sapphire laser which outputs 180 fs, 800 nm pulses. The Coherent Raman was done with 2 narrowband “pump” pulses, and one broadband “probe” pulse. To obtain the narrowband pulses, the fundamental from the Ti:Sapphire is stretched spectrally using diffraction gratings. A slit is then used between the diffraction gratings to select the wavelength and bandwidth of 1-2 nm. This 800 nm narrowband beam is then frequency doubled using a BBO crystal to obtain the 400 nm, 500 fs time width pulse. The broadband pulse was created by a home-built Non-collinear Optical Parametric Amplifier (NOPA). The NOPA allows for broad bandwidth pulses. It was tuned to create 710 nm pulses with approximately 50 nm of bandwidth. This pulse is then frequency doubled through a BBO crystal to produce the 355 nm broadband pulses, with a 45 fs time width. The pulse energies were approximately 50-100 nJ, and were focused to a spot size of ~ 120 micron full width half max (FWHM) in the sample. Polarization of the signal and broadband pulses were set orthogonal to the polarization of the narrowband pulse to repress the raman response from the toluene solvent.

The signals were detected by interferometry on a back illuminated CCD (Princeton Instruments PIXIS 100B) using a 0.3 m spectrograph. The signal was integrated for

3 seconds, and spectra represent 75 averages. To correct for scattered light, a mechanical shutter was placed in the broadband pulse beam, so that “on” and “off” measurements could be taken. The measurement interferograms were then processed using a Fourier transform algorithm to select out and process the signal spectra.

5.3 Results and Discussion