Electronic Supporting Information I. Experimental Methods Chemicals and Methods 4-Thiothymidine (4-thioThd, 99% purity) was obtained from Carbosynth Limited, Berkshire, UK and used as received. 1-butyl-1-methylpyrrolidium bis(trifluoromethylsulfonyl) imide, [BMPyrr][NTf 2 ], was obtained from Sigma-Aldrich (≥ 98.0% purity and ≤ 0.2% water content). Acetonitrile (99.9%) was bought from Fisher Scientific. Steady-state absorption and emission spectra were measured at room temperature using a Cary

more »

... 00 and Cary Eclipse spectrophotometers (Varian, Inc.), respectively. Background corrections were performed by subtracting a solvent-only scan under the same experimental conditions, as described elsewhere. 1 Fluorescence spectra were taken at medium PMT voltage with slit-widths of 5 nm and averaging times of 3.2 s. The absorption of the solutions for the emission experiments was 0.2 at the 340 nm excitation wavelength in a 1 cm 2 cell. The slitwidths were increased to 10 nm for the excitation spectra. The pump-probe femtosecond transient absorption spectrometer and data analysis procedure have been described in detail elsewhere. 2 In short, a Quantronix Integra-i/e 3.5 Laser generating 100 fs pulses at 800 nm with a repetition rate of 1 kHz is used to pump an optical Supplementary Material (ESI)