Abstract
The photochromism of N‐phenyl‐rhodaminelactame (3′ ,6′ ‐bis‐(diethylamino)‐2‐phenyl‐spiro[1H‐isoindole‐1,9′[9H] Xanthaen]‐3‐[2H]‐one) was investigated in solutions of acetonitrile, dioxane and mixtures of dioxane with H20, methanol and ethanol by laser flash time resolved absorption measurements. The colouring reaction kinetics and the relative quantum yield of the red product were measured using nanosecond excimer laser excitation at 308 nm. In most cases, first order kinetics was.observed. The first order rate constants and the quantum yield were found to be dependent on the solvent and the number of laser pulses that had previously irradiated the solution. The thermal fading reaction occurred on a millisecond timescale and showed large deviations from first order kinetics in all solvents. In addition, picosecond laser pulses at 285 nm were used to measure the fluorescence lifetime of the S1 state of the rhodamine derivative with picosecond resolution. Single exponential decays were observed in all solvents and the lifetime ranged between 3 and 6 nanoseconds. We conclude that the photoreaction does not proceed directly from the S1 state, and propose a mechanism for the photochromic process, in which an intermediate decomposes to rotameric isomers of the coloured product, which are not equilibrated on the millisecond time scale of the reverse reaction.
Highlights
Since the first studies on photochromism of organic molecules a wide variety of compounds have been investigated in this area because of their possible applications in optical switches and optical storage devices. 2 The most important groups of compounds are the spiropyrans,3’4 triarylmethanes,5 polymethincolours,6 and substances that show e-z isomerization of double bonds. 7 In most of these systems the photochromic process proceeds by photochemically induced heterolytic bond fissure to a normally coloured zwitterionic structure followed by thermally induced reformation of this bond back to a colourless form of the molecule
We propose a more detailed mechanism of the photochromic process unifying microsecond absorption measurements and picosecond fluorescence measurements
The spectrum of the coloured form was obtained by illuminating the fresh solution with 50 laser pulses (308 nm, 150 mJ, 50 Hz) and subsequently recording the UV/VIS absorbance
Summary
Since the first studies on photochromism of organic molecules a wide variety of compounds have been investigated in this area because of their possible applications in optical switches and optical storage devices. 2 The most important groups of compounds are the spiropyrans,3’4 triarylmethanes, polymethincolours, and substances that show e-z isomerization of double bonds. 7 In most of these systems the photochromic process proceeds by photochemically induced heterolytic bond fissure to a normally coloured zwitterionic structure followed by thermally induced reformation of this bond back to a colourless form of the molecule. 8. Mechanism 1-gives the simplest form for the photochromic process for the title compound. In this work we tested if this simple mechanism can explain the photochromic behaviour of this compound by investigating the influence of solvents and irradiation time on the relative quantum yield and reaction rate. The laser beam was focussed by a cylindrical lens into a rectangular quartz cell with 2 mm pathlength along the excitation axis. Behind the quartz cell the reference light was focussed onto the slit of a Zeiss MQ 3 monochromator. A distributed feedback dye laser (285 nm wavelength, 10 ps pulsewidth, 50 #j energy) was used for excitation of the sample. The fluorescence signal was detected by a streak camera (Hamamatsu C 1537) in the range of [400-500] nm (maximum of fluorescence emission) perpendicular to the excitation beam (40 ps time resolution)
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