Giovanna Favaro

Thermally reversible photoconversion of spiroindoline-naphtho-oxazines to photomerocyanines: a photochemical and kinetic study

Abstract In this paper, a treatment of the spectroscopic data from thermoreversible photochromic systems is described which allows the molar absorption coefficient of the metastable photomerocyanine, the quantum yield of the colour-forming reaction and the kinetic parameters of the thermal bleaching to be determined. The results obtained for five spiroindoline-oxazines indicate that “colourability” on steady state irradiation depends on the photomerocyanine molar absorption coefficient, quantum field and bleaching rate coefficient. For the molecules investigated, the “colourability” markedly depends on temperature, due to the temperature dependence of the bleaching reaction. The results obtained are compared with literature “colourability scales”, determined empirically by comparison of different systems excited by pulsed light.

Competition between vibrational relaxation and photochemistry: relevance of vibronic quantum effects.

Abstract Two molecules showing photochemistry but no fluorescence have been investigated at 80 K in a rigid matrix regarding the behavior of the quantum yield for bond fragmentation as a function of the vibrational/vibronic level and electronic excited state. A new equation was developed to determine the photochemical quantum yield under ambient conditions (80 K). The levels/bands involved were those within a given vibrational progression, in different progressions as well as in combination. The yield was low (Φ = 0.1) with excitation into the n = 0 level of S1 but very rapidly increased with excitation into higher levels whether they were harmonics or combination levels. A parallel result was observed upon excitation into S2. Vibrational relaxation/deactivation occurs only between levels of the same vibrational progression. Deactivation from the 0 level of S2 does not occur via levels of S1. The photochemically active modes correspond to the vibrational modes present in the region of the molecule where bond breakage occurs. These results add further proof of the complex nature and number of processes that can occur within excited states of photochemically active molecules.

Photocyclisation of 2-pyridyl phenyl ketone. A reaction driven by hydrogen bonding

The photochemistry of 2-pyridyl phenyl ketone (2-PPK) has been studied in water, ethanol and other organic solvents of different polarity and hydrogen-bonding ability. Steady-state and laser flash photolysis (ps and ns resolution) techniques were used to determine the photoproducts and the reaction intermediates. The reactive state is the lowest n,π* carbonyl triplet. Three different photoreaction paths were recognised leading to cyclisation and reduction products. The relative contribution to the overall reaction by the different pathways is governed by the hydrogen-bonding ability of the solvent.

Unusual UV (λexc = 303 nm) and visible (λexc = 574 nm) activated photochromism of an indeno-fused naphthopyran

In this work a completely novel photochromic mechanism exhibited by an indeno-fused naphthopyran (chromene) is presented. The studied molecule is the first case of a chromene-type compound exhibiting both photochromism and thermochromism. These phenomena were investigated in ethanol. Thermochromism was detected by monitoring absorption spectra in the 285–320 K temperature range: the enthalpy of the thermocoloration reaction was determined (ΔH = 25 kJ mol−1) and the equilibrium constant was estimated (Keq ≈ 10−3). For the photochemical investigation, carried out in the 200–270 K temperature range, continuous monochromatic light was used for excitation. The photocoloration was observed under two distinct stimulations: UV-irradiation (λexc = 303 nm), which is normally used for photochromic chromene activation, and visible-irradiation (λexc = 574 nm) of the thermally equilibrated solution. Two coloured species are involved in the photochemical and thermal processes. One of them, P, is photochemically produced by UV irradiation of the colourless form, while the other, T, is present in thermal equilibrium with the closed form. When the thermally equilibrated solution is irradiated with visible light, T is converted to P. This system not only results in a cyclic on–off chromogenic device, which can be switched-on by UV radiation, but it also exhibits on–off functionality using lower energy visible light. Such behaviour makes this photochromic system exceptionally efficient upon exposure to sunlight.