Cooperative Photochemical Reaction in Molecular Crystal Induced by Intense Femtosecond Laser Excitation:  Photochromism of Spironaphthooxazine

Abstract

Photochromism of indolinospironaphthooxazine (SNO) microcrystalline powder was investigated by steady state and time-resolved diffuse reflectance spectroscopy using a femtosecond laser as a light source. When laser intensity is weak, the photoinduced ring-opening reaction in a picosecond time region and subsequent thermal ring-closure one in a nanosecond scale take place in the crystal, not leading to permanent photocoloration of the powder. The same sample shows photochromism upon intense femtosecond laser excitation. The absorption spectra after laser excitation are similar to those of the photocolored form in solution. The spectral shape is independent of the laser fluence, whereas the yield increases nonlinearly with the fluence. To reveal the photocoloration mechanism, we examined the excitation fluence dependence of the transient absorption spectra and elucidated the photocoloration behavior by femtosecond double pulse excitation varying the delay of the two pulses. These experimental results show that cooperative interactions between the excited states and short-lived intermediates are important to generate a long-lived photomerocyanine. Femtosecond induced photocoloration will be discussed in terms of transient and local lattice deformations, and a cooperative photochemical reaction model is proposed.

Unexpected chromogenic properties of 1,3,3-trimethylspiro(indoline-2,3′-[3H]naphtho [2,1-b][1,4]oxazine) in the solid phase: photochromism, piezochromism and acidichromism

“…16 Since the bands observed in solid phase overlap, even if produced by different stimulations, they can be assigned to the same M isomer, surrounded by a similar micro-environment. The spectral properties of M in different media are summarized in Table 1.…”

“…(12) and (14)] steps, eqns. (15) and (16) are obtained, which describe the overall kinetics in diluted acid solution and in concentrated acid solution, respectively:…”

“…In fact, even if photochromism and piezochromism were expected for two cationic spiropyrans in crystalline form, 15 spirooxazine and spiropyran powders have never been found to show colouration under either UV irradiation or pressure. Only recently, by using intense UV pulsed excitation (femtosecond time scale), 16,17 was photocolouration observed in solid SO and other spiro compounds. Under weak-intensity laser excitation, a non-planar open form was produced, which absorbed at low energy (around 740 nm) and thermally faded to the ground state spiro isomer without yielding the trans-planar M. In order to obtain the transoid planar structure, which absorbs around 600 nm, more intense irradiating light was needed.…”

Photochromic properties of phenanthroline-annulated spirooxazine in the solid state

“…15 An interesting effect is the formation of the short lived (characteristic lifetime about several nanoseconds) isomers of the nonplanar photocol ored form upon irradiation with femtosecond laser pulses of microcrystalline powders of the spiro compounds that exhibit no photochromic properties under the conditions of steady state irradiation. [16][17][18][19] In this work, the photochromic transformations of phenanthroline annulated 3,3 trimethylspiro[indoline 2,2´ bipyrido [3,2 f] [2,3 h] [1,4]benzoxazine] (SPO1) 20 in the polycrystalline state (see Scheme 1) were found and studied.…”

“…37 The character istic lifetime of the nonplanar cisoid isomers formed upon the photoexcitation of spiropyrans and spirooxazines is several nanoseconds in the polycrystalline state. [16][17][18] The broad distribution of molecules of the B form over planar transoid isomeric forms appears as a result of local melting of a microcrystal of SPO1 under the action of a laser pulse. The effect of nonequilibrium distribution over isomeric forms is manifested in the transient absorp tion spectra and kinetics of formation of the absorption of the В form in experiments on laser flash photolysis (see Figs 1 and 5).…”