Cooperative Photochemical Reaction Mechanism of Femtosecond Laser‐Induced Photocoloration in Spirooxazine Microcrystals

Abstract

Microcrystalline powders of spirooxazine and spiropyran compounds do not show photocoloration under steady‐state illumination, whereas they undergo photochromism on intense femtosecond laser‐pulse excitation. We investigated the characteristic mechanism of the crystalline photochromism by studying the photocoloration of spironaphthooxazine (SNO) and its chloro‐substituted derivative (Cl‐SNO) with our femtosecond diffuse‐reflectance spectroscopic system. In particular, femtosecond double‐pulse excitation using 390+780‐nm pulses and 390+390‐nm pulses, with a variable time interval between the two pulses, was applied to reveal an intermediate species involved in the photocoloration. Although 780‐nm excitation of an intermediate produced by 390‐nm excitation did not lead to isomerization, the 390+390‐nm excitation resulted in photocoloration. The yield for SNO decreased on increasing the interval from 40 ps to 5 ns, while that for Cl‐SNO was constant. The photocoloration mechanism in the crystalline phase is considered from the viewpoint of the time‐dependent density of short‐lived transient species, and it is concluded that cooperative interactions of excited states and nonplanar open forms play an important role in femtosecond laser‐induced photochromism in these crystals.

Spironaphthoxazine switchable dyes for biological imaging

“…The ring-opening kinetics observed for SO are significantly different from those previously reported from studies of the ultrafast photochemistry of spirooxazines. 44 , 53 – 57 For instance, in the absence of the ester substituent, the corresponding spirooxazine converts to its open form with τ = 1 ns, and was shown by time-resolved resonance Raman to proceed through multiple open-form intermediates. 29 The substitution of the N -methyl by an isopropyl group resulted in a highly solvent-dependent photo-isomerization reaction.…”

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).…”

Organic Photomechanical Materials

“…The development of techniques for the rapid and selective inversion of molecular populations is a problem in the field of quantum control 94. There have already been some intriguing results in the use of chirped femtosecond pulses to increase photoreaction yields in spiropyrans,95 as well as using sequential absorption phenomena to drive spirooxazine reactions that do not occur naturally in the crystalline state under low fluence excitation 96. It is likely that novel laser excitation schemes can increase the rate and selectivity of the molecular photoreaction that powers the material’s photomechanical response.…”