The cis-cisoid X Isomer

In 1962, Fischer and co-workers reported that ultraviolet irradiation of spiropyrans results in formation of an unstable isomer, X, of the colored modification and proposed the existence of an open cis-cisoid isomer.16 The geometry of this isomer resembles very closely that of the parent spiropyran, i.e the central C3=C4 bond has a cis-cisoid conformation  

The role of the X isomer in the photocoloration of 6-nitro- l, 3, 3 -trimcthyl-spiro 2H-1 -bcnzopyran-2,2 -indoline (6-nitro-BIPS, 6) has been discussed by several authors. 

In a picosecond transient absorption spectroscopy study of 6-nitro-BIPS derivatives, Krysanov and Alfimov8 identified a transient absorbing at 440 nm and proposed its formation within 8 ps from the excited singlet state SPNo2 of the 

In 1990 Zhang el alv identified, by means of picosecond and femtosecond transient electronic absorption spectroscopy, a metastable species X that formed in less than 100 fs after excitation, at 295 nm, of 6-hydroxy-1, 3, 3 -trimethyl-spiro 2//-l-bcnzopyran-2,2 -indolinc (6-OH-BIPS, 7). 

In that study, it was empathized that the lifetime of the X isomer determined the extent of the SP degradation. 

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In a quantitative study of the photostability of spiropyrans, Malkin et al.11 concluded that the photodegradation quantum yield d is independent of the heterocyclic part and is determined instead by the structure of the chromene part of SPs. According to these authors, photodegradation of nitro-substituted SPs originates from the reaction of the cis-cisoid X isomer with solvent or impurities (Scheme 6) ... 

According to Malkin et al.11 photodegradation is due to the reaction of the cis-cisoid X isomer with the solvent S and/or impurities (Scheme 20) ... 

The photocoloring reaction for spiroindolinobenzopyrans with a nitro group proceeds mainly via the formation of the excited triplet state of the molecule. The reaction proceeds partly from the triplet state [(SP )3] of the spiropyran to the triplet state (X)3 of the cis-cisoid isomer which subsequently transforms into the CF and partly from (SP )3 to the CF. This process from (X)3 to the colored form is accelerated by the presence of atmospheric oxygen (Scheme 6).2,28 For the photocoloring reaction, the participation of singlet or triplet state depends not only on the substituent but also on the nature of the heterocyclic component. 

As for the multiplicity of the excited state from which the X isomer is generated, we have already mentioned that Krysanov and Alfimov8 proposed a mechanism in which the vibrationally excited triplet state of SPNo2 acts as the precursor to the cis-cisoid ring-opened photoproduct X. On the other hand, Kalisky et al.19 claimed that X is formed from SPNo2 in less than lOps. An excited singlet precursor SP is also postulated for the formation of X, with the... 

From what has been so far reported, it would seem that the intermediacy of the cis-cisoid ring-opened nonplanar intermediate X is much better documented for nitrospiropyrans than for spiroazines, for which the matter is still controversial. No studies have so far been reported on the importance of the X isomer in the photodegradation mechanistic schemes. 

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