6,8-Dinitro spiropyran

The spiroindolinobenzopyran 2 is a classical example of spiropyran and is easily prepared by the condensation of l,3,3-trimethyl-2-methyleneindo-line (Fischer s base) and salicylaldehyde in anhydrous ethanol or benzene (Scheme 2).ia The nucleophilic attack of Fischer s base on the carbonyl group (like an enamine) gives an aldol product, which undergoes ring closure followed by dehydration. This condensation is reversible therefore, an exchange of the salicylaldehyde component of spiropyran with a different salicylaldehyde is possible. For example, when a solution of spiropyran 2 (Scheme 2) was refluxed with 3,5-dinitro-substituted salicylaldehyde, the open form of 6,8-dinitro-BIPS was obtained.2... 

Figure 2 FUR spectta of 6-nitto-BIPS spiropyran closed form, 6-nitto-BIPS photoaggregate, and 6,8-dinitro-BIPS merocyanine.

Spiropyran merocyanine specffa shift markedly to the blue as the solvent polarity increases [4,25], as shown in Fig. 7a for 6,8-dinitro-BIPS merocyanine. This is generally accepted to imply that they have a zwitterionic character caused by the donation of electron density from the indoline nitrogen to the phenolic C9 oxygen [4,25]. The rational behind this assignment to the zwitterion is based simply on the fact that if the zwitterion is in the highest occupied molecular orbital (HOMO) state, then the corresponding quinoidal resonant form is in the lowest unoccupied molecular orbital (LUMO) state and changing the solvent to a more polar one... 

The nanosecond studies that have been performed on similar systens [48-51,104] are apparendy at odds with the previously mentioned femtosecond study. However, 532-nm nanosecond laser pulses have different characteristics compared to a 388-nm femtosecond pulse and this may explain the discrepancies. Again, different substiments can also affect the outcome, and certainly the 6,8-dinitro-BIPS system is not a standard to gauge all other spiropyran systems. In fact, no typical molecule or molecular system exists which can be used as an example for describing the behavior of all other nominally similar systems. 

Takeda et al. [104] found that the steady-state irradiation of A-octadecyl 6,8-dinitro-BlPS resulted in the formation of the closed form. This is in agreement with the femtosecond studies. However, the fade reaction showed three components when monitored at 500 nm in 1,2-dichloroethane. The first component was manifest as a bleach that occurred within the instrument response time of 25 nsec. This was followed by two longer components with 240-nsec and 3.4 psec lifetimes. The spiropyran ring-closed form evolved with two exponential components with lifetimes of 620 nsec and 4 p-sec. The Af-octadecyl group may affect the outcome compared to the A-methylated form. 

Nitro-substitution especially at the 6-position of BIPS opens up a triplet pathway for photo-isomerization. This pathway runs in parallel to the singlet manifold. This increases the yield and, in turn, may lead to photo-aggregation that is observed for these compounds. Photochemical ring closure to the spiropy-ran form is more efficient for these 6-nitro-substituted compounds. The photochemistry of 6-nitro-BIPS merocyanine is similar to that of unsubstituted BIPS(s) however, the 6,8-dinitro compound efficiently cyclizes upon excitation to form the spiropyran closed form via a singlet manifold. 

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