Merocyanine isomers

The reverse reaction, the photochemical ring opening of sphopyranes (22b), takes place by absorption ia the short-wave uv region of the spectmm and the merocyanine isomer (22a) is obtained. The electron transition of (22a) is ia the visible spectral region, whereas (22b) is colorless. As a result, the dye solution can change from colorless to a colored solution (87,88). These photochromic reactions can be used for technical appHcations (89). 

Much discussion about the photo-formation of the merocyanines is based on the possibility of four stable merocyanine isomers that are trans about the 13-bond on the methine bridge. Commonly, these are referred to as TTC, CTT, CTC, and TTT [6-8,28,36] and they are shown in Scheme 4. Note that these are possible or hypothesized structures. Corner et al. even suggest that isomers cis about the central 3-methine bond may have stability and equilibrate with the more planar isomers [14,46-51]. Certainly, the four isomers cis about the cental (3-methine bond could have transient stability. 

Another explanation for their resonance Raman results could be a change in the zwitterionic nature of the merocyanine isomers in the different solvents which may result in changes in the Raman transition probabilities, or the spectral changes could be due to solvent shifts of the absorption spectrum, resulting in a change in the relative contribution of the different vibrational modes to each resonance Raman spectrum. We note that in the same article, the authors report the transient absorption spectra of the merocyanine forms, which clearly show that the BIPS spectrum in cyclohexane has more discrete vibrational modes than are observed in the polar solvents, which show more spectral broadening. Al-... 

Clearly, the TC isomer has its spectrum red-shifted compared to the TT isomeric form. Although spectra for individual isomeric forms of spiro-oxazine and spiropyran merocyanines are not available, it has been shown that the different isomers have very different spectra. In fact, Abe and co-workers [89] have shown that the merocyanine of spiro-oxazine can be converted photochemically between two states having different absorption spectra. Even earlier work carried out at low temperature and or with visible-light irradiation suggested that the spiropyran merocyanine isomers also exhibit significant differences between their absorption spectra [6-8]. 

Table 2 Selected Bond Lengths for the 6,8-Dinitro DIPS TTT Merocyanine Isomer...

Schneider et al. [63] investigated the photochemistry of the spiro-oxazine merocyanines pumping and probing at 570 nm in acetonitrile. The found that the solution bleached within the <5-psec pulse duration. The bleached state recovered with at least a biexponential behavior, and from their fluorescence decay measurements, three exponentials were required to fit the decay. They attribute these findings to the possibility of three merocyanine isomers that are in equilibrium. Their compounds feature geminal ethyl groups on the indoline moieties and this may influence the system as compared to NOSIl. 

In summary, the merocyanine lifetime depends on its particular isomeric form and this, itself, is affected by the solvent and substituents. The longest merocyanine isomer singlet-state lifetimes reported are 4 nsec and the shorter ones are tens of picoseconds. After excitation of the merocyanine, a bleached state can form, which leads to the closed form in polar solvents, but the full ring closure is blocked in nonpolar media and the bleached state recovers to its original state. The bleached state is possibly an isomer cis about the central p-methine bond. Scheme 17 describes the transient bleaching of the merocyanine state. 

Aramaki et al.30 examined the photochromic reactions of spirooxazines by picosecond time-resolved Raman spectroscopy. Vibrational resonance Raman spectra of the merocyanine isomer(s) recorded over a 50-ps-1.5-ns interval did not change. This indicated that the open ring opening to form a stable merocyanine isomer or the distribution of isomers31 was complete within 50 ps and that the isomer(s) distribution remained unchanged for at least 1.5 ns. 

Such rate of decoloration was initially interpreted as being the sum of two or three exponential rate proc ses, each char terized by its own rate constant (kj > k2 > ks), which should correspond to a different merocyanine isomer (Kg. 2). 

The existence of four merocyanine isomers had indeed been demonstrated previously The decoloration rate equation can be written... 

It is therefore admitted that the photochemically induced contraction/length recovery phenomena are related mainly to the photochemical interconversion of the merocyanine isomers, and at a smaller extent to local thermal effects (20-30%). 

They found that the first merocyanine isomer is formed in its ground state in 0.9-1.4ps. The internally hot (molecular temperature ofca 900 K) primary merocyanine (MC) product isomerizes on the picosecond time scale to give a distribution of MC isomers. The open merocyanine form of both SPs and SOs may, in principle, originate from either a singlet ( SP SO) or triplet (3SP 3SO) excited state of the closed spiro structure. 

Figure 1. Photochemical reaction process of 6-hydroxy-1, 3, 3 -trimethylspiro[2H-1 -benzopyran-2,2 -indoline] (6-OH-BIPS 7). The reaction proceeds along the diabatic surface (dashed line) i.e., direct dissociation of the Cipjro-0 bond takes place. B is one of the several possible merocyanine isomers, and X refers to the initial metastable product formed soon after the C,pjro—O bond cleavage. (Reprinted from Ref. 10 with permission of the American Chemical Society.)...

Upper part Second harmonic generation (SHG). Lower part Optical absorption of the merocyanine isomer at 7=532 nm. Adapted from Atassi et al. [59] with permission from the American Chemical Society. 

The photo-, thermo- and solvatochromic properties of 2,3-dihydro-1 ,3 ,3 -trimethyl-6-nitrospiro[l-benzopyran-2,2 -17/-indole] (BSP) and its photo-induced merocyanine isomer (MC) were investigated in phosphonium based ILs by UV-vis absorption spectroscopy and the kinetics and thermodynamics of the BSPMC equilibrium were found to be sensitive to the nature of the anion. For example, the MC Xmax shifted from 560 nm to 578 nm in solutions of [Me(C4H9)3P][tos] and [Ci4H29(C6Hi3)3P][dca], respectively. The BSP isomer was highly favoured at equilibrium in the ILs studied. 

In the present paper we review some of our findings on the reversible photo-induced wettability changes of polymer surfaces doped with photochromic spiropyran molecules. We illustrate how the hydrophilicity of the surfaces is enhanced upon UV laser irradiation when the embedded non-polar spiropyran molecules convert to their polar merocyanine isomers and how this process is reversed upon irradiation with green laser pulses. Moreover, we show that, when the doped polymer surfaces are micropattemed by soft molding lithography or by interferometric... 

Other applications of photoswitchable materials on SAMs are the UV-controlled complexation of metal ions to the merocyanine isomer of spiro-naphthoxazine-thioethers (Fig. 41a) [214]. The long saturation times for the pure spironaphthoxazine-SAM at complexation with zinc ions is an indication for a densely packed spironaphthoxazine-monolayer, with only a small fraction of open merocyanine isomer due to steric constraints. 

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