Structural Features of SP Molecules

The efficiency of this hyperconjugation must depend essentially on the geometric configuration of the LEPs of the N(l ) and 0 atoms and the Cspi[O-0 and Cspin,-N(l ) bonds, on the one hand, and on the electronic state ofthe N and 0 atoms, on the other hand. In all of the SPs studied, the spiro center has a sterically strained structure. Because of the rigidity of the tetrahedral structure of the spiro center and the similar conformation of the indoline ring, the Cspiro—O bond makes an angle (t) of 165-171° with the LEP of the N(l) atom (Table 1). 

These structural features lead to different orientations of the 7t-electron density of the O-atom LEP and the Cspjro—N(l ) bond (angle A, Table 1). In SP2, SP3, SP4b, SP6, and SP7, the corresponding angle is only 116-130° in SP4a and SP9-SP11, it is 141-145° and in SPS and in SP1, it is 160.2° Thus, the k(0) a [Cspiro—N(l )] interaction, which has an opposite effect to the nlNOOl-o Cspiro-O) interaction, will increase with an increase of the A value, which means that the benzopyran moiety is bent more toward R.  

The Nature of Structural Changes in SP Molecules upon Photoexcitation 

The specific structural features of SPs affect the photochromic properties of these compounds. Since photophysical and photochemical properties of organic molecules depend essentially on their structure in the excited state, we shall examine the nature of the structural changes in these molecules upon photoexcitation. The nature of such changes, in turn, depends on the type of electronic transition that converts the molecule into a photochemically active state (PCAS). The converstion of SP1-SP4 molecules to a PCAS upon UV irradiation with k 300 nm is known to be due to electronic transitions of the n-n type in molecules without EA substituents and of the nn -jt 7t type in molecules with EA substituents in the benzopyran moiety.29 These transitions are characterized by intramolecular charge transfer (ICT), mainly from the l orbital of the oxygen atom and the 7t orbital of the C(3)=C(4) bond to the phenyl group and the EA substitutents. 

This ICT must lead to a decrease of electron density on the 0(1) atom. For 277-chromenes, charge transfer from the 0 atom during the S0 - S3 transition was found to be 0.060 e [CNDO(S) method].30 A decrease of the charge on the O atom is accomplished at the expense of a electrons of the Cspiro—O bond, which provides the driving force for further polarization. As was shown above, this type of redistribution of electron density leads to an increase of the [N(r)]- T (cspiro-o) interaction in SP molecules and a further contraction of the CspitoN(l ) bond and weakening and elongation of the Cspiro-0 bond in the excited state. 

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