Tautomerization in the Lowest Excited Singlet State

Fluorescence of porphycene embedded in rigid media was found to be depolarized, both at room temperature in the poly(vinyl butyral) matrix [79] and at low temperatures in glasses [30, 80[ (Fig. 8.11). In a rigid environment, where the reorientation of an excited chromophore is not possible, the direction of the Sq-Sj transition moment should be the same in absorption and emission, leading, for excitation into Sj, to the anisotropy value of 0.4. Instead, the observed values 

The depolarization of fluorescence has been observed at temperatures around 100 K not only for 1, but also for the two derivatives, lb and Ic. in contrast, the measurements performed under the same conditions for le revealed no sign of depolarization. The textbook values of the anisotropy were obtained, i.e. 0.4 and about -0.2 for excitation into Sj and S2, respectively (Fig. 8.11). The octaethyl derivative le is the porphycene with the largest separation (2.80 A) between the hydrogen-bonded nitrogen atoms and should therefore exhibit the slowest tautomerization kinetics. It was thus concluded that the reduced anisotropy values observed in three different porphycenes are caused by excited state tautomerization [30, 80]. As shown in Fig. 8.12, the interconversion between the two trans tautomers changes the direction of the transition moment. Therefore, only a part of the excited state population emits fluorescence polarized parallel to that of the transition moment in Sg-Sj absorption (this fraction should approach 0.5 if the tautomerization is fast compared to the excited state lifetime). For the remaining frac- 

Second, careful analysis of anisotropy data is useful not only with respect to investigation of structural and kinetic aspects of tautomerization, but also as a means to obtain detailed spectroscopic information about transition moment directions. Both procedures will be described below in more detail. 

Tautomerization as a Tool to Determine Transition Moment Directions in Low Symmetry Molecules 

The directions of transition moments in every chromophore are dictated by molecular symmetry. For the cis tautomers of porphycene ( 2 point group), only three mutually orthogonal transition moment directions are allowed. On the other hand, the trans form is of 2 symmetry and, therefore, any direction in the molecular plane is possible, as well as the direction perpendicular to the plane. The determination of transition moment directions in such low symmetry molecules is not an easy task. However, in the case of narcissistic type of reactions exemplified by trans-trans conversion in 1, one can take advantage of an additional symmetry element introduced by the tautomerization process. Double hydrogen transfer converts the molecule into its image, with the horizontal and vertical mirror symmetry planes perpendicular to the molecular plane (Fig. 8.13). Thus, tautomerization results in the rotation of each in-plane transition moment direction. The angle of rotation is twice the value of the angle formed by a particular transition moment with the horizontal (or vertical) in-plane axis. It can be shown [80] that, for a fast excited state process, which results in equal population of both trans tautomers, the measured fluorescence anisotropy r will be expressed by the formula  

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