Excited porphycene

The fluorescence excitation spectrum of parent porphycene isolated in an ultracold supersonic jet consists of doublets, separated by 4.4 cm i (Fig. 8.6). This behavior contrasts with that of porphyrin, which exhibits normal behavior, with single peaks corresponding to particular vibronic transitions [73]. Upon replacing one or two inner protons with deuterons, the splitting disappears (which, given the experimental resolution, means that it becomes less than 0.1 cm i). Adding... 

For alkyl-substituted porphycenes la and lb, with smaller N-N separation and thus stronger hydrogen bonds, one would expect a larger tuimeling splitting. The experiments in the supersonic jets reveal a quite complicated pattern, different from that of parent porphycene [76]. Two peaks, separated by 12.5 cm are observed in the 0-0 region of the fluorescence excitation spectrum (Fig. 8.8). This doublet structure does not disappear for singly and doubly deuterated compounds. Each of the peaks consists of doublets in undeuterated species and is associated with a different vibronic structure. Two low frequency modes (32 and... 

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-... 

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 ... 

The differences in tautomerization in the lowest excited singlet and triplet states of 1 and 2 reflect the pattern observed for the ground state. In contrast to porphyrin, both trans and cis structures are detected in porphycenes, separated by a very small energy gap. The tautomerization barriers are also much lower in 1. Finally, at least in the case of the lowest excited singlet state, synchronous transfer of two hydrogen atoms seems to be preferred over a stepwise mechanism, even when both forms are present (la and lb). 

Ground- and excited-state tautomerism in porphycenes 06ACR945. 

One way of proving this tautomeric reaction in the excited state is by measurement of fluorescence anisotropy depolarization. For porphycene, this turns out to be temperature dependent. From these measurements it could be demonstrated... 

Figure 3.32 Excitation and tautomerism of porphycene. (Reproduced from Berkesi et al.

Og = 2.55 A and bg = 3.61 A). A molecule can be switched between two orientations through thermal- or STM-induced excitation. (c) Chemical structures of a porphycene molecule with the cis tautomer state. The switching between the two orientations in (a,b) corresponds to a cis-cis tautomerization. (Reproduced from Kumagai et al. [31]. Copyright (2014), with permission of Nature Publishing.)... 

---