Tautomerization in 2,7,12,17‐Tetraphenylporphycene and 9‐Amino‐2,7,12,17‐tetraphenylporphycene: Influence of Asymmetry on the Direction of the Transition Moment

Abstract

Femtosecond transient absorption anisotropy studies have been performed for two porphycenes of different symmetry. In 2,7,12,17-tetraphenylporphycene, the chemical identity of two trans forms implies a change in the S(0)-S(1) transition-moment direction upon tautomerization. Exploiting this phenomenon, the rates of double hydrogen transfer in both the S(0) and S(1) states (1.4×10(12) s(-1) and 2.7×10(11) s(-1) , respectively) have been determined by performing time-resolved anisotropy studies. In the asymmetric 9-amino-2,7,12,17-tetraphenylporphycene, tautomerization occurs with a similar rate in the ground state. In the S(1) state, the reaction is hindered in its vibrationally relaxed form, but the excitation spectra suggest that it may occur for an unrelaxed molecule. Unlike all porphycenes that have been studied so far, 9-amino-2,7,12,17-tetraphenylporphycene does not reveal significant changes in anisotropy owing to intramolecular double hydrogen transfer; rather, the transition-moment directions are similar in the two tautomeric forms. Analysis of the molecular orbitals allows for an explanation of the "locking" of the transition moments: it is due to a large splitting of the two HOMO orbitals, which retain the order of their energies in the two tautomers.

Fluorinated Porphycenes: Synthesis, Spectroscopy, Photophysics, and Tautomerism

“…This happens when the energy difference between HOMO and HOMO‐1 orbitals becomes sufficiently large. Such situation has been described for 9‐amino‐2,7,12,17‐tetraphenylporphycene . Inspection of Figure shows that the most likely candidate for transition moment locking is molecule 4 .…”

“…Time resolved anisotropy measurements indicate that tautomerization of 2 leads to a significant rotation of the dipole transition moments, similarly to the behaviour of symmetrical porphycene derivatives. In this respect 2 is similar to 9‐acetoxy‐2,7,12,17‐tetra‐ n ‐propylporphycene and different from 9‐amino‐2,7,12,17‐tetraphenyl‐porphycene, in case of which the directions of dipole transitions moments are “locked” by the amino group located at position 9 and rotate by only a few degrees.…”

A drastic substituent effect on the emission properties of quinone diimine models and valuable insight into the excited states of emeraldine

“…10 This time scale is also similar to photoinduced H-tautomerisation processes, 11 which interestingly in 1 must occur via a double H-transfer somewhat similar to porphyrin-like species. 12 Moreover, it was also reported that S 1 relaxation of 4-aminodiphenylamines, used as models for leucoemeraldine, occurs via N-C bond twisting at B1.6 ps. 13 Thus, the kinetic data for 1 and 2 are consistent with this mechanism but do not exclude the plausible double H-transfer for 1.…”

3,6,13,16-Tetrasubstituted Porphycene: The Missing Link in Porphycene Chemistry

“…The absorption spectrum of porphycene 1 reveals a broad Soret type band with a maximum at 376 nm and a very unusual four-banded Q-type spectral pattern with peaks at 540, 561, 584, and 622 nm (Figures , S7, and S8). This unusual Q-type spectral pattern may be attributed to the tautomerization process, which is so far noticed for 9-amino-2,7,12,17-tetraphenylporphycene. , In comparison with porphycene 2 , the lowest energy Q-band of prophycene 1 is blue-shifted by 14 nm. The UV–vis spectra of all of the metalloporphycenes exhibit an unusual splitting pattern.…”