Axial fabrications of subporphyrins have been conveniently accomplished by the reaction of B(methoxo)triphenylsubporphyrin with Grignard reagents such as aryl-, heteroaryl-, ferrocenyl-, β-styryl-, phenylethynyl-, and ethylmagnesium bromides. The axial groups thus introduced are not conjugated with the subporphyrin core. This situation leads to effective fluorescence quenching of subporphyrins when the axial group is strongly electron donating such as 4-dimethylaminophenyl and ferrocenyl groups.

Porphyrins are one of the most intensively studied families of compounds owing to their role as key pigments in photosynthesis. [1] Nowadays, the electronic and vibrational relaxation processes that occur within porphyrins are thoroughly understood. Furthermore, it is now known that modifications of the peripheral substituents and altering the identity of the metal atom present in the central cavity have a significant impact on the photophysics of the porphyrin system. These vital pieces of information have made it possible to unravel the fundamental mechanism of photosynthesis as well as to develop applications of porphyrins across a wide range of research disciplines.In organic photovoltaic (OPV) cells, for instance, the efficiency of the light-to-current conversion processes is limited by the amount of energy dissipated in the early stages following light absorption. The excess photon energy is usually dissipated by ultrafast internal conversion (IC) and vibrational energy redistribution. [2] Thus, a complete understanding of the dynamics of higher excited states is essential for an improvement of the performances of OPV devices. Gaining insight into the dynamics of higher excited states of molecular systems has been difficult, as time-resolved absorption spectroscopy measurements often give rise to congested spectra. On the other hand, because of a lack of spectral congestion, fluorescence spectroscopy and timeresolved fluorescence decay profiles allow the unequivocal assignment of dynamics observed for excited states. In this regard, the observation of emissions from higher excited states has offered a unique opportunity to thoroughly understand the mechanism of electronic relaxation processes. Porphyrins are amongst a few molecules that display dual fluorescence from the second excited singlet state. By virtue of porphyrin S 2 fluorescence, the dynamics of higher excited states of porphyrins, such as free-base tetraphenylporphyrin (H 2 TPP), magnesium tetraphenylporphyrin (MgTPP), zinc tetraphenylporphyrin (ZnTPP), and zinc phtalocyanine tetrasulfonate (ZnPcS 4 ), have been extensively studied. [3] In recent years, subporphyrins, which may be considered as ring-contracted porphyrin analogues because of a regular arrangement of three constitutional pyrrole rings and three carbon atoms in an alternate fashion, have emerged as a novel class of promising functional pigments. [4] Unlike for porphyrins, little is known about the photophysical behavior of subporphyrins, and until now, the observation of their S 2 fluorescence has not been reported. Thus, information regarding the higher excited states of subporphyrins is of great interest in its own right, and would lead to an in-depth understanding of the nature of the excited electronic states of subporphyrins.Although meso-triaryl-substituted subporphyrins exhibit intriguing optical properties, such as nonplanar aromaticity and exceptionally strong electronic coupling with aryl substituents in the meso position, their orbitals have features similar to those of ...

S2‐Fluoreszenz: Subporphyrine sind faszinierende Verbindungen, aber die Eigenschaften ihrer höherliegenden angeregten Zustände waren bislang nicht bekannt. Die S2‐Fluoreszenz dieser Subporphyrine wurde jetzt untersucht; interne Umwandlung des S2‐Zustandes in den S1‐Zustand geschieht innerhalb von ungefähr 300 fs. Die zwei Übergangsdipolmomente der entarteten S2‐Zustände liegen trotz der C3‐Symmetrie des Subporphyrins orthogonal zueinander vor.

Die S2‐Fluoreszenz und die photophysikalischen Eigenschaften der höherliegenden angeregten Zustände von Subporphyrinen werden von A. Osuka, D. Kim und Mitarbeitern in ihrer Zuschrift auf beschrieben. Diese S2‐Fluoreszenz von Subporphyrinen war bislang nicht beobachtet worden. Gleichermaßen wird der Große Wagen meist als ein Sternbild von sieben Sternen beschrieben. Ein achter Stern Alcor existiert jedoch ebenfalls, wird aber aufgrund seiner Nähe zu dem Stern Mizar nur schwerlich erkannt.

The S 2 fluorescence … … and the photophysical behavior of the higher excited states of subporphyrins are described by A. Osuka, D. Kim, et al. in their Communication on page 12632 ff. The S 2 fluorescence of subporphyrins had not been described to date. Similarly, the Big Dipper is recognized as an asterism of seven stars. However, the eighth star, Alcor, definitely exists, although its close proximity to the star Mizar renders it more difficult to resolve.