Nonlinear Optical Properties and Excited-State Dynamics of Highly Symmetric Expanded Porphyrins

Abstract

A strong correlation among calculated Nucleus-Independent Chemical Shift (NICS) values, molecular planarity, and the observed two-photon absorption (TPA) values was found for a series of closely matched expanded porphyrins. The expanded porphyrins in question consisted of [26]hexaphyrin, [28]hexaphyrin, rubyrin, amethyrin, cyclo[6]pyrrole, cyclo[7]pyrrole, and cyclo[8]pyrrole containing 22, 24, 26, 28, and 30 pi-electrons. Two of the systems, [28]hexaphyrin and amethyrin, were considered to be antiaromatic as judged from a simple application of Hückel's [4n + 2] rule. These systems displayed positive NICS(0) values (+43.5 and +17.1 ppm, respectively) and gave rise to TPA values of 2600 and 3100 GM, respectively. By contrast, a set of congeners containing 22, 26, and 30 pi-electrons (cyclo[n]pyrrole, n = 6, 7, and 8, respectively) were characterized by a linear correlation between the NICS and TPA values. In the case of the oligopyrrolic macrocycles containing 26 pi-electron systems, a further correlation between the molecular structure and various markers associated with aromaticity was seen. In particular, a decrease in the excited state lifetimes and an increase in the TPA values were seen as the flexibility of the systems increased. Based on the findings presented here, it is proposed that various readily measurable optical properties, including the two-photon absorption cross-section, can provide a quantitative experimental measure of aromaticity in macrocyclic pi-conjugated systems.

Nonlinear Optical Properties of Porphyrins

“…[151] In expanded porphyrins this can be attributed to the large p-conjugation, a more flexible conformation as a result of more meso links and an increase in the number of heteroatoms (e.g., S, Se). Similar data were reported by Osuka's group for related systems [73,152] and for N-confused hexaphyrins. [153] Especially the cyclo-8 expanded systems 111 [154] reached exceptionally high values.…”

“…A typical case is a recent report on the synthesis and nonlinear optical properties of seven different expanded porphyrins (61)(62)(63)(64)(65). [73] The preparation of these expanded porphyrins followed different synthetic pathways, for example compounds 61, 63, 64 were synthesized by acid-catalyzed condensations of the appropriate precursor affording the expanded porphyrins in moderate yields. Other compounds used for optical studies are related to the classic sapphyrin and texaphyrin systems.…”

“…These clearly show the interrelationship between aromaticity, polarizability, and conformational aspects of the p-conjugation pathway on the TPA values. [73] Macrocycle distortion significantly affects the optical limiting properties of a porphyrin. [124b] Studies have also been reported for baskethandle porphyrins 87 with short straps.…”

Zn(II)porphyrin Helical Arrays: A Strategy to Overcome Conformational Heterogeneity by Host-Guest Chemistry

“…Additionally, referring to the previous reports, we can obtain information on delocalized π-conjugation pathway, excitonic coupling and molecular planarity from two-photon absorption (TPA) measurement. [14][15][16] In order to check the enhanced excitonic interactions accompanied by structural rigidity and delocalized π-conjugation, we have performed TPA measurement by an open aperture femtosecond Z-scan method with excitation at 800 nm, as shown in Figure 5. As indicated in Table 1, the TPA cross-section σ (2) values of ZNH remain intact with elongated molecular length, whereas that of ZNHG increases dramatically being consistent with the elongated π-conjugation length arising from reduced dihedral angle between porphyrin moieties.…”

Ein kernmodifiziertes Rubyrin mit meso‐Aryl‐Subsituenten und Phenanthren‐anellierten Pyrrolringen als hoch konjugierter NIR‐Farbstoff und Hg2+‐Sonde

“…[13] Um einen Farbstoff zu erhalten, der bei deutlich höheren Wellenlängen absorbiert, erwies sich die von uns kürzlich auf Tetrapyrrolporphyrine angewendete Anellierung von Phenanthrenringen an die bPyrrol-Position [25] [8,9,24,26,27] …”