title: 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.
authors.affiliation: Department of Chemistry and Center for Ultrafast Optical Characteristics Control, Yonsei University, Seoul 120-749, Korea.
authors.forename: Zin Seok
authors.initials: ZS
authors.lastname: Yoon
authors.forename: Jung Ho
authors.initials: JH
authors.lastname: Kwon
authors.forename: Min-Chul
authors.initials: MC
authors.lastname: Yoon
authors.forename: Mi Kyoung
authors.initials: MK
authors.lastname: Koh
authors.forename: Su Bum
authors.initials: SB
authors.lastname: Noh
authors.forename: Jonathan L
authors.initials: JL
authors.lastname: Sessler
authors.forename: Jeong Tae
authors.initials: JT
authors.lastname: Lee
authors.forename: Daniel
authors.initials: D
authors.lastname: Seidel
authors.forename: Apolonio
authors.initials: A
authors.lastname: Aguilar
authors.forename: Soji
authors.initials: S
authors.lastname: Shimizu
authors.forename: Masaaki
authors.initials: M
authors.lastname: Suzuki
authors.forename: Atsuhiro
authors.initials: A
authors.lastname: Osuka
authors.forename: Dongho
authors.initials: D
authors.lastname: Kim
mesh_terms: D015394:Molecular Structure; D055095:Optics and Photonics; D017785:Photons; D011166:Porphyrins / Q000737:chemistry*; D013050:Spectrometry, Fluorescence; D019265:Spectroscopy, Near-Infrared
chemical_list: D011166:Porphyrins