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