Photophysical properties of core-modified expanded porphyrins: nature of aromaticity and enhancement of ring planarity.
We have investigated the excited-state dynamics and nonlinear optical properties of representative core-modified expanded porphyrins, tetrathiarubyrin, tetraselenarubyrin, pentathiaheptaphyrin, tetrathiaoctaphyrin, and tetraselenaoctaphyrin, containing 26, 30, and 34 pi electrons using steady-state and time-resolved absorption and fluorescence spectroscopic measurements along with femtosecond Z-scan method, with a particular attention to the photophysical properties related to molecular planarity and aromaticity. Core-modification of macrocycles by sulfur and selenium leads to NIR-extended steady-state absorption and fluorescence spectra and short-lived excited-state due to the heavy-atom effect in time-resolved spectroscopic experiments. Large negative nucleus-independent chemical shift values ranging from -13 to -15 ppm indicate that all molecular systems are highly aromatic. The observed enhancement of two-photon absorption cross-section values over 10 (4) GM for core-modified hepta- and octaphyrins is mainly attributable to their rigid and planar structures as well as their aromaticity. Overall, the observed spectroscopic and theoretical results consistently demonstrate the enhanced molecular planarity of core-modified expanded porphyrins compared with their corresponding all-aza expanded porphyrins.
Yoon
Min-Chul
MC
Department of Chemistry, Yonsei University, Seoul 120-749, Korea.
Misra
Rajneesh
R
Yoon
Zin Seok
ZS
Kim
Kil Suk
KS
Lim
Jong Min
JM
Chandrashekar
Tavarekere K
TK
Kim
Dongho
D
D015394:Molecular Structure; D011166:Porphyrins / Q000737:chemistry*; D013050:Spectrometry, Fluorescence
D011166:Porphyrins