PERSPECTIVE pubs.acs.org/JPCL
Defining Spectroscopic Features of Heteroannulenic Antiaromatic Porphyrinoids Sung Cho,† Zin Seok Yoon,† Kil Suk Kim,† Min-Chul Yoon,† Dong-Gyu Cho,§ Jonathan L. Sessler,*,‡ and Dongho Kim*,† †
Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 120-749, Korea, §Department of Chemistry, Inha University, Incheon 402-751, Korea, and ‡Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station - A5300, Austin, Texas 78712-0165
ABSTRACT A new set of spectroscopic tools is proposed that may be used to distinguish antiaromatic compounds from their corresponding aromatic congeners. This prediction is based on a detailed analysis of the optical and photophysical properties of a matched set of expanded porphyrins. In particular, the antiaromatic porphyrinoids having [4n] π-electrons within their conjugation pathway exhibit distinct photophysical features that differ dramatically from what is observed for the corresponding aromatic congeners. The clear diagnostic differences seen between the antiaromatic and aromatic compounds leads us to propose that the spectroscopic methods detailed in this Perspective could emerge as general tools that may be used to characterize the electronic characteristics of complex systems for which a number of potential electronic states can be envisioned on the basis of simple line formulas or analyses of π-electron populations.
W
ith a storied history dating back to the days of early interest in benzene, the concept of aromaticity continues to excite the imagination of chemists, even as its importance has become increasingly apparent in fields as diverse of biomedicine and materials science. Not surprisingly, therefore, considerable effort has been devoted to exploring and understanding aromaticity. As a consequence, aromaticity has come to be defined in terms of five key experimental parameters, namely, energetics, structure, reactivity, magnetism, and spectroscopic features.1 While the chemical and physical properties of many canonical aromatic systems have been investigated in detail, a complete understanding of the underlying structure-property relationships, including H€ uckel's [4n þ 2] rule and other types of quantitative indices of aromaticity,2-7 still remains elusive.8,9 This is even more true in the case of antiaromaticity.
conjugated, cyclic systems that are thermodynamically less stable than their corresponding acyclic analogues with [4n] πelectrons.10 Intriguing as is this concept, it remains somewhat nebulous. In particular, there is a dearth of spectroscopic studies involving putative antiaromatic molecules, presumably due to the limited number of such systems, with even fewer having the photochemical stability needed to allow for investigation using optical spectroscopy and related techniques. In this Perspective, we discuss new spectroscopic tests that we believe can be used to define antiaromatic systems and distinguish them from their corresponding aromatic congeners. As detailed below, our approach is based on the results of an in-depth analysis of several archetypical expanded porphyrins. Expanded porphyrins are macrocyclic heteroannulenes that, as their name implies, are large analogues of porphyrins. Typically, they contain more than four pyrroles within their macrocyclic cores and are characterized by well-defined conjugation pathways. The particular expanded porphyrins chosen for inclusion in the present study were all stable under the conditions of analysis, and their use has allowed us to discover a set of spectroscopic parameters that we suggest will emerge as defining features of antiaromatic systems. Expanded porphyrins have attracted considerable attention recently for both their fundamental properties and the potential utility in various practical applications.11 Among the
The concept of aromaticity continues to excite the imagination of chemists, even as its importance has become increasing apparent in fields as diverse of biomedicine and materials science.
Received Date: January 13, 2010 Accepted Date: February 11, 2010 Published on Web Date: February 19, 2010
The word “antiaromaticity” was first introduced by Ronald Breslow, who described antiaromatic molecules as being
r 2010 American Chemical Society
895
DOI: 10.1021/jz100039n |J. Phys. Chem. Lett. 2010, 1, 895–900
PERSPECTIVE pubs.acs.org/JPCL
Table 1. Spectroscopic Properties of Aromatic (1-3) and Antiaromatic (4-6) Expanded Porphyrins Na
point group
NICS (ppm)b
AICDc
λabs (nm)d
λflu (nm)e
σ(2) (GM) f
τS (ps)g
τT (μs)i
1
22
C2v
-14.9
0.098
712
716
2900
2430
14.9
2
22
D2h
-11.4
0.088
794
842
2100
430
16.0
3
26
C2h
-15.4
0.089
926
968
9100
560
146.0
4 5
20 24
C2v C3h
þ22.8 þ20.2
0.148 0.143
552 554
-j -j
1200 1200
8.7
