C O M M U N I C A T I O N S
The enhancement of δmax for 1 relative to those for 2 and 3b
suggests substantial electronic coupling between the porphyrin and
squaraine moieties; this idea is confirmed by the DFT frontier
orbitals(FigureS3).Thecomplexityof1andtheporphyrin-squaraine
coupling are anticipated to lead to a large number of low-lying
excited states; indeed, Figure 1a indicates the presence of several
1PA states. Therefore, the broad 2PA spectrum of 1 is likely to
arise from overlap of several transitions. The calculations support
this idea, as they reproduce the main features of the experimental
spectrum of 1, indicating peaks with δ ) ∼6.9 × 103 and 9.7 ×
103 GM at transition energies between those of the two strong 1PA
states. The observation of large δ over a 750 nm-wide wavelength
range suggests that 1 could have applications in broadband NIR
pulse suppression.
Acknowledgment. We gratefully acknowledge the support of
the National Science Foundation (through ECS 0524533, the STC
program under Agreement DMR-0120967, and a graduate research
fellowship to S.A.O.), the U.S. Army Research Laboratory
(W911NF0420012, 50372-CHMUR), the U.S. Army Research
Office (50372-CHMUR), DARPA (MORPH N00014-06-1-0897),
and EPSRC. We thank Dr. Craig Smith and Dr. Michael Frampton
for insightful discussions.
Supporting Information Available: Experimental details; 1H NMR
and COSY spectra of the aromatic region of 1; fluorescence spectra
for 1, 2, and 3b; details of quantum-chemical calculations for 1; and
complete refs 2, 4a, and 6a. This material is available free of charge
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