We
present a comprehensive description of the unique properties
of newly developed phthalocyanines (Pcs) containing main-group elements
that absorb and emit in the near-IR region. Group 16 (S, Se, and Te)
elements and group 15 (P, As, and Sb) elements were used as peripheral
and central (core) substituents. With the introduction of group 16
elements into free-base Pc, a red-shift of the Q-band was observed,
as a result of the electron-donating ability of group 16 elements
particularly at the α positions. An X-ray crystallographic analysis
of α-ArS-, ArSe-, and ArTe-linked free-base Pcs was also successfully
performed, and the relationship between structure and optical properties
was clarified. When a group 15 element ion was introduced into the
center of the Pc ring, the resulting Pcs showed a single Q-band peak
beyond 1000 nm (up to 1056 nm in CH2Cl2). In
particular, [(ArS)8PcP(OMe)2]+ and
[(ArS)8PcAs(OMe)2]+ exhibited a distinct
fluorescence in the 960–1400 nm region with moderate quantum
yields. The atomic radius of the group 15 element is important for
determining the Pc structure, so that this can be controlled by the
choice of group 15 elements. Electrochemical data revealed, while
MO calculations suggested, that the red-shift of the Q-band is attributable
to a decrease of the HOMO–LUMO gap due to significant and moderate
stabilization of the LUMO and HOMO, respectively. The effect of peripheral
substutuents and a central P(V) ion on the Q-band shift was independently
predicted by MO calculations, while the magnitude of the total calculated
shift was in good agreement with the experimental observations. The
combination of spectral, electrochemical, and theoretical considerations
revealed that all of the central group 15 elements, peripheral group
16 elements, and their positions are necessary to shift the Q-band
beyond 1000 nm, indicating that the substitution effects of group
15 and 16 elements act synergistically. The Pcs having Q-bands beyond
1000 nm in this study also had stability under aerobic conditions
comparative to that of CuPc, which is presently being widely used
in consumer products.