Novel boron(III) subphthalocyanines (SubPcs) soluble in organic solvents containing a variety of
donor and acceptor substituent groups have been synthesized by boron trihalide-induced cyclotrimerization of
adequately substituted derivatives of phthalonitrile in 1-chloronaphthalene. The choice of the substituents on
the 1,2-dicyanobenzene derivatives has been made taking into account the high reactivity of the Lewis acid
BCl3 toward many functional groups. Considering this limitation, we set out to synthesize phthalodinitriles
equipped with iodo, nitro, alkyl- or arylthio, alkyl- or arylsulfonyl groups that are sufficiently stable under the
required reaction conditions and also provide an easily accessible set of acceptor/donor substituents. The quadratic
and cubic hyperpolarizabilities of these compounds as well as their linear optical and electrochemical properties
have been measured by several techniques, including EFISH (at two wavelengths), HRS, and THG, steady-state and time-resolved absorption and fluorescence, laser-induced optoacoustic calorimetry, time-resolved
near-infrared emission spectroscopy, and cyclic voltammetry. βHRS has been measured at 1.46 μm, where the
contamination from the multiphoton-induced fluorescence can be ruled out. βHRS reachs high values that markedly
depend on substitution. It shows a clear enhancement with the acceptor character of the substituents, the highest
values being obtained for the compounds bearing the strongest acceptor groups. They are comparable or even
superior to many efficient second-order compounds. A main outcome of these results is that an adequate
choice of the substituents offers a promising route for optimization of the quadratic response of the SubPcs.
This kind of compounds is less prone to aggregation than their expanded analogues, the phthalocyanines,
fluoresces with quantum yields ca. 0.25, lower than those typical for phthalocyanines, and has larger triplet
quantum yields. The triplet-state lifetime is in the 100-μs time range, long enough for efficient oxygen quenching.
Indeed, subphthalocyanines sensitize singlet molecular oxygen, O2(1Δg), with quantum yields ranging from
0.23 to 0.75. The ground-state oxidation potentials are similar to those of phthalocyanines, while the reduction
potentials are clearly more negative; i.e., they are more difficult to reduce. In contrast, electronically excited
subphthalocyanines are more easily oxidized than the corresponding phthalocyanines by ca. 500 mV which
results in lower photostability, especially in polar solvents.
