Synthesis and nonlinear optical, photophysical, and electrochemical properties of subphthalocyanines

Novel boron(III) subphthalocyanines (SubPcs) soluble in organic solvents co ntaining a variety of donor and acceptor substituent groups have been synth esized by boron trihalide-induced cyclotrimerization of adequately substitu ted derivatives of phthalonitrile in 1-chloronaphthalene. The choice of the substituents on the 1,2-dicyanobenzene derivatives has been made taking in to account the high reactivity of the Lewis acid BCl3 toward many functiona l groups. Considering this limitation. we set out to synthesize phthalodini triles equipped with iodo, nitro, alkyl- or arylthio, alkyl- or arylsulfony l groups that are sufficiently stable under the required reaction condition s and also provide an easily accessible set of acceptor/donor substituents. The quadratic and cubic hyperpolarizabilities of these compounds as well a s their linear optical and electrochemical properties have been measured by several techniques, including EFISH (at two wavelengths), HRS, and THG, st eady-state and time-resolved absorption and fluorescence, laser-induced opt oacoustic calorimetry, time-resolved near-infrared emission spectroscopy, a nd cyclic voltammetry. beta(HRS) has been measured at 1.46 mu m, where the contamination from the multiphoton-induced fluorescence can be ruled out. b eta(HRS) reachs high values that markedly depend on substitution. It shows a clear enhancement with the acceptor character of the substituents, the hi ghest values being obtained for the compounds bearing the strongest accepto r groups. They are comparable or even superior to many efficient second-ord er compounds. A main outcome of these results is that an adequate choice of the substituents offers a promising route for optimization of the quadrati c response of the SubPcs. This kind of compounds is less prone to aggregati on than their expanded analogues, the phthalocyanines, fluoresces with quan tum yields ca. 0.25, lower than those typical for phthalocyanines, and has larger triplet quantum yields. The tripler-state lifetime is in the 100-mu s time range, long enough for efficient oxygen quenching. Indeed, subphthal ocyanines sensitize singlet molecular oxygen, O-2((1)Delta(g)), with quantu m yields ranging from 0.23 to 0.75. The ground-state oxidation potentials a n similar to those of phthalocyanines, while the reduction potentials are c learly more negative; i.e., they are more difficult to reduce. In contrast, electronically excited subphthalocyanines are more easily oxidized than th e corresponding phthalocyanines by ca. 500 mV which results in lower photos tability, especially in polar solvents.