Unmasking the chemistry and properties of non-peripherally octasubstituted phthalocyanines

Abstract

The paper reviews the development of the chemistry, properties and applications of 1,4,8,11,15,18,22,25- (or non-peripheral) octasubstituted phthalocyanines that has been undertaken at the University of East Anglia over the last quarter of a century. The research has proved valuable for initiating and developing a number of areas of phthalocyanine science, not least because the compounds exhibit a number of properties seldom shared by Pc derivatives with other substitution patterns. In particular the compounds are obtained isomerically pure and the substituents' location at the non-peripheral sites as opposed to the peripheral positions ensures superior solubility and lower aggregation in most hydrophobic solvents. A further significant difference between isomeric non-peripheral (np)- and peripheral (p)-octasubstituted compounds is the effect of substituents on the energy of the Q-band, those at the former positions leading to more significant bathochromic shifts of this absorption band into the near infra-red. Examples of np-octasubstituted derivatives exhibit columnar mesophase behavior, a property shared by their p-octasubstituted counterparts. The favorable solubility of the np-octasubstituted compounds, including examples in which one of the four benzenoid rings bears different substituents from those on the other three (so called AAAB structures), in organic solvents facilitates their deposition as thin film formulations. A high degree of molecular organisation within Langmuir-Blodgett (LB) films of amphiphilic AAAB compounds has been investigated using various methods. Spin-coated films of certain AAAA and AAAB series of compounds have been shown to contain layer structuring and purpose designed AAAB compounds have been used in formation of self-assembled monolayers (SAMs). The use of LB and spin-coated films within electrical devices is described. Various applications of SAMs deposited on gold surfaces have been explored including gas sensing. Recent attention has focused on their use, when deposited onto gold nanoparticles, for applications in PDT, an approach complementing more conventional PDT studies. The principal challenge for obtaining the np-octasubstituted compounds is in preparing the precursor phthalonitriles. Though 3,6-dialkoxyphthalonitriles are readily prepared, early access to the 3,6-dialkylphthalonitriles was somewhat laborious. However, more convenient syntheses of the latter are now available through metal catalyzed cross-coupling reactions. Interesting contrasts in the reactivity of 3,6-dialkylphthalonitriles and 4,5-dialkylphthalonitriles have been uncovered. Thus reactions of the former with excess lithium in alcoholic solvents or with excess MeMgBr provide particularly ready access to examples of macrocycles that are hybrids of the Pc and tetrabenzoporphyrin structures. This level of reactivity is very different among the 4,5-dialkylphthalonitriles. Unexpectedly limited reactivity of np-octahexylsubstituted Si(OH)2Pc in condensation reactions has been exploited to provide a basis for the stepwise construction of m-oxo linked hetero-chromophore arrays. Finally, the unexpected construction of the novel EPR active Cd2Pc3 complexes from both np- and p-octaalkyl substituted CdPcs provided examples where both series behaved similarly – in contrast to behavior exhibited by alternatively substituted CdPcs investigated to date.