Study of substituent effects on the photoconductivity of soluble 2,(3)- and 1,(4)-substituted phthalocyaninato- and naphthalocyaninatotitanium(IV) oxides

Soluble alkyl (II, 8a,b), fluoroalkyl,(4a), and fluoroalkoxy (4b,c, 8c) 1,(4)- or 2,(3)-substituted phthalocyaninato- and linear 2,(3)- and-angular 1,(2)-annulated naphthalocyaninatotitanium(IV) oxides 10, 12, and 14 were synthesized and characterized with regard to their spectroscopic, photophysical, and photochemical properties. While alkyl- and fluoroalkoxy-substituted compounds are highly soluble in nonpolar solvents, e.g., hexane, fluoroalkyl-substituted compounds are better soluble in polar aprotic solvents such as acetone. The stability against photooxidation in solution is' enhanced on going-from alkylated phthalocyanines 1,(4)-(C(5)H(11))(8)PcTiO (8a), 1,(4)-(C(6)H(13))(8)PcTiO (4b), and 2,(3)-(C(4)H(9))(8)PcTiO (II) to fluorinated phthalocyanines 2,(3)-(CF(3))(4)PcTiO (4a), 2,(3)-(CF(3)CH(2)O)(4)PcTiO (4b), and 2,(3)-(CF(3)CH(2)O)(8)PcTiO (4c), from phthalocyanines to naphthalocyanines (tert-butyl)(4)-2,(3)-NcTiO (10), 1,(2)-NcTiO (12), and (tert-butyl)(4)-1,(2)-NdTiO (14), and on going from 2,(3)-substituted 4a-c to 1,(4)-substituted phthalocyanines 8a-c. Thin films of these compounds, prepared by either vacuum deposition or spin casting, are classified into three types according to increasing intermolecular pi-pi interactions. Type alpha films, characterized:by absence of exciton splitting, are formed from 1,(4)-substituted phthalocyanines 8a-c. These films show low dark conductivities and photoconductivities and are considerably sensitive to photooxidation. Type beta films, characterized by weak exciton splitting, are formed from fluorinated phthalocyanines 4a-c as well as from rapidly deposited 2,(3)-substituted phthalocyanines II and the unsubstituted PcTiO (I). These films show enhanced photoconductivity and are generally more stable against photooxidation than type alpha films. Type gamma films, formed by slow deposition of II, 10 and unsubstituted phthalocyanine I, are classified by a largely red-shifted B-band absorbing in the near-IR. These films are highly photosensitive as well as stabilized against photooxidation. Steady-state photoconductivities and dark conductivities in thin films are strongly dependent on oxygen partial pressure. Alkylated PcTiO's such as 8a, 8b, and II are found to be p-type conductors (positive oxygen influence on conductivities) like unsubstituted PcTiO (I), whereas angularly annulated naphthalocyanines such as 12 and 14 as well as fluorinated PcTiO's 4a-c are n-type conductors (negative oxygen influence on conductivity). These findings are rationalized by comparison with experimental and theoretical literature data.