Phthalonaphthalocyanines: New Far-Red Dyes for Spectral Hole Burning

Abstract

Mixed phthalocyanines carrying one anthracene (Pc3An) or substituted naphthalene nucleus (Pc3NcR2, R = H, OCH3, or SC12H25) are proposed as low-temperature photochroms for spectral hole burning. Solubility of these compounds in polymers was greatly enhanced by introducing the 2,4-dimethyl-3-pentoxy substituent to the remaining three benzopyrrolic fragments. The wavelengths and intensities of the Q transitions (S1 and S2) were measured for two prototropic tautomers having different position of the pair of inner protons. The average energy of the two lowest transitions is very similar in both tautomers, although the S1−S2 splitting is much smaller in the less stable form. The relative equilibrium concentration of the tautomers at room temperature depends on the electron releasing properties of the substituents. This allows one to predict the positions of protons in each form. Absorption dichroism of stretched polyethylene films was used in order to establish the direction of the S1 and S2 transition dipole moments in the molecular framework. The tautomers can be completely converted into each other at 10 K by light with quantum yields of (1−2) × 10-3 and (5−8) × 10-3, respectively, depending on the direction of the process. Most probably the phototransformation occurs in the vibrationally relaxed triplet state via the tunneling of a single proton which results in an intermediate state with cis-configuration of protons. The photochemically accumulated less stable form decays at higher temperatures (T) as a result of a thermally activated tunneling process at characteristic T of 115 and 153 K for protonated and deuterated Pc3Nc, respectively. The strength of linear electron−photon coupling (EPC), which is crucial from the point of view of spectral hole burning, is characterized by Debye−Waller factors (DWFs) about 0.6−0.75, depending on the compound and the polymer matrix. The T dependence of quasihomogeneous hole width (Γqh) obeys a power law with coefficients 2.5 ± 0.5 (between 6 and 30−45 K). In different polymer hosts, the DWF increases and the hole width decreases in the following order: polystyrene > poly(vinyl butyral) > polyethylene. The strength of EPC for the lowest transitions is similar in both tautomeric forms. A slight enhancement of the EPC strength in the series of dyes Pc3Nc(OCH3)2 ∼ Pc3Nc < Pc3Nc(SC12H25)2 < Pc3An is correlated with the increase of electron-withdrawing power of substituents, plausibly, as a result of the increase of dipole moment change upon electronic excitation. Spectroscopic properties, phototautomerization quantum yields, and the EPC strength of mixed phthalocyanines were compared with those of chlorin and porphyrins.