Near-Infrared-Emitting Phthalocyanines. A Combined Experimental and Density Functional Theory Study of the Structural, Optical, and Photophysical Properties of Pd(II) and Pt(II) α-Butoxyphthalocyanines

The structural, optical, and photophysical properties of 1,4,8,11,15,18,22,25-octabutoxyphthalocyaninato-palladium(II), PdPc(OBu)8, and the newly synthesized platinum analogue PtPc(OBu)8are investigated combining X-ray crystallography, static and transient absorption spectroscopy, and relativistic zeroth-order regular approximation (ZORA) Density Functional Theory (DFT)/Time Dependent DFT (TDDFT) calculations where spin−orbit coupling (SOC) effects are explicitly considered. The results are compared to those previously reported for NiPc(OBu)8(J. Phys. Chem. A2005, 109, 2078) in an effort to highlight the effect of the central metal on the structural and photophysical properties of the group 10 transition metal octabutoxyphthalocyanines. Different from the nickel analogue, PdPc(OBu)8and PtPc(OBu)8show a modest and irregular saddling distortion of the macrocycle, but share with the first member of the group similar UV−vis spectra, with the deep red and intense Q-band absorption experiencing a blue shift down the group, as observed in virtually all tetrapyrrolic complexes of this triad. The blue shift of the Q-band along the MPc(OBu)8(M = Ni, Pd, Pt) series is interpreted on the basis of the metal-induced electronic structure changes. Besides the intense deep red absorption, the title complexes exhibit a distinct near-infrared (NIR) absorption due to a transition to the double-group 1E (π,π*) state, which is dominated by the lowest single-group 3E (π,π*) state. Unlike NiPc(OBu)8, which is nonluminescent, PdPc(OBu)8and PtPc(OBu)8show both deep red fluorescence emission and NIR phosphorescence emission. Transient absorption experiments and relativistic spin−orbit TDDFT calculations consistently indicate that fluorescence and phosphorescence emissions occur from the S1(π,π*) and T1(π,π*) states, respectively, the latter being directly populated from the former, and the triplet state decays directly to the S0surface (the triplet lifetime in deaerated benzene solution was 3.04 μs for Pd and 0.55 μs for Pt). Owing to their triplet properties, PdPc(OBu)8and PtPc(OBu)8have potential for use in photodynamic therapy (PDT) and are potential candidates for NIR light emitting diodes or NIR emitting probes.