Effects of Benzoaanulation and α-Octabutoxy Substitution on the Photophysical Behavior of Nickel Phthalocyanines: A Combined Experimental and DFT/TDDFT Study

The photophysical properties of a group of Ni(II)-centered tetrapyrroles have been investigated by ultrafast transient absorption spectrometry and DFT/TDDFT methods in order to characterize the impacts of α-octabutoxy substitution and benzoannulation on the deactivation pathways of the S1(π,π*) state. The compounds examined were NiPc, NiNc, NiPc(OBu)8, and NiNc(OBu)8, where Pc = phthalocyanine and Nc = naphthalocyanine. It was found that the S1(π,π*) state of NiNc(OBu)8 deactivated within the time resolution of the instrument (200 fs) to a vibrationally hot T1(π,π*) state. The quasidegeneracy of the S1(π,π*) and 3(dz2,dx2-y2) states allowed for fast intersystem crossing (ISC) to occur. After vibrational relaxation (ca. 2.5 ps), the T1(π,π*) converted rapidly (ca. 19 ps lifetime) and reversibly into the 3LMCT(π,dx2-y2) state. The equilibrium state, so generated, decayed to the ground state with a lifetime of ca. 500 ps. Peripheral substitution of the Pc ring significantly modified the photodeactivation mechanism of the S1(π,π*) by inducing substantial changes in the relative energies of the S1(π,π*), 3(dπ,dx2-y2), 3(dz2,dx2-y2), T1(π,π*), and 1,3LMCT(π,dx2-y2) excited states. The location of the Gouterman LUMOs and the unoccupied metal level (dx2-y2) with respect to the HOMO is crucial for the actual position of these states. In NiPc, the S1(π,π*) state underwent ultrafast (200 fs) ISC into a hot (d,d) state. Vibrational cooling (ca. 20 ps lifetime) resulted in a cold (dz2,dx2-y2) state, which repopulated the ground state with a 300 ps lifetime. In NiPc(OBu)8, the S1(π,π*) state deactivated through the 3(dz2,dx2-y2), which in turn converted to the 3LMCT(π,dx2-y2) state, which finally repopulated the ground state with a lifetime of 640 ps. Insufficient solubility of NiNc in noncoordinating solvents prevented transient absorption data from being obtained for this compound. However, the TDDFT calculations were used to make speculations about the photoproperties.