Orbital Analysis and Excited-State Calculations in an Energy-Based Fragmentation Method.

Covalently bound molecular arrays composed of porphyrins or related pigments have gained a lot of interest as components of artificial light-harvesting systems and molecular photonic devices. The large size of these arrays, however, makes their theoretical investigation employing the ab initio or density functional methodologies difficult. Energy-based fragmentation methods (EBF) represent a set of conceptually simple approaches to theoretical investigation of large systems and were therefore chosen as a tool to study these systems. Here a new approach to EBF, EBF-MO, is introduced that enables one to obtain orbitals and orbital energies and to perform population analysis and excited-state calculations of large systems composed of hundreds of atoms. This approach was implemented into a parallel program, JETT, and the benchmark calculations have shown its accuracy and applicability to the ground- and excited-state calculations of systems containing transition metals and extended π-conjugation. EBF-MO was then applied to the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) calculations of ground- and excited-state properties of a porphyrin-based molecular photonic wire composed of 472 atoms and 4265 basis functions at the B3LYP/LANL08,6-31G* level. The TDDFT calculations have revealed the character of the excited states, and the unidirectionality of the excitation energy transfer across the array relevant to its signal transfer function. The computational approaches introduced here have widened the applicability of the ab initio and density functional methodologies to calculations of extended systems such as natural and artificial light-harvesting systems and molecular photonic devices.