Optical properties of the phosphorescent trinuclear copper(I) complexes of pyrazolates: insights from theory

Theoretical investigations have been performed to explore the optical properties of {[3,5-(CF3)(2)Pz]Cu}3 in monomeric and dimeric forms using TD-DFT approaches. The emission of all complexes originates from the lowest triplet excited-states (T1), and the corresponding emissive states are assigned as the mixture of the metal-centered charge transfer and ligand-to-metal charge transfer. The features of the emission spectrum are clarified in detail. The bulk emission spectrum of complex is mainly determined by the stacked dimers rather than the individual monomers. The predicted maximum emission wavelength (lambda(em)) are in good agreement with experimental values, indicating that the phosphorescence bands can be assigned to two different conformations for the neighboring stacked dimers sharing the same monomer in the complex. Energy transfer from T1 of one stacked dimer to the neighboring one is responsible for the disappearance of the shoulder, leaving only the main peak upon heating. With the aim to reveal the conformational dependence for the triplet excited-state emission spectrum, the optical properties of various stacked dimers with different conformations are investigated by varying the relative arrangements through changing inter-monomer distance or rotational angles for the dimer which is responsible for the main peak emission. Calculation results suggest that the shortest intermolecular Cu...Cu distance plays an important role in the emission spectra of the vertical- and tilting-movement dimers, which is ascribed to the variation of the energy gap for the frontier molecular orbitals involved in the main emitting transition. The blue shift of lambda(em) in parallel-movement and rotational dimers can be traced back to the variation of the mutual spatial orientation. Therefore, the modulation of the extent of movement or rotational angles for stacked dimers by external perturbations creates new possibilities for the design of molecular light-emitting devices.