Exciton Coupling and Charge Transfer Dynamics in Zn(II) Complexes of π-Extended Dipyrrins

Dipyrrins form a group of versatile chromophores, which find use as laser dyes as well as in light harvesting and bioimaging applications. The mode of the central ion coordination and the ensuing molecular geometry play a key role in the photophysics of dipyrrins, whereby some complexes are brightly fluorescent and some completely lack emissivity. However, the relationship between the structure and excitation dynamics in dipyrrins is still poorly understood. Here, we used a range of spectroscopic methods to investigate the photophysics of Zn(II) complexes of <i>meso</i>-Ar-2,2'-di-<i>tert</i>-butoxycarbonyl-dibenzodipyrrins (BDP; Ar = 4-MeO₂C-C₆H₄). In particular, two-dimensional electronic spectroscopy (2DES) was used to characterize the initial excited states in a homoleptic <i>bis</i>-dipyrrinate Zn(BDP)₂, in which two dipyrrin ligands are oriented in a nonorthogonal geometry. From the position of the peaks in the 2DES spectra and spectral modeling, the initial excited states of Zn(BDP)₂ were assigned to excitonic states. The low oscillator strength, associated with excitation to the lower excitonic state, is responsible in part for the weak emissivity of Zn(BDP)₂, contrasting the bright fluorescence of <i>mono</i>-dippyrinate Zn(BDP)X. Femtosecond (fs-), nanosecond (ns-) transient absorption (TA), and time-resolved fluorescence spectroscopies were used to monitor the solvent-dependent evolution of the excitonic states, which appear to evolve into an intermediate state possibly with charge transfer character. Taken together, our findings reveal a significant impact of both structural and environmental factors on the photophysics of dipyrrins and present the first example of the application of 2DES to investigate excitonic states in a system where the interacting chromophores are held together via coordination of an optically neutral metal ion. On a broader scale, we demonstrate that nonorthogonal <i>bis</i>-dipyrrin complexes constitute a versatile model for studying exciton coupling and associated energy and charge dynamics.