Ground State versus Excited State: Discrepancy in Electronic Communication in a Series of meso–meso Two‐Atom‐Bridged Diporphyrins

Abstract

The focal point of this work is the photophysical characterization of three meso-meso two-atom-bridged diporphyrins. Detailed investigations by means of cyclic voltammetry, absorption, fluorescence, and femto-/nanosecond transient absorption spectroscopy revealed the discrepancy in electronic communication in a series of meso-meso two-atom-bridged porphyrins in the ground state and in the excited state. In the ground state, the azo bridge facilitates the strongest electronic communication between the two porphyrins. In the excited state, however, the ethene bridge induces the strongest coupling, followed by the imine and azo bridges.

Investigating Electronic Communications in meso–meso Ethene‐Bridged Unsymmetrical Diporphyrins

“…Owing to the steric repulsions between the hydrogen atoms of the ethene bridge sp 2 carbon atoms and those at the β‐positions of the porphyrins a wide distribution of dihedral angles must be assumed ( θ ). To this end, the fluorescence of 1 stems from three different populations of the following conformers: twisted (⊥), intermediate ( II ), and planar (‖) (Figure ) …”

“…This clearly demonstrates bridge‐assisted electronic communication. Similarly to what was previously reported for 1 (Figures S11–S13, Supporting Information), the transient absorption spectra of 2 and 3 show ground‐state absorption bleaching in the region of Soret‐ and Q band absorptions and intense transient absorptions in the NIR …”

“…Figure S20, Supporting Information, presents a mechanistic model, in which the processes taking place in 1 upon 420 nm excitation of 1 , are visualized. The model is based on the conclusions drawn from viscosity‐, temperature‐, and coordination‐dependent experiments performed in our previous work . Using this particular model in the context of the current work, the two shortest lifetimes of 1.8±0.1 and 17±0.2 ps are assigned to solvent relaxation and conformational changes from less to more planarized conformers, respectively.…”

“…In stark contrast, ethene‐bridges reveal the strongest interactions in the excited state; followed by the imine‐ and azo‐bridges. Considering a balanced electronic communication between the two porphyrins in their ground and excited states, ethene‐bridges are the most versatile . In light of the aforementioned, we opted for the current work on heterometallic diporphyrins for ethene‐bridges.…”

“…In the diporphyrins, dipole couplings along the long molecular axis B z generate two excitonic states; but, only the lower‐energy transition is allowed. Meanwhile, the mutually orthogonal B x and B y dipoles fail to couple . In short, porphyrin arrays reveal excitonic splitting of the Soret‐band absorptions and the splitting reflects the degree of porphyrin–porphyrin interactions.…”