Soret Band Splitting as an Optical Fingerprint of Qubit Architecture in Spin-Active Porphyrin Dimers.

The Soret band splitting of meso-meso-linked porphyrin dimers provides a quantitative optical signature of heterometallic two-qubit architecture. Time-dependent density functional theory calculations on [VO(TrPP)]2 and [Cu(TrPP)]2 (TrPP = 5,10,15-triphenylporphyrinate) show that the homodimer splitting vanishes at the orthogonal geometry, where the frontier a2-derived HOMOs are degenerate, producing a symmetry-protected node in excitonic coupling. In contrast, the heterodimer [VO(TrPP)-Cu(TrPP)] retains a two-peak Soret structure in orthogonal geometry because of the intrinsic reduction of the symmetry that cannot be removed by rotation. A two-chromophore exciton model reproduces this splitting quantitatively. Broken-symmetry DFT yields a superexchange magnetic coupling, |J| ≈ 10-2 cm-1, 5 orders of magnitude below the Soret splitting, confirming that optical and magnetic interactions are decoupled. The Soret profile and J together provide independent, complementary observables to characterize porphyrin-based two-qubit architectures.