Charge-Transfer Switching in Cofacial Chromophore Dimers: A Comparative Study of Porphin, Chlorin, and Perylenediimide

Charge-transfer character in cofacial chromophore dimers is quantified from first-principles multireference wavefunctions using the charge-fluctuation measure σ(NA), the standard deviation of the electron count on one monomer. Three systems are compared at interplanar distances of 3.0-7.0 Å within CAS(16,16)/STO-3G active spaces: Mg-porphin, Mg-chlorin, and perylenediimide (PDI). All three systems exhibit low ground-state charge-transfer character at short distances (σ ≤ 0.15). However, chlorin and PDI achieve near-complete charge neutrality at large distances (σ < 0.02), while porphin retains a residual charge-transfer floor (σ ≈ 0.05). Upon photoexcitation, chlorin and PDI undergo a 4.5-5.8× increase in charge-transfer character (σ = 0.40-0.63 for S1), functioning as charge-transfer switches. The variational ground-state energy for the Mg-chlorin dimer agrees with full configuration interaction to within 0.99 mHa, confirming chemical accuracy. These results identify the ground-state charge-transfer floor and the excited-state switching contrast as two independent descriptors for chromophore dimer design and suggest criteria for organic photovoltaic materials based on cofacial chromophore stacking.