Ultrafast Excimer Formation and Solvent Controlled Symmetry Breaking Charge Separation in the Excitonically Coupled Subphthalocyanine Dimer

Abstract

Knowledge of the factors controlling excited state dynamics in excitonically coupled dimers and higher aggregates is critical for understanding natural and artificial solar energy conversion. In this work, we report ultrafast solvent polarity dependent excited state dynamics of the structurally well-defined subphthalocyanine dimer, μ-OSubPc₂ . Stationary electronic spectra demonstrate strong exciton coupling in μ-OSubPc₂ . Femtosecond transient absorption measurements reveal ultrafast excimer formation from the initially excited exciton, mediated by intramolecular structural evolution. In polar solvents the excimer state decays directly through symmetry breaking charge transfer to form a charge separated state. Charge separation occurs under control of solvent orientational relaxation.

Keywords: excimer; excited states; subphthalocyanine; symmetry breaking charge separation; ultrafast dynamics.

Figure 1

a) DFT optimized chemical structure of the SubPc monomer and μ‐OSubPc₂ dimer. b) Absorption (solid lines) and emission (λ _ex=530 nm, dash lines) spectra of monomer (black) and μ‐OSubPc₂ dimer (red) in toluene. Spectra are shown peak normalized.

Figure 2

a,b) Transient absorption spectra of μ‐OSubPc₂ dimer in toluene (TL) and DMF respectively at different timescales, with insets showing early time data. Excitation was at 546 nm and the dimer concentration was 12 μM. c,d) EADS recovered from global analysis for the TA datasets in TL and DMF respectively. Inset shows data and global analysis fits at different probe wavenumbers. Note that data near 18600 cm⁻¹ are perturbed by scattered light (shown by grey area).

Figure 3

Schematic representation of μ‐OSubPc₂ photophysics. Direct excitation is to a Frenkel exciton state, where excitation is shared over the dimer. This is followed by sequential formation of the μ‐OSubPc₂ excimer and its decay by symmetry breaking charge separation to form a charge separated state, which is accessible only in polar solvent. The first step thus involves an evolution in the wavefunction of the Frenkel exciton state to favor charge resonance forms in the excimer. This involves evolution on intramolecular coordinates. The excimer subsequently decays along a solvation coordinate in polar solvents, with the initial step arising from an asymmetric fluctuation in the solvent environment.