Hidden excimer formation in the gas-phase photodynamics of a BN-doped phenanthrene

Replacing CC units by isoelectronic BN motifs provides a powerful strategy to tune the electronic structure and excited-state chemistry of polycyclic aromatic hydrocarbons (PAHs). Here, we combine multiphoton ionization spectroscopy, time-resolved photoelectron imaging, ion velocity-map imaging, and quantum-chemical calculations to disentangle the monomer and dimer photophysics of 4a,4b-azaboraphenanthrene. The monomer exhibits a structured S₁ ← S₀ spectrum with an origin at 22 880 ± 15 cm^-1, corresponding to 2.837 eV, and pronounced activity in low-wavenumber deformation modes. Photoelectron spectroscopy yields an adiabatic ionization energy of 7.18 ± 0.02 eV. While the structured spectrum, high fluorescence quantum yield, small computed geometry changes, and weak spin-orbit couplings all point to a long-lived monomer S₁ state, time-resolved photoelectron images reveal an additional picosecond component. Ion imaging shows that this component originates from dissociative ionization of the molecular dimer, which projects dimer excited-state dynamics into the monomer mass channel. Computations identify the initially excited dimer state as a bright H-aggregate-like exciton, followed by ultrafast S₂ → S₁ internal conversion and subsequent structural relaxation toward an excimeric S₁ minimum. The experimentally observed ≈15 ps time constant is therefore assigned to excimer formation in the neutral dimer.