Vibrationally-resolved spectroscopic studies of electronically excited states of 1,8-naphthalic anhydride and 1,8-naphthalimide: a delicate interplay between one ππ* and two nπ* states

The spectroscopic and dynamic properties of the lower electronically excited states of 1,8-naphthalic anhydride and 1,8-naphthalimide have been studied in supersonically cooled molecular beams using nanosecond Resonance Enhanced MultiPhoton Ionization (REMPI) spectroscopic techniques in combination with quantum chemical calculations. The excitation spectra of these compounds show near – and even below – the apparent 0–0 transition to a strongly allowed electronic state, previously assigned as the S₁(2¹A₁(ππ*)) state, a plethora of vibronic transitions that cannot simply be rationalized in terms of the Franck–Condon vibronic activity of that particular state. Instead, it is shown that the ¹B₁(nπ*) state, which was previously reported to be S₃ for vertical excitation, is adiabatically the lowest excited singlet state. Interactions between this ‘dark’ state and the ‘bright’ 2¹A₁(ππ*) state lead to intensity borrowing of transitions to ‘dark’ state levels that thus show up in the excitation spectra. A complicating factor is that, apart from the coupling of these two singlet states, a relatively strong spin–orbit coupling between the 2¹A₁(ππ*) and ³B₁(nπ*) states is also present. We show that the latter state has a slightly higher adiabatic excitation energy than the former state in 1,8-naphthalic anhydride but lies energetically below the 21A1(ππ*) state in 1,8-naphthalimide. Concurrently, we find that the decay dynamics of the excited states of 1,8-naphthalimide are entirely dominated by intersystem crossing, while in 1,8-naphthalic anhydride both internal conversion to the ground state and intersystem crossing occur, albeit the former loses importance once the excitation energy exceeds that of the ³B₁(nπ*) state.