Excited Electronic State Mixing in 7-Azaindole. Quantitative Measurements Using the Stark Effect

Abstract

Stark effect measurements of the +280 cm(-1) vibronic band at ∼286 nm in the high resolution S1-S0 fluorescence excitation spectrum of 7-azaindole (7AI) in a molecular beam show that the permanent (electric) dipole moment (PDM) of the upper state vibrational level reached in this transition is 4.6 D, twice as large as the PDM of the zero-point level of the S1 state. This large difference is attributed to state mixing with a more polar state. EOM-CSSD calculations suggest that this more polar state is σπ* in nature and that it crosses the ππ* state in energy along the coordinate connecting the two potential energy minima. Such state mixing apparently provides more facile access to conical intersections with the ground state, and subsequent hydrogen atom detachment reactions, since independent studies by Sakota and Sekiya have shown that the N-H stretching frequency of 7AI is significantly reduced when it is excited to the +280 cm(-1) vibrational level of the S1 state.

Electronic spectra of azaindole and its excited state mixing: A symmetry-adapted cluster configuration interaction study

“…27 In fact, the recent theoretical study using EOM-CCSD theory overestimates this state by 0.7 eV. 20 Given these uncertainties, the SAC-CI calculation is again in reasonable agreement with experiment. The predicted oscillator strengths reveal that 1 L a is more intense than the 1 L b state, as it has been concluded in previous theoretical studies.…”

“…27 A recent EOM-CCSD study found the excitation energy of this state as 4.85 eV and the adiabatic excitation energy for this state is calculated as 4.67 eV. 20 By considering the deviations of all these theoretically calculated values from the experimental one, the value obtained using SAC-CI is reasonable.…”

“…46 The same basis set, the 6-311+G(2d,p) augmented with two sets of Rydberg sp functions for carbon and nitrogen, that was used for the earlier TD-DFT calculations 28 was also utilized here. The obtained results are given in Table I 20 concluded that the lowest lying A ′′ state is the n-π * state (see also the SAC-CI results in Sec. III B).…”

“…Using UV/vis spectroscopy in an argon matrix, Ilitch 12 predicted the n-π * state at 4.54 eV, while theoretical studies using complete active space second-order perturbation theory (CAS-PT2) and equation-of-motion coupled cluster with single and double excitation (EOM-CCSD) found the same state at 5.27 and 5.40 eV, respectively. 15,20 By using jet-cooled spectroscopic study, Kim and Bernstein 21 suggested a strong vibronic mixing between L b π-π * and n-π * states. However, through similar experimental studies, Huang et al 13 concluded that S 1 π-π * (L b ) state undergoes mixing with the L a (π-π * ) state and not with the n-π * state.…”

“…In fact, it has been shown in a recent combined experimental and theoretical study that the n-π * state is mixing with the π-π * (L b ) state. 20 Furthermore, the other valence excited and Rydberg states, especially π-σ * N−H (hereafter referred to as the π-σ * state), which are supposed to be involved in the photochemistry of these types of molecules, 22,23 do not appear to have been studied before. The π-σ * state, which is highly mixed with π-3s Rydberg state and lies close in energy to the low-lying π-π * states, plays a crucial role in both spectroscopic and photophysical properties of indole and in many aromatic biomolecules, and hence, studying this π-σ * state along with the low-lying Rydberg states in azaindole is important.…”