Substituted naphthalenes have a wide range of applications. In particular, naphthols are photoacids since their acidity increases several folds upon photoexcitation. The OH substituent on the naphthalene ring also introduces further complexity in the electronic structure, with dramatically different effects with the substitution of the 1- or 2-position of the aromatic ring (i.e., in 1-naphthol (1N) or 2-naphthol (2N), respectively). Although 1N and 2N have similar acidities in the ground electronic state, their photoacidities are strikingly different, suggesting the involvement of excited states of different character in the stabilization of the conjugate base. Nevertheless, a molecular-level understanding of the photoacidity of naphthols remains controversial. We characterized the 1Lb, and 1La excited states of naphthols by using time-dependent density functional theory (TDDFT), configuration interaction with singles (CIS), and equation-of-motion coupled cluster singles and doubles (EOM-CCSD) methods. TDDFT fails dramatically at predicting the energy and relative ordering of the 1La and 1Lb excited states as observed experimentally, while EOM-CCSD accurately predicts the excited states as characterized by natural transition orbital analysis.<br/>The implication of the wrong description of the excited states not only affect the energetics of the excited states but also dramatically influence the relative populations of the isomers in the excited states leading to misinterpretation of the dynamics of the excited states.<br/>This limitation of TDDFT is attributed to the absence of correlation from doubly excited configurations as well as the inconsistent description of excited electronic states of naphthol photoacids revealed by excitation analysis using an exciton model based on the one-electron transition density matrix. We show that one exciton quantity is of particular importance, electron-hole correlation coefficient. Besides, revealing the electron-hole dynamics, this correlation coefficient would also serve as a perfect metric for benchmarking purposes of different quantum mechanical methods as shown for photoacids.

Can TDDFT describe the excited electronic states of naphthol photoacids?

256th National Meeting (2018)

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