Computational Evidence of Inversion of¹L_aand¹L_b-Derived Excited States in Naphthalene Excimer Formation fromab InitioMultireference Theory with Large Active Space: DMRG-CASPT2 Study

Abstract

The naphthalene molecule has two important lowest-lying singlet excited states, denoted (1)La and (1)Lb. Association of the excited and ground state monomers yields a metastable excited dimer (excimer), which emits characteristic fluorescence. Here, we report a first computational result based on ab initio theory to corroborate that the naphthalene excimer fluorescence is (1)La parentage, resulting from inversion of (1)La and (1)Lb-derived dimer states. This inversion was hypothesized by earlier experimental studies; however, it has not been confirmed rigorously. In this study, the advanced multireference (MR) theory based on the density matrix renormalization group that enables using unprecedented large-size active space for describing significant electron correlation effects is used to provide accurate potential energy curves (PECs) of the excited states. The results evidenced the inversion of the PECs and accurately predicted transition energies for excimer fluorescence and monomer absorption. Traditional MR calculations with smaller active spaces and single-reference theory calculations exhibit serious inconsistencies with experimental observations.

Low-Lying ππ* States of Heteroaromatic Molecules: A Challenge for Excited State Methods

“…Due to the fundamental importance of oligoacenes, the conundrum has gained significant interest in the literature. 5,17−31 Large improvements of the L a excitation energies were later reported with the use of range-separated hybrid functionals, 19−21,24 however at the expense of deteriorating the L b excitation energy values. 21 The difficulty to provide a balanced description was attributed to the significant impact of contributions from double-excitation (mainly for L b ), that cannot be properly described with the standard adiabatic TD-DFT implementations.…”

Multireference exciplex binding energies: Basis set convergence and error

“…Characterizing aromatic exciplexes involves a number of challenges for electronic structure methods. Several methods fail to give the correct ordering for the valence π → π * excitations of linear acene monomers . Calculating binding energies presents further difficulties.…”

“…Exciplex stabilization is fundamentally a noncovalent interaction, and thus binding energy calculations are prey to the same errors that plague calculations involving van der Waals or hydrogen bond interactions. The well‐known failure of most DFT functionals to account for stabilizing dispersion interactions can contribute to severe underestimation of exciplex binding energies when corrections are not included . Further errors can be attributed to basis set effects.…”

“…Perturbatively‐corrected complete active space methods, such as second‐order complete active space perturbation theory CASPT2 and second‐order n ‐electron valence perturbation theory (NEVPT2), have the potential to address the problem of describing the excited states encountered in exciplexes. They have successfully captured the static and dynamic correlation involved valence excitations of acene monomers and dimers . Exponential scaling with active space size has limited multireference perturbation theory approaches to small molecules, but it is likely that increased computing power and the possibility of achieving lower scaling using techniques such as the density matrix renormalization group (DMRG) approach will allow these computational methods to be used in increasingly larger systems, including noncovalently‐bound dimers and supramolecular complexes.…”

“…CASPT2 excitation energies have been calculated using both the TZVP and aug‐cc‐pVTZ basis sets for a database of small molecules, and the performance of basis sets of different sizes and families for excited state energies and wave function descriptors of tetracene has been assessed . Basis set effects on the excited state noncovalent interactions of aromatics, though, have not been examined in detail; studies have been limited to a single atomic natural orbital basis or to double‐ and triple‐zeta basis sets without diffuse functions . No studies of aromatic excimer or exciplex interactions have reported results from basis sets of quadruple‐zeta quality or higher, and the effects of diffuse functions on description of noncovalent interactions have not been systematically examined.…”