A Density Functional Theory Study of an Exciplex: Pyridine and Benzene

Abstract

An exciplex formation of pyridine and benzene complex in cofacial configuration was investigated at time‐dependent density functional theory level within Tamm–Dancoff approximation using ωB97X functional in conjunction with 6–311++G(d,p) basis sets. Unlike the benzene excimer, calculated potential energy surfaces for the lowest‐lying singlet excited states for this complex show double minima, corresponding to a simple excited complex and an exciplex, respectively. While the former is associated with the localized exciton characterized as the n–π* transition of pyridine, the latter is linked to the significant electronic coupling between the π–π* transitions of constituting individual molecules. Despite its obvious presence, the net charge transfer character from benzene to pyridine is not such significant as expected; wave function delocalization of exciton and hence reduced orbital energy gap are found to play more pivotal role in the exciplex formation of this complex.

A Density Functional Theory Study of an Exciplex II: Benzene and Tricyanobenzene

“…In particular, the difference between the CT character from Bz to TCBN and that from TCBN to Bz, i.e., net CT character, increases from 0 to ca. 37% at the equilibrium; recalling that the net CT character in the Bz–Pyr complex remains less than 10%, the enhancement in the CT character of Bz–TCNB complex is noteworthy. In the case of Bz–Pyr complex, the energy offset between the HOMOs and that between LUMOs are calculated to be only ca.…”

“…In the case of Bz–Pyr complex, the energy offset between the HOMOs and that between LUMOs are calculated to be only ca. 0.37 and 0.23 eV, respectively, facilitating the delocalization of orbitals over the complex. On the other hand, the calculated energy offset between the HOMOs of Bz and TCNB is ca.…”

“…As a result, the complete CT characters of the complexes are hardly observed in particular at the equilibrium distance. However, such significant CT characters in the low‐lying singlet states are absent in the case of Bz–Pyr complex . Therefore, the substantial energy offsets between the FMOs of constituting molecules play an essential role in stabilizing the CT states and enriching the CT characters in the low‐lying states.…”

“…In the previous study of benzene–pyridine complex, we reported that the exciplex formation was driven by the strong electronic coupling between the S 1 state of benzene (Bz) and S 3 state of pyridine (Pyr), similarly to the benzene excimer. In this complex, in addition, the net CT character from the donor to the acceptor remained marginal; unlike what is commonly believed, this suggests that the exciplex formation is not necessarily associated with the CT from the donor to acceptor.…”

Simulating Excited-State Complex Ensembles: Fluorescence and Solvatochromism in Amine-Arene Exciplexes

“…[28][29][30] Advances in quantum chemistry methods have facilitated accurate modeling of excited states in isolated molecules as well as in the condensed phase. Excimer and exciplex structures have been computed by geometry optimization on excited-state potential energy sur-faces (PESs) using configuration interaction singles (CIS), 31 time-dependent density functional theory (TDDFT), 6,9,19,30,[32][33][34][35][36] equation-of-motion methods for electronically excited states based on coupled-cluster singles and doubles (EOM-EE-CCSD), 7 complete active space self-consistent field (CASSCF), and the algebraic diagrammatic construction scheme to second-order with resolution of identity and spin-opposite scale (RI-SOS-ADC(2)) methods. 37 The exciplex state can be characterized by a single structure (i.e., minimum on the excited-state PES), 30 however, such description does not account for the fluxional character of these complexes due to a relatively shallow PESs that may feature multiple accessible minima.…”