The quenching of the excitonic splitting in hydrogen-bonded molecular dimers has been explained recently in terms of exciton coupling theory, involving Förster's degenerate perturbation theoretical approach [P. Ottiger, S. Leutwyler, and H. Köppel, J. Chem. Phys. 136, 174308 (2012)]. Here we provide an alternative explanation based on the properties of the adiabatic potential energy surfaces. In the proper limit, the lower of these surfaces exhibits a double-minimum shape, with an asymmetric distortion that destroys the geometric equivalence of the excitonically coupled monomers. An effective mode is introduced that exactly reproduces the energy gain and amount of distortion that occurs in a multi-dimensional normal coordinate space. This allows to describe the quenched exciton splitting as the energy difference of the two (S(1) and S(2)) vibronic band origins in a one-dimensional (rather than multi-dimensional) vibronic calculation. The agreement with the earlier result (based on Förster theory) is excellent for all five relevant cases studied. A simple rationale for the quenched exciton splitting as nonadiabatic tunneling splitting on the lower double-minimum potential energy surface is given.

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Articles you may be interested inExcitonic splitting and vibronic coupling in 1,2-diphenoxyethane: Conformation-specific effects in the weak coupling limit J. Chem. Phys. 138, 204313 (2013); 10.1063/1.4807300The S 1/S 2 exciton interaction in 2-pyridone·6-methyl-2-pyridone: Davydov splitting, vibronic coupling, and vibronic quenching J. Chem. Phys. Analysis of the S 2 ← S 0 vibronic spectrum of the ortho-cyanophenol dimer using a multimode vibronic coupling approach The S 2 ← S 0 vibronic spectrum of the ortho-cyanophenol dimer (oCP) 2 is analyzed in a joint experimental and theoretical investigation. Vibronic excitation energies up to 750 cm −1 are covered, which extends our previous analysis of the quenching of the excitonic splitting in this and related species [Kopec et al., J. Chem. Phys. 137, 184312 (2012)]. As we demonstrate, this necessitates an extension of the coupling model. Accordingly, we compute the potential energy surfaces of the orthocyanophenol dimer (oCP) 2 along all relevant normal modes using the approximate second-order coupled cluster method RI-CC2 and extract the corresponding coupling constants using the linear and quadratic vibronic coupling scheme. These serve as the basis to calculate the vibronic spectrum. The theoretical results are found to be in good agreement with the experimental highly resolved resonant two-photon ionization spectrum. This allows to interpret key features of the excitonic and vibronic interactions in terms of nodal patterns of the underlying vibronic wave functions. C 2015 AIP Publishing LLC. [http://dx

We present a generalization of the transition state search using chemical dynamics simulations (TSSCDS) methodology (discussed in a previous study) which allows the topographical characterization of intermolecular potential energy surfaces (IPES) for non‐covalently bound complexes (vdW‐TSSCDS). Starting from a single random input geometry, we show that vdW‐TSSCDS is able to globally and automatically locate stationary points of an IPES, even in limiting cases such as extremely flat regions or nontrivial topologies (eg, bifurcation points). The basic idea is the expression of the connectivity matrix in block structure, where diagonal blocks correspond to the isolated fragments and off‐diagonal blocks provide the intermolecular connectivity. To this end, we introduce a new definition of bound or not, in a non‐covalent sense, utilizing an extra set of van der Waals distances, which encompasses all kinds of non‐covalent distances. To discuss the use of the vdW‐TSSCDS method, we present a series of 2‐body van der Waals systems, namely, Ar‐Benzene (3D), N2‐Benzene (6D) and H2O‐Benzene (9D). Finally, we further illustrate its capabilities by presenting some applications for n‐body problems (n > 2), (H2O)2‐Benzene (12D) and (H2O)3‐Benzene (21D), as well as to a reactive, fully‐flexible, system (Benzene‐NO2)+ (39D) in which the simultaneous breaking/formation of both covalent and non‐covalent interactions takes place.

The interplay between excitonic and vibronic coupling in hydrogen-bonded molecular dimers leads to complex spectral structures and other intriguing phenomena such as a quenching of the excitonic energy splitting. We recently extended our analysis from that of the quenching mechanism to the theoretical investigation of the complete vibronic spectrum for the ortho-cyanophenol dimer. We now apply the same approach to the vibronic spectrum of the 2-pyridone dimer and discuss the assignment of vibronic lines to gain insight into the underlying coupling mechanism. This is based on potential energy surfaces obtained at the RI-CC2/aug-cc-pVTZ level. They are used for the dynamical analysis in the framework of a multi-mode vibronic coupling approach. The theoretical results based on the quadratic vibronic coupling model are found to be in good agreement with the experimental resonant two-photon ionization spectrum.

A brief pedagogic rederivation is given of basic exciton coupling theory, taking the nuclear coordinates to be fixed. This is then extended to take variations of these coordinates into account by adopting suitable multimode coupling models and extracting the transfer of excitation energy from the populations of the locally excited states. The dynamical problem thus defined is solved numerically in a fully quantal manner. Two doubly hydrogen bonded dimers of (hetero)­aromatic systems are selected as representative cases whose electronic excitation spectra have been analyzed previously based on ab initio data, and good agreement with experiment has been found. The numerical calculations of the electronic populations reveal a complex time dependence of the excitation transfer that is far from being oscillatory or exponential. For localized excitation, the short-time behavior can be understood in terms of the quenched excitonic energy splitting, while for delocalized excitation a complex time dependence with rapidly changing features results. Some of these can be interpreted in terms of the vibronic structure of the excitation spectra. The importance of the quenched excitonic splitting for the short-time behavior of the excitation energy transfer is emphasized.

The S/S splitting of the m-cyanophenol dimer, (mCP) and the delocalization of its excitonically coupled S/S states are investigated by mass-selective two-color resonant two-photon ionization and dispersed fluorescence spectroscopy, complemented by a theoretical vibronic coupling analysis based on correlated ab initio calculations at the approximate coupled cluster CC2 and SCS-CC2 levels. The calculations predict three close-lying ground-state minima of (mCP): The lowest is slightly Z-shaped (C-symmetric); the second-lowest is <5 cm higher and planar (C). The vibrational ground state is probably delocalized over both minima. The S → S transition of (mCP) is electric-dipole allowed (A → A), while the S → S transition is forbidden (A → A). Breaking the inversion symmetry by C/C- or H/D-substitution renders the S → S transition partially allowed; the excitonic contribution to the S/S splitting is Δ = 7.3 cm. Additional isotope-dependent contributions arise from the changes of the m-cyanophenol zero-point vibrational energy upon electronic excitation, which are Δ(C/C) = 3.3 cm and Δ(H/D) = 6.8 cm. Only partial localization of the exciton occurs in the C/C and H/D substituted heterodimers. The SCS-CC2 calculated excitonic splitting is Δ = 179 cm; when multiplying this with the vibronic quenching factor Γ = 0.043, we obtain an exciton splitting Δ = 7.7 cm, which agrees very well with the experimental Δ = 7.3 cm. The semiclassical exciton hopping times range from 3.2 ps in (mCP) to 5.7 ps in the heterodimer (mCP-h)·(mCP-d). A multimode vibronic coupling analysis is performed encompassing all the vibronic levels of the coupled S/S states from the v = 0 level to 600 cm above. Both linear and quadratic vibronic coupling schemes were investigated to simulate the S → S/S vibronic spectra; those calculated with the latter scheme agree better with experiment.

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