^{1}and Martin Schütz

^{1,a)}

### Abstract

The lowest electronically excited states of the aniline dimer and trimer related to the lowest transition of the monomer are investigated by applying time-dependent coupled cluster theory, primarily at the level of the (spin-component-scaled) CC2 model. Minimum energy structures in the vicinity of the Franck–Condon points were determined on the individual potential energy surfaces. For the dimer we find an excimer and a head-to-tail configuration (with the monomers substantially displaced relative to the ground state minimum) for the lowest (dark) and second lowest (bright) states, respectively. The excitation is delocalized on both chromophores for both of these states. For the trimer three distinct minima with quite different hydrogen-bonding arrangements are found for the three lowest states. In strong contrast to the dimer the excitation here is clearly localized on the individual aniline chromophores for each of these three states. One of the three geometries is rather similar to the ground state minimum, while the two others are rather different and thus have presumably quite small Franck–Condon factors. It can be expected that only the electronic origin of the first conformer can eventually be detected in the absorptionspectrum of the trimer, provided that it is separated by high-enough barriers from other, energetically lower configurations.

Financial support from the Deutsche Forschungsgemeinschaft (DFG) under Project No. SCHU 1456/3 is gratefully acknowledged.

I. INTRODUCTION

II. COMPUTATIONAL METHODS

III. RESULTS AND DISCUSSION

A. The aniline dimer:

B. The aniline trimer:

IV. CONCLUSIONS

### Key Topics

- Polymers
- 51.0
- Excited states
- 21.0
- Ground states
- 20.0
- Absorption spectra
- 16.0
- Hydrogen bonding
- 13.0

## Figures

Aniline dimer minimum energy geometries in the ground and the excited state, calculated with SCS-CC2 response in the basis set, together with the respective binding energies in kcal/mol.

Aniline dimer minimum energy geometries in the ground and the excited state, calculated with SCS-CC2 response in the basis set, together with the respective binding energies in kcal/mol.

Aniline trimer minimum energy geometries in the ground and the excited state, calculated with SCS-CC2 response in the basis set, together with the respective binding energies in kcal/mol.

Aniline trimer minimum energy geometries in the ground and the excited state, calculated with SCS-CC2 response in the basis set, together with the respective binding energies in kcal/mol.

Density differences, calculated at the ground state geometry, between and (left) and and (right). The red isosurface represents a value of 0.001 and the blue one a value of −0.001. (Upon excitation the electron density moves from blue to red regions.)

Density differences, calculated at the ground state geometry, between and (left) and and (right). The red isosurface represents a value of 0.001 and the blue one a value of −0.001. (Upon excitation the electron density moves from blue to red regions.)

Density differences of geometries between excited and ground states. Left column: at ground state minimum; right column: at respective excited state minima. The red isosurface represents a value of 0.001 and the blue one a value of −0.001. (Upon excitation electron density moves from blue to red regions.)

Density differences of geometries between excited and ground states. Left column: at ground state minimum; right column: at respective excited state minima. The red isosurface represents a value of 0.001 and the blue one a value of −0.001. (Upon excitation electron density moves from blue to red regions.)

## Tables

Selected geometry parameters of the individual minima. denote parallel and perpendicular components of the distance vector between the centers of the two phenyl rings of . denotes the individual NH–N H-bonding distances. denotes the dihedral angle describing the displacement of the ipso C atom out of the plane defined by the five other C atoms of the individual monomers, and denotes the dihedral angle measuring the nonplanarity of a group. Distances are given in Å and angles are in degrees.

Selected geometry parameters of the individual minima. denote parallel and perpendicular components of the distance vector between the centers of the two phenyl rings of . denotes the individual NH–N H-bonding distances. denotes the dihedral angle describing the displacement of the ipso C atom out of the plane defined by the five other C atoms of the individual monomers, and denotes the dihedral angle measuring the nonplanarity of a group. Distances are given in Å and angles are in degrees.

rms geometry differences between two individual aniline dimer and trimer geometries. All the geometries are calculated with SCS-CC2 response in the AO basis set. The values are given in Å.

rms geometry differences between two individual aniline dimer and trimer geometries. All the geometries are calculated with SCS-CC2 response in the AO basis set. The values are given in Å.

Counterpoise corrected interaction energies , BSSE , relaxation energies (calculated within monomer basis), and binding energies . Calculations were performed with SCS-CC2 response in the and AO basis sets at the geometries. For the binding energies the relaxational energies were used. All values are given in kcal/mol.

Counterpoise corrected interaction energies , BSSE , relaxation energies (calculated within monomer basis), and binding energies . Calculations were performed with SCS-CC2 response in the and AO basis sets at the geometries. For the binding energies the relaxational energies were used. All values are given in kcal/mol.

Adiabatic excitation energies in eV, calculated with SCS-CC2 response in the AO basis set, between ground state (GS) and excited state (XS). In parentheses the vertical excitation energies at the Frank–Condon point of the respective state are given. Experimental values according to Ref. 7 are also included for comparison.

Adiabatic excitation energies in eV, calculated with SCS-CC2 response in the AO basis set, between ground state (GS) and excited state (XS). In parentheses the vertical excitation energies at the Frank–Condon point of the respective state are given. Experimental values according to Ref. 7 are also included for comparison.

Counterpoise corrected interaction energies, calculated at the individual dimer geometries optimized by using the SCS, and the unscaled CC2 response method, in the AO basis set. All values are given in kcal/mol.

Counterpoise corrected interaction energies, calculated at the individual dimer geometries optimized by using the SCS, and the unscaled CC2 response method, in the AO basis set. All values are given in kcal/mol.

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