In this section, an introductory study of molecular vibrations and, indirectly, relaxation dynamics, of single molecule HAT6 is made by extending previous ground-state computational work to excited states, while retaining the molecular tails [15, 16, 28, 29]. The model calculations are validated by IR and UV absorption measurements of HAT6 in solution at room temperature [30]. The goals are three-fold:

(i) To determine the electronic excitation with the largest oscillator strength in HAT6;

(ii) Modelling the molecular structure of the selected excited state;

(iii) Understanding effects of electron-phonon coupling by comparing the vibrations of the ground state (GS) and the selected excited state (ES1).

However, a thorough investigation of the vibronic and electronic aspects of the DLC-CT HAT6-TNF is presented in Sect. 6.3.1.5.

It has been found [30] that the HOMO-to-LUMO transition dominates the tar­geted ES1 with the largest oscillator strength (*1). The calculated excitation — energy using the time-dependent DFT matches perfectly the measured one at 3.76 eV. The most important feature is that ES1 can be approximated as simply a HOMO-to-LUMO excitation, the HOMO-to-LUMO contribution in ES1 being * 80 %. As far as changes in the PES in going from GS to ES1 are concerned, these can be explored directly by comparing the structural parameters of the two elec­tronic-states. Results of the structural analyses can be summarized as follows: the structural distortion of ES1, compared to GS, corresponds to geometrical changes mainly in the aromatic core of HAT6. The oxygen atoms are of central importance,

since they connect the aromatic core to the alkyl tails, and due to their electro­negativity, they could play an electronic role in the charge-transport process. Bond angles involving the oxygen atoms show changes that are comparable in amplitude to those within the aromatic core. The alkyl tails are, as expected, less sensitive to the ES1 structural distortion, but nevertheless, a change is found in the chain structure [30]. These changes reflect the different minima of the PES of GS and ES1, but gradients of the PES around local minima also change, and these are probed by the molecular vibrations. The HAT6 molecule with 144 atoms has 426 normal modes. The D3h symmetry of the molecule results in six irreducible rep­resentations (irreps) with the following distribution of modes; 42 (A’1), 41 (A’2), 166 (E’), 30(A”1), 29 (A”2), 118 (E”). The modes corresponding to E’ and E” are doubly degenerate. Modes with A”2 and E’ symmetry are IR active. Figure 6.14 compares the calculated IR spectrum with that measured. The agreement is very good allowing the excited state of the molecule and its vibrations to be studied with more confidence.

Figure 6.15 shows the calculated IR spectra for GS and ES1 over the whole spectral range (195 active modes) and, in more detail, in the restricted spectral range from 550 to 1,700 cm-1 (118 active modes). It is in this range that the most striking differences are observed between GS and ES1 and, indeed, differences occur throughout this part of the spectrum. However, we have selected bands with the strongest IR intensities (and high dipole strengths), these being denoted: I, II, III, and IV. Modes I—III are composed of single peaks (pairs of degenerate modes), whereas mode IV is composed of 5 peaks/frequencies. With the exception of II, the intensities of the modes are greater in ES1 than in GS. The frequency shifts in going from GS to ES1 are about 10 cm-1 for modes II-IV but for mode I the shift is 29 cm 1. The related atomic displacements for GS and ES1 modes in the four

frequency bands can be analysed. For mode I, which has the biggest frequency shift, the GS mode is an out-of-plane mode, whereas the ES1 mode is an in-plane mode. Accordingly, the displacements of the alkoxy tails are significantly different, being polarised in the planes perpendicular to the tail directions in GS and being polarised along the tails in ES1. For modes II-IV, the frequency shifts are smaller and the similarity of modes between GS and ES1 is stronger, with mode III being almost identical in the two electronic states. In these three bands, all modes show in-plane polarisation of the molecular cores. Mode II involves deformations of the aromatic cores and C-O stretches, with corresponding responses in the alkoxy tails, and the displacement patterns change between GS and ES1. For mode IV, the main difference between GS and ES1 involves the pattern of C-C stretches in the aro­matic core, but there is a notable difference in the chain displacements. It is evident from this investigation of GS and ES1 molecular vibrations that core and tail vibrations are coupled.

In the intermediate frequency range that has been considered, deformations of the aromatic core are accompanied by complex displacements of the alkoxy tails, and these are not simple rigid-body motions. Accordingly, the QENS study reported in Sect. 6.3.1.2, in which diffusive dynamics on the ps timescale were probed, proved in a consistent way, that motion of the alkyl tails is driven by the core dynamics. The alkyl chain (beyond the O atom) is also found to play a significant role in the molecular distortion and change in molecular vibrations upon electronic excitation. The vibrational spectrum in the excited electronic state, which has the strongest oscillator strength, is considerably different from that in the ground state, despite the size and overall flexibility of the system. This result, supported by the good agreement with measured IR and UV spectra, is encouraging for gaining insight into the technologically important relaxation processes within the conduction band that lead to serious efficiency losses in solar-cell applications. After all, most energy present in the incoming light is dissipated and converted to
heat due to fast relaxation from higher excitations towards the lower band edge. In this context Sect. 6.3.1.5 focuses also on some resonant Raman measurements which are related to the excited-state molecular structure via the vibrations that constitute the atomic shifts between the ground and excited electronic-state structures.