Theoretical Investigation of the Absorption Properties of 3,6-Bis(thiophen-2-yl)diketopyrrolopyrrole Derivatives in the Solid State
Organic Molecular Chemistry - Synthesis and Characterisation
Final Report Abstract
Polar chromophores have a wide area of application. They can be used as semiconductors in organic photovoltaics, field-effect transistors, light-emitting diodes (OLEDs) as well as for application in bioimaging materials. Hence, gaining insight into the interchromophoric interactions leading to changes of the optical properties of dye aggregates is crucial and of large interest. In this project, the Essential States Model (ESM) has been applied and extended to study the optical absorption properties of merocyanine dye stacks, representing an important class of donor-acceptor (DA) chromophores. In general, DA chromophores are described as a superposition of two states, i.e. the neutral and zwitterionic structure. However, our analysis reveals that a simple two-states model is not sufficient to describe the absorption properties and the permanent and transition dipole moments of a merocyanine chromophore. Therefore, the common model was extended by including an additional bridge state, which can be regarded as an intermediate state of the intramolecular charge transfer from the donor to the acceptor unit. In this way, it is possible to reproduce the absorption spectrum including the vibronic progression as well as to obtain reasonable values of the permanent and transition dipole moments of the chromophores. Furthermore, the model was also applied for defined double and quadruple stacks of merocyanine chromophores with an antiparallel orientation of the ground state dipole moments. Indeed, the spectra could be simulated with the same parameters as used for the monomeric chromophore. The interchromophoric interactions were described by means of a Coulomb interaction between point integer charges. In this way, polarizability effects are considered, which is not the case in the exciton model and which is crucial for the description of aggregates comprising polar chromophores. The results revealed that the spectral changes observed for the double stack with respect to the monomer are best described by a redistribution of oscillator strength of the vibronic bands. Thus, vibronic coupling plays an important role for merocyanine dyes, which is often neglected. Furthermore, our analysis shows that vibronic coupling is also responsible for the broadening of the absorption spectrum in the case of the quadruple dye stack. Notably, the dipolar character of the chromophores increases in the stacks caused by the polarizability effect of the adjacent chromophore within the stacks. In addition, the optical properties of squaraine dimers have been investigated. Toward this goal, the absorption spectra have been simulated for different chromophore arrangements and the effect of polarizability on the absorption features has been studied. The theoretical investigations revealed that the observed non-symmetric Davydov splitting with respect to the monomer transition energy arises from the interactions between three-charge distributions. However, the size of the splitting is determined by the interactions between only two-charge distributions of the chromophore. The model was verified by comparison to the experimental data of a covalently linked squaraine dimer. Indeed, the absorption and fluorescence spectrum could be adequately described and the absorption bands assigned to the different Davydov components. Besides, the ESM suggests a new type of aggregate, in which both Davydov components are above the monomer transition energy, which cannot be described within the exciton model. The results of this project may help to better interpret the absorption spectra of DA and DAD chromophores and their respective aggregates, from which we can gain insight into the electronic and vibronic couplings that ultimately determine energy transfer. This is of particular interest since polar chromophores are gaining more and more attention for applications such as semiconductors in organic electronics as well as for bioimaging.
Publications
- J. Phys. Chem. C 2019, 123, 18654
D. Bialas, C. Zhong, F. Würthner, F. C. Spano
(See online at https://doi.org/10.1021/acs.jpcc.9b04430) - J. Phys. Chem. C 2019, 123, 18734
C. Zhong, D. Bialas, C. J. Collison, F. C. Spano
(See online at https://doi.org/10.1021/acs.jpcc.9b05297)
