Simulations of absorption spectra of conjugated oligomers: role of planar conformation and aggregation in condensed phase

ABSTRACT

This Review highlights the structure/property relationship underlying the morphology modulation through various factors towards the exploration of light-absorbing materials for efficient utilisation of solar power. Theoretical study using a combination of molecular dynamics imulations and the time-dependent density functional theory demonstrated that the planarity plays an important role in tuning spectral properties of oligomer aggregates. The aggregation-induced blue-shift in absorption spectra of oligothiophenes and the red-shift for oligofluorenols were rationalised in a unified way from the reduced (and increased) content of planar conformations in molecular aggregates. The planarity versus non-planarity of oligomers can be modulated by introduction of alkyl side chain or steric bulky substituents. The substitution with various groups in the ortho-position of azobenzene leads to the distorted backbone, breaking symmetry, and hence the red-shift in spectra, expanding the application in biological systems with visible light absorption. The donor–acceptor substituent groups in conjugated oligomers can increase the degree of planarity, electron delocalisation and polarisation, and charge separation, giving rise to the red-shift in spectra and enhancement in polarisability and charge mobility for device applications. The solvent dependent and pH-sensitive properties and intramolecular hydrogen bonds also caused the shift of absorption spectra with the appearance of planar conformers.

KEYWORDS:

• Aggregation effect
• substituent effects
• D–π–A
• pH-dependence
• solvent effects
• hydrogen bonding

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant Nos. 21290192, 21273102, 21673111 and 21703118), Shandong Provincial Natural Science Foundation, China (No.ZR2017MB038). We are grateful to the High Performance Computing Center of Nanjing University for doing the quantum chemical calculations in this paper on its IBM Blade cluster system.

Supporting Information

The Hammett constants, computational details of MD simulations, rotation potential of AZO, concentration-dependent spectra shifts of UV/Vis spectra, and three different solvent models to simulate spectra in solution.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

National Natural Science Foundation of China [grant number 21290192], [grant number 21273102], [grant number 21673111], [grant number 21703118]; Shandong Provincial Natural Science Foundation, China [grant number ZR2017MB038].