Spectroscopy of Donor-Acceptor Dyes

Abstract

A number of organic donor-acceptor dyes were investigated using spectroscopic and computational methods. This class of materials can form charge-transfer excited-states which result in a number of interesting properties. These properties have shown potential in solar cells, non-linear optics, display technologies and photo-dynamic therapy technologies.

The first part of this work investigated donor-acceptor interactions using benzothiadiazole-triphenylamine materials. These materials showed a low energy band in electronic absorption data around 400 nm. Analysis of this band revealed changes of absorption wavelength (350 to 430 nm) and extinction coefficient (8,000 to 25,000 L mol-1 cm-1) with structural modifications. Resonance Raman spectroscopy and computational analysis indicated this transition was charge-transfer in nature. Dual-band emission was observed with some compounds in polar solvents (420 and 600 nm). The relative intensity of the two bands could be changed by varying solvent polarity. The high energy emission band was assigned as n→π* in nature, and the low energy emission band was assigned as charge-transfer in nature.

Benzothiadiazole-triphenylamine materials were subject to a number of ground state computational methods. Absorption and Raman parameters of six well known density functional theory functionals were compared. It was found that Hartree-Fock exchange contributions around 25% were best for predicting absorption parameters. Raman cross-sections were best predicted by the range separated functionals CAM-B3LYP and ωB97XD (19-65 and 22-100% Hartree-Fock exchange contribution, respectively). Excited state calculations were also explored. These used two different solvation formalisms: a linear-response approach and a state-specific approach. The more intensive state-specific approach provided better predictions of emission wavelengths (within 0.2 eV compared to around 0.5 eV).

In the second part of this work, the acceptor unit was changed to hexazatrinaphthalene, which has a lower reduction potential. These materials showed a low energy band in electronic absorption data around 500 nm. This band showed changes of absorption wavelength (430 to 500 nm) and extinction coefficient (19,000 to 43,000 L mol-1 cm-1) with structural modifications. Resonance Raman spectroscopy and computational analysis indicated this lowest energy transition was charge-transfer in nature. The `fill-in' observed between 400 and 500 nm also appeared to be of a similar nature in resonance Raman data. In these materials, a strong and solvatochromic emission band is observed from 500 to 750 nm. Regardless of the number of donor units, the solvatochromic behaviour was similar between the materials. This indicates that the emissive excited states are similar, and these have been assigned as a HATN.--TPA.+ couples.

In the final part of this work, both the acceptor and donor were changed. These materials used an indandione-diethylaniline framework which showed a strong, low energy absorption feature around 600 nm. This feature showed changes of absorption wavelength (500 to 750 nm) and extinction coefficient (23,000 to 67,000 L mol-1 cm-1) with structural modifications. Resonance Raman spectroscopy and computational analysis identified two distinct transitions within the lowest energy absorption feature in most compounds. The higher energy transition (550 nm) was assigned as charge-transfer and the lowest energy transition (650 nm) was more delocalised in nature. In general, these compounds were weak, dual-emitters. A high energy emission (450 nm) and a low energy emission (650 nm) were observed. The high energy emission band was assigned as n→π* in nature and the low energy emission band is assigned as delocalised in nature.