Issue 4, 2016

Localised and delocalised excitons in star-like squaraine homo- and heterotrimers

Abstract

Exciton coupling of localised chromophore states within covalently bound superchromophores is a viable strategy to modify optical properties such as spectral broadening and red-shifting of absorption bands. These are desirable properties for e.g. organic photovoltaic applications. Attaching three squaraine dyes to a central nitrogen core in a star-shaped manner leads to the formation of superchromophores that may form localised and delocalised excitons upon photoexcitation. In this work we investigated two homotrimers, two heterotrimers and a heterodimer formed by the combination of two different squaraines SQA and SQB. Due to exciton coupling the two homotrimers display a red shift of the main absorption band by about 1000 cm−1 compared to their monomeric reference compounds. On the other hand, the heterotrimers show a broadening of the absorption spectra with three peak maxima at the exciton manifold band. In fluorescence experiments the homotrimers display signals similar to the emission of the monomeric compounds but red shifted. However, the heterotrimers and the heterodimer show, beside emission from the delocalised lowest energy state, an additional signal that overlaps strongly with the absorption. Excitation and time-dependent emission spectra of the hetero compounds indicate that this emission stems from a localised higher energy state. This interpretation is corroborated by transient absorption measurements with fs-time resolution.

Graphical abstract: Localised and delocalised excitons in star-like squaraine homo- and heterotrimers

Supplementary files

Article information

Article type
Paper
Submitted
12 Nov 2015
Accepted
11 Dec 2015
First published
18 Dec 2015

Phys. Chem. Chem. Phys., 2016,18, 2646-2657

Author version available

Localised and delocalised excitons in star-like squaraine homo- and heterotrimers

H. Ceymann, M. Balkenhohl, A. Schmiedel, M. Holzapfel and C. Lambert, Phys. Chem. Chem. Phys., 2016, 18, 2646 DOI: 10.1039/C5CP06917C

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