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.

Exciton Transport in Molecular Aggregates – From Natural Antennas to Synthetic Chromophore Systems

“…Thus, taking symmetry breaking and vibronic coupling into account is essential in order to describe the fluorescent properties properly. In contrast, star‐like chromophore trimers such as N(SQA) 3 or N(SQB) 3 possess degenerate lowest energy states from which fluorescence is allowed (Φ f = 35% and 10%) . The exciton coupling in N(SQA) 3 and N(SQB) 3 is 630 and 600 cm −1 , respectively.…”

“…Indolenine squaraines SQB with a dicyanovinylene squaric acid core possess a banana shape because of steric reasons (see Figure 13 and 14 ). Similar to the SQA oligomers, the dimer [SQB] 2 , the trimer [SQB] 3 and the polymer [SQB] n display a progressive red‐shift of the lowest energy absorption band compared to the monomer . The monomer and the trimer also show high fluorescence quantum yields and were used as efficient emitters in NIR OLED devices .…”

“…In contrast, star-like chromophore trimers such as N(SQA) 3 or N(SQB) 3 possess degenerate lowest energy states from which fluorescence is allowed (Φ f = 35% and 10%). [223] The exciton coupling in N(SQA) 3 and N(SQB) 3 is 630 and 600 cm −1 , respectively. In mixed trimers such as N(SQA)(SQB) 2 or N(SQA) 2 (SQB) dual fluorescence is found from delocalized excitonic states and from squaraine localized states.…”

“…Similar to the SQA oligomers, the dimer [SQB] 2 , the trimer [SQB] 3 and the polymer [SQB] n display a progressive red-shift of the lowest energy absorption band compared to the monomer. [223,224] The monomer and the trimer also show high fluorescence quantum yields and were used as efficient emitters in NIR OLED devices. [225,226] Again, as in the SQA cases the exciton coupling (estimated from the bandwidth) decreases slightly with the number of SQB chromophores (dimer: 800 cm −1 , trimer: 636 cm −1 , polymer: 620 cm −1 ).…”

Star‐Shaped Push‐Pull Molecules with a Varied Number of Peripheral Acceptors: An Insight into Their Optoelectronic Features

“…Specifically, Ceymann et al. have studied tripodal molecules with different squaraine chromophores linked to a central nitrogen core (heterotrimers) . Lambert et al.…”