Folding and fluorescence enhancement with strong odd-even effect for a series of merocyanine dye oligomers.

Xiaobo Hu,Matthias Grüne,Joachim O. Lindner,David Bialas,Frank Würthner,Alexander Schulz

doi:10.1039/d1sc01678d

Abstract

A series of merocyanine (MC) oligomers with a varying number of chromophores from two to six has been synthesized via a peptide synthesis strategy. Solvent-dependent UV/vis spectroscopic studies reveal folding processes for the MC oligomers driven by strong dipole–dipole interactions resulting in well-defined π-stacks with antiparallel orientation of the dyes. Whilst even-numbered tetramer 4 and hexamer 6 only show partial folding into dimeric units, odd-numbered trimer 3 and pentamer 5 fold into π-stacks of three and five MC units upon decreasing solvent polarity. In-depth 2D NMR studies provided insight into the supramolecular structure. For trimer 3, an NMR structure could be generated revealing the presence of a well-defined triple π-stack in the folded state. Concomitant with folding, the fluorescence quantum yield is increased for all MC oligomers in comparison to the single chromophore. Based on radiative and non-radiative decay rates, this fluorescence enhancement can be attributed to the rigidification of the chromophores within the π-stacks that affords a pronounced decrease of the non-radiative decay rates. Theoretical investigations for the double and triple dye stacks based on time-dependent density functional theory (TD-DFT) calculations indicate for trimer 3 a pronounced mixing of Frenkel and charge transfer (CT) states. This leads to significant deviations from the predictions obtained by the molecular exciton theory which only accounts for the Coulomb interaction between the transition dipole moments of the chromophores.

Highlights

In order to direct a stepwise folding into well-de ned merocyanine stacks, a sophisticated design of the molecular backbone is required
Our employed spacer moieties were selected based on the following considerations: (1) the diamide pyridine motif[33,34] connecting the MC units should support the dipolar aggregation of the chromophores via a hydrogenbonded turn motif (Fig. 1b)
Our UV/vis spectroscopic investigations revealed that a hydrogen-bonded diamide pyridine turn moiety supported the folding for oligomers 2–6 by strong dipole–dipole interactions between the MC chromophores in low-polarity solvents

Summary

Introduction

Dye aggregates currently receive increasing attention due to their high relevance for a variety of applications, e.g. in organic photovoltaics,[1,2] organic light-emitting devices[3,4] and lightharvesting systems for arti cial photosynthesis.[5,6] In order to gain fundamental understanding of the intermolecular interactions leading to the desired functional properties, wellde ned aggregates with precise control of the chromophore arrangement and aggregate size are desired.[7,8,9] The importance of de ned chromophore arrangements is demonstrated by natural light-harvesting complexes, in which photosynthetic pigments are organized into discrete structures stabilized by a protein shell, which ensures an efficient light-harvesting process and charge separation.[10,11] it is obvious that the realization of such complex structures is beyond the scope of current supramolecular chemistry. The spectrum of trimer 3 in chloroform is characterized by a further blue-shi ed absorption band at 477 nm (Fig. 2b, green line), indicating the presence of merocyanine stacks larger than a dimer, i.e. a triple stack.

Results

Conclusion