Self-assembly of carboxylic acid appended naphthalene diimide derivatives with tunable luminescent color and electrical conductivity

Mijanur Rahaman Molla, Dominik Gehrig, Lisa Roy, Valentin Kamm, Ankan Paul*, Frederic Laquai, Suhrit Ghosh

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

74 Scopus citations

Abstract

Self-assembly of a series of carboxylic acid-functionalized naphthalene diimide (NDI) chromophores with a varying number (n=1-4) of methylene spacers between the NDI ring and the carboxylic acid group has been studied. The derivatives show pronounced aggregation due to the synergistic effects of H-bonding between the carboxylic acid groups in a syn-syn catemer motif and π stacking between the NDI chromophores. Solvent-dependent UV/Vis studies reveal the existence of monomeric dye molecules in a "good" solvent such as chloroform and self-assembly in "bad" solvents such as methylcyclohexane. The propensity of self-assembly is comparable for all samples. Temperature-dependent spectroscopic studies show high thermal stability of the H-bonding-mediated self-assembled structures. In the presence of a protic solvent such as MeOH, self-assembly can be suppressed, suggesting a decisive role of H-bonding, whereas π stacking is more a consequence of than a cause for self-assembly. Syn-syn catemer-type H-bonding is supported by powder XRD studies and the results corroborate well with DFT calculations. The morphology as determined by AFM is found to be dependent on the value of n; with increasing n, the morphology gradually shifts from 2D nanosheets to 1D nanofibers. Emission spectra show sharp emission bands with relatively small Stokes shifts. In addition, a rather broad emission band is observed at longer wavelengths because of the in situ formation of excimer-type species. Due to such a heterogeneous nature, the emission spectrum spans almost the entire red-green-blue region. Depending on the value of n, the ratio of intensities of the two emission bands is changed, which results in a tunable luminescent color. Furthermore, in the case of n=1 and 3, almost pure white light emission is observed. Time-resolved photoluminescence spectra show a very short lifetime (a few picoseconds) of monomeric dye molecules and biexponential decays with longer lifetimes (on the order of nanoseconds) for aggregated species. Current-voltage measurements show electrical conductivity in the range of 10-4 S cm-1 for the aggregated chromophores, which is four orders of magnitude higher than the value for a structurally similar NDI control molecule lacking the H-bonding functionality.

Original languageEnglish (US)
Pages (from-to)760-771
Number of pages12
JournalChemistry - A European Journal
Volume20
Issue number3
DOIs
StatePublished - Jan 13 2014

Keywords

  • aggregation
  • chromophores
  • hydrogen bonds
  • luminescence
  • self-assembly

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Organic Chemistry

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