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

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.

Fig. 1. (a) Resonance structures and donor–acceptor character with large dipole moment of reference merocyanine 1. (b) Chemical structure of MC oligomers 2–6 in the folded state. The dashed lines indicate the hydrogen bonding between the amide protons and the pyridine nitrogen atom of the linker moiety. (c) Schematic representation of the folding process of 2–6 into discrete π-stacks driven by dipole–dipole interactions.

Scheme 1. Synthetic route towards MC oligomers 2–6 employing a peptide synthesis strategy starting with the deprotection of the carboxylic group of precursor 7 and subsequent amino acid coupling with compound 8. The procedure was repeated until the desired oligomer length was obtained. Finally, the target compound was obtained by condensation reaction with building block 9 in the last step. Reaction conditions for the deprotection step: NaOH_aq, THF/MeOH, 2 h; reaction conditions for the amino acid coupling: HBTU, DIPEA, CH₂Cl₂, 1 h.

Fig. 2. Solvent-dependent UV/vis spectra of MC oligomers (a) 2, (b) 3, (c) 4, (d) 5 and (e) 6 in CHCl₃/MCH solvent mixtures at 20 °C (c ∼ 10⁻⁶ M) starting from pure CHCl₃ (green line) with stepwise increase of the amount of methylcyclohexane to 70% (red line). The arrows indicate the spectral changes upon decreasing solvent polarity and the dashed spectrum in panel (a) shows the absorption spectrum of reference chromophore 1 in pure CHCl₃. In addition, the schematic representation of the folding process is displayed below the respective UV/vis spectra.

Fig. 3. Excerpts of the ¹H NMR spectra (400 MHz, amide and aromatic region) of (a) monomer 1, (b) dimer 2, (c) trimer 3, (d) tetramer 4 and (e) pentamer 5 in CDCl₃ at 295 K.

Fig. 4. (a) Excerpt of the ¹H–¹H ROESY spectrum (600 MHz) of trimer 3 in CDCl₃ at 295 K. Positive and negative signals are displayed in blue and green, respectively. (b) Schematic representation of the triple stack of 3 with antiparallel orientation of the chromophores. The correlations between protons 7 ↔ 9 (red rectangles in the ¹H–¹H ROESY spectrum) of neighboring MCs are indicated by red double-headed arrows and 6 ↔ 9 correlations (black rectangles in the ¹H–¹H ROESY spectrum) by black double-headed arrows. Only one set of correlation is shown for clarity.

Fig. 5. (a) Excerpt of the ¹H–¹H ROESY spectrum (600 MHz) of trimer 3 in CDCl₃ at 295 K. (b) Simulated NMR structure of 3 based on ¹H–¹H ROESY data (carbon: green, oxygen; red, nitrogen: blue and hydrogen: white) with enlarged section showing the hydrogen bonds between the carbonyl oxygen atom of the merocyanine acceptor moiety and nearby amide protons 1^C (brown) and 13^B (green). Dodecyl chains are replaced by methyl groups for clarity. (c) Chemical structure of 3 with significant correlations between protons indicated by double-headed arrows.

Fig. 6. Calculated UV/vis spectra of (a) 2 and (c) 3 on the TD-DFT level of theory and the molecular exciton model of interacting transition dipole moments according to Kasha. For both methods, vertical transitions are displayed as sticks and convoluted spectra are given as solid lines. Transitions with significant oscillator strength were assigned with arrows. TD-DFT transition densities of the most relevant states are plotted for (b) 2 and (d) 3. Additionally, schematic representations of orientation and magnitude of monomer transition dipole moments are depicted as obtained by Kasha theory.

Fig. 7. Normalized UV/vis absorption (black line), excitation (blue and red solid line) and emission spectra (blue and red dashed line) of reference merocyanine (a) 1, (b) dimer 2 and (c) trimer 3 in CHCl₃ (298 K, c = 3.5 × 10⁻⁶ M). Colored arrows indicate excitation or emission points of the corresponding emission or excitation spectra, respectively, with the same colors.

Fig. 8. (a) Fluorescence quantum yields of reference MC 1 and MC oligomers 2–6 in CHCl₃/MCH 30 : 70 (black) and CHCl₃/liquid paraffin 30 : 70 (blue) at rt. (b) Calculated radiative (green) and non-radiative rate constants of MC 1–6 in CHCl₃/MCH 30 : 70.