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.

Solvophobically‐Driven Merocyanine Dye Assembly: Predominant Dipole‐Dipole Interactions Over Hydrogen‐Bonding

“…This was in agreement with the VT UV‐vis data (Figure 2c) that also showed retention of spectral signature of the aggregate even at 90 °C, which was further supported by the VT dynamic light scattering (DLS) data that showed no change in the particle size when the same solution was heated to higher temperature (Figure 4a, vide infra). Molecular arrangement of MC−OH in aqueous medium was rationalized from the 1 H− 1 H nuclear Overhauser effect spectroscopy (NOESY) (Figure 3c) [10,14a,b] . As the DMSO‐d 6 signals overlapped with some of the proton signals of MC−OH , the experiment was conducted with 10% CD 3 OD/D 2 O, which also showed desired downfield shift for the aromatic protons with increasing temperature (Figure S5a), alike 10% DMSO‐d 6 /D 2 O mixture, confirming π‐stacking at ambient temperature.…”

“…The evidence of strong π‐stacking was further confirmed by solvent dependent 1 H NMR spectroscopy studies (Figure 3a). MC−OH in the aggregated state in 10% DMSO‐d 6 /D 2 O showed significant upfield shift of the aromatic protons ( δ= 8.14 ppm) [10,14a,b] with respect to its molecularly dissolved state in DMSO‐d 6 ( δ =8.82 ppm) due to the shielding effect of the adjacent chromophores, indicating the occurrence of strong π‐stacking between the MC−OH dyes. Antiparallel stacking was validated from the blue‐shifted UV‐vis spectrum of MC−OH even in 10% DMSO/water (Figure S4).…”

Fluorescent supramolecular polymers of barbiturate dyes with thiophene-cored twisted π-systems

“…Although this pronounced change of the absorption spectra of 1 is a typical signature of non-emissive H-type aggregates, the aggregated 1 and 2 in the present study are both highly emissive and maintain radiative rate constants comparable to the molecularly dispersive state, which clearly excludes the exciton coupling upon simple H-type aggregation (Table S1 † ). 12 As another explanation for the absorption spectral change, one may quote distinct vibrational signature of absorption bands observed for J- and H-type aggregation of dyes, which cannot be explained by conventional exciton theory. 13 However, the two absorption bands of 1 and 2 are not vibrational transition but rather ascribable to charge-transfer transition according to the TD-DFT study.…”

Folding‐induced Fluorescence Enhancement in a Series of Merocyanine Hetero‐Folda‐Trimers

“…First evidence for such behavior has been observed in previous research on selfassembled MC oligomers [19][20][21] and on a series of MC dye foldamers bearing up to six identical chromophores of type R (symbol R for the red-colored solutions of this dye, for definition see Figure 1). [22,23] However, these dye oligomers suffer strongly from the Htype exciton coupling in the co-facially stacked folded state, thereby enabling only modest fluorescence enhancements with fluorescence quantum yields up to 2 % in chloroform/ methylcyclohexane (MCH). [23] Because the best results in our previous work were accomplished for trimers, we herein explore a series of trimers composed of the previously applied dimethine MC dye R (abbreviation related to the red color) and a newly synthesized elongated tetramethine analogue B (abbreviation related to the blue color) (Figure 1 a).…”

“…[22,23] However, these dye oligomers suffer strongly from the Htype exciton coupling in the co-facially stacked folded state, thereby enabling only modest fluorescence enhancements with fluorescence quantum yields up to 2 % in chloroform/ methylcyclohexane (MCH). [23] Because the best results in our previous work were accomplished for trimers, we herein explore a series of trimers composed of the previously applied dimethine MC dye R (abbreviation related to the red color) and a newly synthesized elongated tetramethine analogue B (abbreviation related to the blue color) (Figure 1 a). Whilst fluorescence properties of such hetero-foldamers were hitherto never investigated, our previous analyses of exciton coupling in hetero MC aggregates suggested a more beneficial preservation of oscillator strength for the lowest exciton state of these co-facially stacked dyes, thereby enabling higher radiative rates compared to their homo-foldamer congeners.…”