Enhanced Room‐Temperature Photoluminescence Quantum Yield in Morphology Controlled J‐Aggregates

Abstract Supramolecular assemblies from organic dyes forming J‐aggregates are known to exhibit narrowband photoluminescence with full‐width at half maximum of ≈9 nm (260 cm−1). Applications of these high color purity emitters, however, are hampered by the rather low photoluminescence quantum yields reported for cyanine J‐aggregates, even when formed in solution. Here, it is demonstrated that cyanine J‐aggregates can reach an order of magnitude higher photoluminescence quantum yield (increase from 5% to 60%) in blend solutions of water and alkylamines at room temperature. By means of time‐resolved photoluminescence studies, an increase in the exciton lifetime as a result of the suppression of non‐radiative processes is shown. Small‐angle neutron scattering studies suggest a necessary condition for the formation of such highly emissive J‐aggregates: the presence of a sharp water/amine interface for J‐aggregate assembly and the coexistence of nanoscale‐sized water and amine domains to restrict the J‐aggregate size and solubilize monomers, respectively.


Exciton-exciton annihilation
Exciton-exciton annihilation is a non-radiative interaction between two excitations, leading to an additional de-population of the excited state and thus a change in fluorescence lifetime. [1] We here present a quick estimate if exciton-exciton annihilation needs to be considered in our system. The laser is operated at 532 nm with 20 MHz repetition rate with an average power of 2.2 W / cm 2 . Each laser pulse would carry an energy of E = 110 nJ over an area of 1 cm 2 , which corresponds to n = 3 × 10 11 photons / cm 2 . At a lifetime of 140 ps and a laser repetition rate of 20 MHz, we can safely assume that we only need to look at one pulse. The samples were probed in a 1.5 m thick cuvette, which would average to N = 9 × 10 14 molecules / cuvette on an area of 1 cm 2 (1 mM solution). With an absorption cross section of a dye molecule  = 6.25 × 10 -17 cm 2 , [1] and an estimated coherence length of N = 10 molecules, the laser power is by far not sufficient to induce exciton-exciton annihilation.

Section S1: Small Angle Scattering on Microemulsions, Teubner-Strey formula
Teubner and Strey obtained the static scattering intensity distribution I(q) of micro-emulsions from a Landau theory. [2] The formula takes the following form: with  2  being the mean square fluctuation of the scattering density and q is the scattering vector. The correlation length of the fluctuations  is derived from the correlation function and a ⁄ , ⁄ .  The scattering profile of J-aggregates after addition of 5 volume % HA is a superposition of J-aggregates and D 2 O/HA (95/5) blend solution ( Figure S8). Compared to the pure samples, the J-aggregate scattering intensity was increased by orders of magnitude, but did not change its general shape. On the other side, the scattering contribution from the D 2 O/HA solution decreased significantly. One must conclude that HA forms complexes with the J-aggregate, an HA environment will increase the scattering contrast, while a D 2 O environment will decrease it. Scattering intensity is proportional to the scattering contrast squared. The scattering intensity profile from the D 2 O/HA solution could be fitted best with a general gaussian coil as form factor. The particles interact strongly and a mass fractal with a Gaussian cut-off as structure factor was introduced.
The form factor is given by [3] : whereby R g is the radius of gyration and  is the excluded volume parameter.
The structure factor is given as mass fractal with Gaussian cut-off [4] : The fitting parameters to the data are given in Table S2. Both datasets were fitted simultaneously with the same set of parameters (besides I 0 ).

Section S3: Variation in the amine water blend
In the main text we linked the morphology of the water/HA to the phase of the dye TDBC (emissive J-aggregate, non-emissive J-aggregate or monomer). The emission behavior was investigated in detail for the dye in a blend with EA and HA. Here we screen the aggregation behavior of the dye TDBC in a couple of additional water/amine blends. In the blends the different phases have a characteristic color appearance. An emissive J-aggregate is also formed with Propylamine and Octylamine, while non-emissive J-aggregates seem to dominate in Triethylamine and N-Methyldioctylamine (see Figure S11). The dye also shows a very different behavior in the amines alone. The insolubility of TDBC in Triethylamine and N-Methyldioctylamine confirms our claim that monomer solubility in the amine phase supports the formation of emissive J-aggregates. As shown in Figure S12, the difference between ethanolamine and ethylamine is the presence of -OH group and alkyl group, respectively. For 1:1 ratio of water/amine, ṭhere is a stark change in PLQY from 5% to 45%. This is due to reduced dissolution of monomers in ethanolamine compared to the alkyl group in ethylamine.