A Record Chromophore Density in High-Entropy Liquids of Two Low-Melting Perylenes: A New Strategy for Liquid Chromophores

Abstract

Liquid chromophores constitute a rare but intriguing class of molecules that are in high demand for the design of luminescent inks, liquid semiconductors, and solar energy storage materials. The most common way to achieve liquid chromophores involves the introduction of long alkyl chains, which, however, significantly reduces the chromophore density. Here, strategy is presented that allows for the preparation of liquid chromophores with a minimal increase in molecular weight, using the important class of perylenes as an example. Two synergistic effects are harnessed: (1) the judicious positioning of short alkyl substituents, and (2) equimolar mixing, which in unison results in a liquid material. A series of 1-alkyl perylene derivatives is synthesized and it is found that short ethyl or butyl chains reduce the melting temperature from 278 °C to as little as 70 °C. Then, two low-melting derivatives are mixed, which results in materials that do not crystallize due to the increased configurational entropy of the system. As a result, liquid chromophores with the lowest reported molecular weight increase compared to the neat chromophore are obtained. The mixing strategy is readily applicable to other π-conjugated systems and, hence, promises to yield a wide range of low molecular weight liquid chromophores.

Keywords: alkylation; high quantum yield; liquid fluorophores; low melting solids; thermodynamic mixing.

Scheme 1

Synthesis of alkylated perylenes.

Figure 1

a) Structure of two enantiomers of 1‐iso‐butylperylene (2c) shown at a shortest distance (Å). b) Packing of 1‐iso‐butylperylene molecules in a unit cell, image viewed along 111 from refined data.

Scheme 2

Plausible mechanistic pathway for the formation of 2a.

Figure 2

Inversion barrier of 2a versus dihedral angle from DFT calculations.

Figure 3

a,b) Normalized absorbance and c,d) emission spectra of a,c) 1‐alkylperylenes and b,d) 3‐ & 2‐ alkylperylenes in cyclohexane.

Figure 4

a) Differential scanning calorimetry (DSC) thermograms of 1 and 2a‐j representing the initial heating half‐cycle of as‐synthesized powder and of material solidified from DCM (kept at −50 °C for 30 min before the scan; darker lines). The heat flow is normalized to the peak height of each melting endothermic transition. The insets show molecular structures with substituent positions. b) DSC thermograms representing the initial heating half‐cycle for blends of 2a and 2b solidified from DCM (kept at −50 °C for 30 min prior to the first heating scan). Glass transition (T _g) and eutectic transition (T _eutectic) temperatures are marked by vertical dotted lines. c) Phase diagram of mixtures of 2a and 2b. The liquidus (solid lines) are sketched following the peak melting temperature (squares) of each blend. The glass transition and eutectic transition (dashed lines) are sketched following the enthalpy relaxation peak associated with the glass transition (triangles) and the peak temperature for the eutectic point (circles), respectively. The lightly gray area indicates the region where the blend remains liquid. The gray area indicates the region where the blend is partially crystalline and the remaining area indicates the region where the blend is a liquid or a glass. The inset shows a grazing‐incidence wide angle X‐ray scattering (GIWAXS) diffractogram of a 1:1 molar ratio blend of 2a and 2b kept at room temperature for 1 month.

Figure 5

Illustration of the impact of mixing on the melting temperature, which decreases from T_m to $T_m^{\prime }$, and driving force for crystallization at temperature T, which decreases from ΔG _lc to $\mathrm{\Delta }{G}_{\mathrm{lc}}^{\prime }$, where G_c, G_l, and $G_l^{\prime }$ are the Gibbs free energy of the crystalline state (G_c of only one component is shown for clarity), of the single‐component liquid, and of the multicomponent liquid, respectively. Above $T_m^{\prime }$, the multicomponent liquid is thermodynamically stable, whereas below $T_m^{\prime }$ the crystallization kinetics are slowed due to a reduced $\mathrm{\Delta }{G}_{\mathrm{lc}}^{\prime }$.

Figure 6

a,c) Bright‐field and b,d) cross‐polarized optical micrographs of a drop cast 1:1 molar ratio blend of 2a and 2b after keeping at a,b) −18 °C and c,d) −5 °C for 30 days. e) A photograph of a liquid droplet comprising a 1:1 molar ratio blend of 2a and 2b on a needle tip at room temperature.