Entropic Mixing Allows Monomeric‐Like Absorption in Neat BODIPY Films

Abstract Intermolecular interactions play a crucial role in materials chemistry because they govern thin film morphology. The photophysical properties of films of organic dyes are highly sensitive to the local environment, and a considerable effort has therefore been dedicated to engineering the morphology of organic thin films. Solubilizing side chains can successfully spatially separate chromophores, reducing detrimental intermolecular interactions. However, this strategy is also significantly decreasing achievable dye concentration. Here, five BODIPY derivatives containing small alkyl chains in the α‐position were synthesized and photophysically characterized. By blending two or more derivatives, the increase in entropy reduces aggregation and therefore produces films with extreme dye concentration and, at the same time almost solution like absorption properties. Such a film was placed inside an optical cavity and the achieved system was demonstrated to reach the strong exciton‐photon coupling regime by virtue of the achieved dye concentration and sharp absorption features of the film.


Methods and Materials
All reactions were carried out under nitrogen atmosphere unless stated differently. Glassware were oven dried prior to use. Unless indicated otherwise, common reagents, solvents, or materials were obtained from Sigma-Aldrich Chemical Co. and used without further purification. Dry solvents for reactions sensitive to moisture and/or oxygen were obtained through a solvent purifying system (MBRAUN SPS-800). Column chromatography was performed using silica gel (VWR 40 to 63 μm) unless stated otherwise. Flash chromatography was performed by a Teledyne CombiFlash EZ prep using normal-phase silica with a mesh size of 230 to 400, a particle size of 40 to 63 μm, and a pore size of 60 Å. 1 H ( 13 C) NMR spectra were recorded on a Varian 400 spectrometer (400 MHz 1 H; 100 MHz 13 C) at room temperature using CDCl3 (containing tetramethylsilane with 0.00 ppm as an internal reference) as solvent. Coupling constants (J values) are given in Hertz (Hz) and chemical shifts are reported in parts per million (ppm). High-resolution MS was obtained from an Agilent 1290 infinity LC system equipped with an auto sampler in tandem with an Agilent 6520 Accurate Mass Q-TOF LC/MS. Melting points were measured using a BÜCHI Melting Point B-545 instrument. IR spectra were recorded using an INVENIO R instrument from BRUKER.

Optical microscopy
Transmission optical micrographs were recorded with a Zeiss Axioscope 5 equipped with a pair of crossed polarizers.

Sample preparation
Neat films and cavities were prepared on glass substrates (25 × 25 mm), which were precleaned by sonication for 15 min in alkaline solution (0.5% of Hellmanex in distilled water), then rinsed with water and sonicated for 1 h in water and ethanol, respectively. The cleaned glass substrates were dried in an oven overnight prior to use. To avoid any inner filter effects, which are dependent on the thickness of the films, and to have absorbances below 0.1, the neat films as well as the blends were prepared to be very thin (oder of a few 10 th of nm). For neat films, solutions of the BODIPY dyes (cdye = 1.0 mg mL -1 ) in toluene were spin coated (45 sec, R.T., 1500 rpm) (Laurell) on glass substrates. The ratio of the components used in the mixed films is 1:1, 1:1:1 and 1:1:1:1:1 respectively. For the cavity, poly(vinyl alcohol) (PVA, 99+% hydrolyzed, Sigma Aldrich, 20 mg mL -1 ) was dissolved in water and equal amounts of sBu-and IP BODIPY (each 2.25 mg mL -1 ) were dissolved in a toluene solution of poly(2-vinylnaphtalene) (Sigma Aldrich, 0.5 mg mL -1 ) resulting in a mass ratio of BODIPY dyes to polymer of 9:1. The layered structure of PVA, BODIPY and PVA on a silver mirror (120 nm) was made by step wise spin coating the corresponding solution on top of each other (45 sec, R.T., 1200 rpm). The total thickness of the three layers is roughly 115 nm. The optical cavity was sealed by depositing a second silver mirror (20 nm). The silver mirrors were fabricated by vacuum sputtering deposition (HEX, Korvus Technologies). The photonic contribution to the lower polariton is 8% whereas the excitonic contribution is 92%, at incident light beam. The contributions to the upper polariton are vice versa. Figure S1. Cavity structure.

Optical spectroscopy
Reflectance spectra were measured using a spectrophotometer (LAMBDA 950, PerkinElmer) with a universal reflectance accessory. Steady-state emission spectra, excitation spectra and emission lifetimes were measured with a spectrofluorometer (FLS1000, Edinburgh Instrument). For the emission lifetime measurements, the samples were excited by a 475 nm picosecond pulsed diode laser (Edinburgh Instruments). The emission quantum yield were measured using the spectrofluorometer (FLS1000, Edinburgh Instrument) equipped with an integration sphere. As a reference a blank glass substrate was used.

Coupled harmonic oscillator model
The Rabi splitting was extracted by fitting the experimental data to the coupled harmonic oscillator model [1] . In the model, the coupling between one exciton and one photon is described by a 2 × 2 matrix Hamiltonian: Where is the fixed exciton energy, ħΩ is the Rabi splitting, , are the mixing coefficients for the system (Hopfield coefficients) and ( ) is the cavity energy, whose angle dependence follows equation (2).
Where 0 is the cavity energy at normal incidence, is the incidence and is the effective refractive index.     Wavelength (nm) Figure S12. Absorbance (green), emission (blue) and excitation at different emission wavelengths (red) spectra of mixed films.

Synthesis 2-Acetylpyrrole
POCl3 (2.8 mL, 30 mmol, 1.2 eq) was added drop-wise to N,N-dimethylacetamide (2.8 mL, 30 mmol, 1.2 eq) at 0 ᵒ C. The mixture was warmed up to room temperature and stirred until the Vielsmeier reagent was formed. The formed solid was dissolved in 1,2dichloroethane (5 mL) and the solution was cooled down to 0 ᵒ C. Pyrrole (1.7 mL, 25 mmol, 1.0 eq) in 1,2-dichloroethane (10 mL) was added dropwise over a period of 20 min at 0 °C. After the addition was finished the reaction mixture was refluxed for 30 min and afterwards cooled down to room temperature. A solution of NaOAc (10.25 g, 125 mmol, 5.0 eq) in water was added to the reaction mixture and the mixture was refluxed again for 30 min. After the mixture was cooled to room temperature the 2 phase system was separated. The aqueous phase was extracted with DCM (3 x 25 mL). The combined organic phases were washed with water (1 x 50 mL), saturated Na2CO3 solution (2 x 50 mL) and brine (2 x 50 mL). The washed organic phase was dried over Na2SO4 and the solvent was removed under reduced pressure. The product was purified by column chromatography on SiO2 using 20 % EtOAc in hexane as eluent which afforded the product as a white solid (2.29 g, 21 mmol, 84 %

2-Ethylpyrrole
To Recorded data are in accordance with the literature. [3]

N,N-Dimethylbutyramide
Dimethylamine (2 M solution in THF) (52,5 mL, 105 mmol, 1.5 eq) was diluted in DCM (0.2 M) and NEt3 (29.4 mL, 210 mmol, 3 eq) was added to the solution. Butyryl chloride (7.35 mL, 70 mmol, 1 eq) in DCM was added dropwise at 0 °C. The reaction mixture was quenched with a saturated aqueous NH4Cl solution after stirring at room temperature for 18 h. The phases were separated and the aqueous phase was extracted with DCM (2 x 50 mL). The combined organic phases were dried over Na2SO4 and the solvent was removed under reduced pressure. The product was purified using column chromatography (SiO2, 0 -20 % DMA mix/DCM). The product was obtained as a white solid (

1-(1H-Pyrrol-2-yl)butan-1-one
POCl3 (2.81 mL, 30 mmol, 1.2 eq.) was added slowly to N,N -dimethylbutyramide (3.45 g, 30 mmol, 1.2 eq.) at 0 °C. The reaction mixture was stirred for 1 h at room temperature. Afterwards the reaction mixture was cooled down to 0 °C and pyrrole (1.81 mL, 25 mmol, 1 eq) dissolved in 1,2-dichloroethane (5 mL) was added to dropwise over a period of 15 min. The resulting mixture was allowed to warm up to room temperature and stirred for 18 h. An aqueous solution of NaOAc (10.25 g, 125 mmol, 5 eq) was added to the reaction mixture and the 2 phase system was left to stir for 1 h at room temperature. The reaction mixture was extracted with DCM (3 x 2 mL) and the combined organic phases were washed with H2O (2 x 50 mL), Na2CO3 (2 x 50 mL) and brine (2 x 50 mL). The washed organic phases were subsequently dried over Na2SO4 and the solvent was removed under reduced pressure. Flash chromatography on SiO2 using 5 -20% EtOAc in hexane as an eluent afforded the product as colourless crystals (2.87 g, 20.9 mmol, 84 %

2-Butylpyrrole
1-(1H-pyrrol-2-yl)butan-1-one (20 mmol, 2.87 g, 1 eq) was dissolved in isopropanol (150 mL) and stirred for 5 min. NaBH4 (80 mmol, 3.03 g, 4 eq) was added and the reaction mixture was refluxed for 18 h. Afterwards the reaction mixture was concentrated under reduced pressure. The concentrated mixture was subsequently taken up in H2O and Et2O the aqueous phase was extracted with Et2O (3 x 50 mL). The combined organic phases were washed with H2O, dried over Na2SO4 and the solvent was removed under reduced pressure. The product was used without further purification ( Recorded data are in accordance with the literature. [6] General Procedure A: to make the branched alkylated pyrroles a modified literature procedure was used [7] Magnesium powder (1 eq) was stirred in dry Et2O at r.t. MeI (1 eq) in dry Et2O was added dropwise to the reaction mixture over 30 min. The mixture started to reflux and was left to reflux for 1.5 h after the addition was finished. The reaction mixture was cooled down to room temperature and pyrrole (1 eq) in dry Et2O was added over 30 min. The mixture was left to reflux for 1 h after the addition of pyrrole was finished. The alkylbromide (1 eq) was added dropwise to the reaction of the formed pyrrole-grignard reagent. The resulting solution was refluxed for 24 h and then cooled down to room temperature. The cooled reaction mixture was quenched using a saturated NH4Cl solution. The 2 phases were separated and the aqueous phase was extracted using Et2O. The combined organic phases were dried over Na2SO4 and the solvent was removed under reduced pressure. The crude mixture was purified via vacuum distillation. The purification afforded a mixture of 2-alkyl and 3-alkyl substituted pyrrole. The mixture was used for the next step without further purification.

2-Butyl-5-formylpyrrole
POCl3 (0.21 mL, 2.2 mmol, 1.1 eq) in dry toluene (0.3 mL) was added dropwise to N,N-dimethylformamide (0.17 mL, 2.2 mmol, 1.1 eq) in dry toluene (0.4 mL) at 0 °C. The mixture was warmed up to room temperature and stirred until the Vielsmeier reagent was formed. 2-Butylpyrrole (250 mg, 2.0 mmol, 1.0 eq) in DMF (0.5 mL) was added drop-wise over a period of 20 min at 0 °C. After the addition was finished, the reaction mixture was stirred for 1 h at room temperature. The reaction mixture was poured in iced water (20 mL) and NaOHaq was added until the solution was alkaline. The mixture was extracted with CHCl3 (3 x 20 mL) and afterwards dried with Na2SO4. The solvent was removed under reduced pressure and the crude product was purified by column chromatography on silica using 20 % EtOAc in hexane as an eluent which afforded the product as a light yellow solid (254 mg, 1.68 mmol, 84 %). Recorded data are in accordance with the literature. [8] General Procedure B: for the Vielsmeier-Haack Formylation a literature procedure was used [9] POCl3 (1.2 eq) was added dropwise to DMF (1.2 eq) at 0 °C. The mixture was left to warm up to room temperature and stirred until the Vielsmeier reagent was formed. The formed solid was subsequently dissolved in 1,2-dichloroethane and the solution was cooled down to 0 °C. The alkylpyrrole (1.0 eq) in 1,2-dichloroethane was afterwards added dropwise over a period of 30 min at 0 °C. After the addition was finished, the reaction mixture was refluxed for 30 min and cooled to room temperature. An aqueous solution of NaOAc (5.0 eq) was added and the mixture was again refluxed for 30 min. After cooling down to room temperature the 2 phase system was separated and the aqueous phase was extracted with DCM. The combined organic phases were washed with H2O, saturated Na2CO3 solution and brine afterwards dried with Na2SO4 and the solvent was removed under reduced pressure. The product was purified by column chromatography on silica gel using 0 -20 % EtOAc in hexane as an eluent.

2-secButyl -5-formylpyrrole
The compound was synthesized following the general Procedure B using a mixture of 3-and 2-secbutylpyrrole (1.72 g, 14.0 mmol). The product was purified by column chromatography on silica gel using 0 -20 % EtOAc in hexane as an eluent. The product was afforded as an off-white oily liquid (1.08 g, 7.14 mmol, 73%

2-tertButyl-5-formylpyrrole
The compound was synthesized following the general Procedure B using a mixture of 3-and 2-tertbutylpyrrole (containing 50 % of 2-tertbutylpyrrole) (246 mg, 2.0 mmol). The product was purified by column chromatography on silica gel using 0 -15 % EtOAc in hexane as an eluent. The product was afforded as an off-white oily liquid (107 mg, 0.71 mmol, 71 %). Obtained NMR data are in agreement with the literature. [10] α-EthylBODIPY 2-Ethyl-5-formylpyrrole (246 mg, 2 mmol, 1 eq) was dissolved in DCM/pentane [2/1] (1 mL) and the solution was cooled down to 0 °C. POCl3 (0.18 mL, 2 mmol, 1 eq) was added dropwise over a period of 3 min. The resulting mixture was stirred at room temperature for 4 h. Et3N (2.78 mL, 20 mmol, 10 eq) was added dropwise over 5 min at room temperature and the reaction mixture was left to stir for 15 min before cooling the solution down to 0 °C. BF3·OEt2 (2.73 mL, 22 mmol, 11 eq) was added dropwise at 0 °C and the reaction mixture was afterwards stirred at room temperature for 1.5 h. The reaction mixture was poured in Et2O (100 mL) and the organic phase was washed with H2O (4 x 50 mL). The organic phase was dried over Na2SO4 and the solvent was removed under reduced pressure. The product was purified via column chromatography on SiO2 using 10-25 % DCM in hexane as eluent. Obtained data are in agreement with the literature. [9] α-tertButylBODIPY 2-tertButyl-5-formylpyrrole (302 mg, 2.0 mmol, 1 eq) was dissolved in DCM (10 mL) and the solution was cooled down to 0 °C. POCl3 (0.23 mL, 2.4 mmol, 1.2 eq) was added dropwise over a period of 5 min. The resulting mixture was stirred at room temperature for 6 h. Et3N (1.40 mL, 10 mmol, 5 eq) was added dropwise over 5 min at 0 °C and the reaction mixture was left to stir for 20 min before BF3·OEt2 (2.00 mL, 16.0 mmol, 8 eq) was added dropwise at 0 °C. The reaction mixture was afterwards slowly warmed to room temperature and stirred for 12 h. The reaction mixture was passed through a short pad of silica and eluted with DCM. The solvent was evaporated and the residue was again dissolved in DCM (25 mL). H2O was added to the organic phase and the two-phase mixture was stirred at room temperature for 2 h. Afterwards the organic phase was washed with H2O (3 x 50 mL). The organic phase was dried over Na2SO4 and the solvent was removed under reduced pressure. After purification, using column chromatography (SiO2, 5 -20 % DCM in hexane), the product was obtained as a red crystalline solid (42 mg, 0.15 mmol, 15 % Obtained data are in agreement with the literature. [11] General Procedure C: for the BODIPY condensation 2-Alkyl-5-formylpyrrole (1 eq) was dissolved in DCM/pentane [2/1] and the solution was cooled down to 0 °C. POCl3 (2 eq) was added dropwise over a period of 3 min. The resulting mixture was stirred at room temperature for 2.5 h. Et3N (6 eq) was added dropwise over 5 min at room temperature and the reaction mixture was left to stir for 15 min before cooling the solution down to 0 °C. BF3·OEt2 (9 eq) was added dropwise at 0 °C and the reaction mixture was afterwards stirred at room temperature for 30 min. Repeatedly, Et3N (6 eq) was added dropwise over 5 min at room temperature and the reaction mixture was left to stir for 15 min before cooling the solution down to 0 °C. BF3·OEt2 (9 eq) was added dropwise at 0 °C and the reaction mixture was afterwards stirred at room temperature for 1 h. The reaction mixture was poured in Et2O and the organic phase was washed with H2O. The organic phase was dried over Na2SO4 and the solvent was removed under reduced pressure. The product was purified via column chromatography on SiO2 using 10-20 % DCM in hexane as eluent.

α-ButylBODIPY
The compound was synthesized following the general Procedure C using 2-butyl-5-formylpyrrole (0.65 g, 4.3 mmol, 1 eq). The product was purified via column chromatography on SiO2 using 10-15 % DCM in hexane as eluent. The product was obtained as a red crystalline solid (63. 8