Initially, the acceptor, BODIPY 9, was synthesized. For the synthesis of BODIPY 9, pyrrole (6) was condensed with p-iodo benzaldehyde (7) in the presence of a catalytic amount of acid to form corresponding dipyrromethane (8). DDQ was used to oxidize 8 followed by its complexation with BF₃. OEt₂ furnished the acceptor BODIPY 9 (Scheme 1).

Next, the donor molecules TAPP 2 and 3 were synthesized via multicomponent condensation reactions. The acid-catalyzed reaction of p-methoxyaniline (10) and 4-((trimethylsilyl)ethynyl)benzaldehyde (12) formed the corresponding Schiff base, which was simultaneously condensed with 2,3-butanedione using Fe(ClO₄)₃.H₂O as the catalyst to furnish TAPP 13. In another effort, p-methylaniline (11) and 4-((trimethylsilyl)ethynyl)benzaldehyde (12) were condensed in an acid-catalyzed reaction to generate the Schiff base, which was further condensed with 2,3-butanedione to furnish TAPP 14 in good yield (Tasior et al., 2020). Then, both TAPP 13 and 14 were desilylated to obtain the corresponding TAPP 2 and 3, respectively (Scheme 2). All the dyes were characterized by NMR spectroscopy and mass spectrometric analyses. For example, in ¹H NMR spectrum of 13, phenyl protons were resonated as four doublets of four proton integration each in the aromatic region. Characteristic singlets for C-3/6 protons of the TAPP moiety and the methoxy and trimethyl silyl groups were observed at 6.34 ppm, 3.83 ppm, and 0.23 ppm, respectively, which confirmed the structure of 13 (Supplementary Figure S5). The disappearance of a singlet for the trimethylsilyl groups at 0.23 ppm and the appearance of a singlet at 3.08 ppm for the acetylenic protons confirm the formation of TAPP 3 (Supplementary Figure S11). Finally, the structures of TAPP 2 and 3 were confirmed by single-crystal X-ray crystallographic studies, as shown in Figure 1. Crystal structures of TAPP 2 and 3 showed that the pyrrolo[3,2-b]pyrrole units are planar, but the four aryl units remain out of plane, making them electronically non-conjugated with the pyrrolo[3,2-b]pyrrole units.

Finally, Sonogashira coupling of BODIPY 9 with TAPP 2 and 3 separately furnished dyads 4 and 5, respectively (Scheme 2). The structures of both dyads were confirmed by NMR spectroscopy and mass spectrometry analyses. In the ¹H NMR spectrum of dyad 4, all the characteristic peaks for both the TAPP 2 and BODIPY 9 moieties were present (Figure 2; Supplementary Figure S15). The acetylenic proton signal of the TAPP 2 moiety was absent, which confirmed the coupling of the two moieties through the acetylenic bond.

Orientations of the donors and acceptors in the dyads are important for efficient energy transfer between them. Thus, DFT studies were done to optimize the ground state geometries of the dyads 4 and 5 using the B3LYP/6-31G level of theory. These are shown in Table 1; Supplementary Table S4. In both dyads, the pyrrolo-pyrrole units are planar as expected, and the four attached aryl rings remain twisted with respect to the pyrrolo-pyrrole plane. Similar observations were found from the X-ray crystallographic structures discussed vide supra. Furthermore, BODIPY moieties are planar, but the attached 8-phenyl groups are twisted with respect to the BODIPY core. Thus, as a whole, in both dyads, the donor TAPP moieties and the acceptor BODIPYs are twisted with respect to each other; that is, they are not electronically conjugated. This suggests that energy transfer is possible in both dyads 4 and 5. The HOMO-LUMO structures and energies of both dyads are also calculated and tabulated in Table 1; Supplementary Table S4. None of the FMO’s electron densities are distributed over both chromophores. The electron densities in the HOMOs of both dyads are located on the pyrrolo-pyrrole unit and shifted towards the BODIPY core in LUMOs. These also indicate that the TAPP and BODIPY moieties are not conjugated; thus, energy transfer is feasible in both dyads, as discussed below.

Steady-state absorption and fluorescence properties of dyads 4 and 5 were recorded in methanol and compared with that of acceptor BODIPY 1 and donors TAPP 2 and 3. Different photophysical parameters are tabulated in Table 2, and their absorption and fluorescence spectra are shown in Figures 3, 4, respectively.

Donors TAPP 2 and 3 showed the longest absorption maximum (λ_abs) at ∼382 nm and high blue fluorescence (Φ_fl = 0.72), with a fluorescence maximum (λ_fl) at ∼440 nm; that is, the Stokes shifts of both the TAPPs are large (∼3,400 cm⁻¹). On the other hand, the absorption and fluorescence spectra of acceptor BODIPY 1 are red-shifted compared to TAPP 2 and 3. The λ_abs of BODIPY 1 is at 497 nm, and it showed greenish fluorescence with λ_em at 516 nm. Importantly, the fluorescence of BODIPY 1 is quenched (Φ_fl = 0.04) because of high non-radiative decay due to the free rotation of the C-8 phenyl ring (Sunahara et al., 2007).

Photophysical property studies of dyad 4 showed that absorption spectra contain λ_abs peaks of both donor and acceptor moieties at 400 nm and 500 nm, respectively (Figure 3; Table 2). This indicates no/less electronic conjugation between the TAPP (donor) and the BODIPY (acceptor) moieties as seen in the DFT optimized structure discussed vide supra. In fluorescence studies, when the TAPP moiety was excited at 395 nm, no characteristic blue fluorescence of the TAPP moiety or greenish-yellow fluorescence of BODIPY was observed. Complete quenching of TAPP fluorescence clearly indicates an efficient transfer of TAPP excitation energy to the acceptor BODIPY dyes. Due to the non-fluorescent nature of the acceptor BODIPY as discussed vide supra, emission of the BODIPY dyes was also not observed. The energy transfer efficiency was calculated from the decrease in the donor TAPP fluorescence, which showed extremely high energy transfer (99.83%) from TAPP to the BODIPY moiety.

Similar to dyad 4, dyad 5 also showed absorption spectra containing λ_abs peaks of both donor and acceptor moieties at 397 nm and 500 nm, respectively (Figure 4; Table 2). These prove that in dyad 5, the donor and acceptor moieties are not electronically conjugated as also predicted from DFT optimized structures discussed vide supra. While exciting dyad 5 at the donor part (395 nm), no fluorescence was observed either from the donor part or from the acceptor part. The drastic decrease in TAPP fluorescence clearly indicates an efficient transfer of TAPP excitation energy to the acceptor BODIPY dyes, as also seen in the case of dyad 4. As the acceptor BODIPY is non-fluorescent in nature, emission of the BODIPY dyes was also not observed. The energy transfer efficiency was calculated from the decrease in the donor fluorescence, which showed extremely high energy transfer (99.79%) from TAPP to the BODIPY moiety.

Next, the viscosity-sensing ability of these TAPP-BODIPY dyads was investigated. For this, the absorbance and fluorescence of dyad 5 were checked in solvents with higher viscosity, that is, in ethanol, 1-propanol, 1-butanol, 1-nonanol, 1-decanol, and methanol/glycerol mixtures, and these properties were compared with those determined in low polar methanol as discussed before. The absorption spectra of dyad 5 in different n-alcohols and glycerol-methanol (1:1) mixtures were similar to that of methanol (Supplementary Figure S1). However, remarkable changes were observed in fluorescence studies (Figures 5A, B). With excitation at the TAPP moiety (λ_ex = 395 nm), the fluorescence emission from the BODIPY moiety was increased sharply in both 1-decanol and the methanol/glycerol (1:1) mixture, showing greenish-yellow fluorescence (Figures 5A, B, D) compared to its non-fluorescent nature in methanol. In more viscous solvents, intramolecular rotation along C-C bonds connecting TAPP and BODIPY dyes decreases; thus, emission from the BODIPY moiety is enhanced. Importantly, fluorescence enhancement in both the solvents was similar when excited at TAPP as well as the BODIPY moiety in dyad 5. This also confirmed the efficient energy transfer from the TAPP moiety to the BODIPY moiety. In different alcohols as well as in methanol/glycerol mixtures, the fluorescence intensities of dyad 5 increased with an increase in their viscosities. Interestingly, in both studies, the logarithm of fluorescent intensity and the logarithm of viscosity of solution obeyed a linear relationship as per the Förster–Hoffmann equation (Förster and Hoffmann, 1971; Koenig et al., 2016). Furthermore, the temperature-dependent fluorescence of dyad 5 was measured in methanol and a glycerol-methanol (1:1) mixture. It was observed that with increasing temperature of the glycerol-methanol (1:1) mixture, emission from the BODIPY moiety decreases (Figure 5C). (Sen et al., 2022) Now, with increasing temperature, the viscosity of the glycerol-methanol (1:1) mixture decreases; thus, the emission from the BODIPY moiety also decreases (Figure 5C). On the other hand, no/very little change in the fluorescence of dyad 5 is observed with increasing temperature in pure methanol, as the temperature-dependent viscosity change in methanol is negligible (Supplementary Figure S4). This clearly indicates that the fluorescence properties of dyad 5 depend on the viscosity of the medium. This showed the ability of dyad 5 to sense the viscosity as anticipated.

The detailed experimental methods and the data for the characterization of synthesized compounds (¹H and ¹³C NMR spectrum) are given in the Supplementary Material.

p-Methoxy/methyl aniline (1 mmol) and 4-((trimethylsilyl)ethynyl) benzaldehyde (1 mmol) were mixed in glacial acetic acid/toluene (1:10, 4 mL), and the mixture was heated at 50°C for 1 h in a 50 mL Schlenk tube. Then, Fe(ClO₄)₃·xH₂O (0.03 mmol) and 2,3-butadione (0.5 mmol) were added, and the mixture was heated again at 90°C for 12 h. Next, the crude product was dried under reduced pressure and subjected to column chromatography (silica gel, DCM/petroleum ether, 30:70) to furnish the pure products TAPP 13/14, respectively.

TAPP 13 was synthesized by following the general procedure using p-methoxyaniline (123 mg, 1.0 mmol), 4-((trimethylsilyl)ethynyl)benzaldehyde (202 mg, 1.0 mmol), 2,3-butadione (43 μL, 0.5 mmol), and Fe(ClO₄)₃ .H₂O (11 mg, 0.03 mmol). Pure TAPP 13 was obtained as a yellow solid. Yield: 344 mg, (52%), R_f = 0.75 (DCM/hexane, 30:70, v/v); ¹H NMR (500 MHz, CDCl₃): δ 0.23 (s, 18H), 3.83 (s, 6H), 6.35 (s, 2H), 6.89 (d, J = 8.0 Hz, 4H), 7.12 (d, J = 7.0 Hz, 4H), 7.18 (d, J = 8.0 Hz, 4H), 7.30 (d, J = 8.5 Hz, 4H) ppm; ¹³C NMR (125 MHz, CDCl₃): δ -0.03, 55.5, 94.1, 94.6, 105.3, 114.4, 120.2, 126.6, 127.5, 131.8, 132.5, 132.9, 133.6, 135.6, 157.7 ppm; HRMS (ESI/Q-TOF) m/z: [M + H]⁺ Calcd for C₄₂H₄₃N₂O₂Si₂ 663.2857; Found 663.2821.

TAPP 14 was synthesized by following the general procedure using p-methylaniline (107 mg, 1.0 mmol), 4-((trimethylsilyl)ethynyl) benzaldehyde (202 mg, 1.0 mmol), 2,3-butanedione (43 μL, 0.5 mmol), and Fe(ClO₄)₃.H₂O (11 mg, 0.03 mmol). Pure TAPP 14 was obtained as a yellow solid. Yield: 208 mg (66%); R_f = 0.45 (DCM/hexane, 30:70, v/v); ¹H NMR (500 MHz, CDCl₃): δ 0.23 (s, 18H), 2.37 (s, 6H), 6.38 (s, 2H), 7.12–7.15 (m, 12H), 7.30 (d, J = 8.1 Hz, 4H) ppm; ¹³C NMR (125 MHz, CDCl₃): δ -0.03, 20.9, 94.5, 94.8, 105.3, 120.3, 125.1, 127.6, 129.8, 131.7, 132.4, 133.7, 135.5, 135.7, 137.3 ppm; HRMS (ESI/Q-TOF) m/z: [M + H]⁺ Calcd for C₄₂H₄₃N₂Si₂ 631.2959; Found 631.2964.

TAPP 13/14 (0.16 mmol) and K₂CO₃ (0.5 mmol) were mixed in dry MeOH:DCM (1:1, 10 mL), and the mixture was stirred at room temperature for 24 h. The reaction mixture turned yellow, was extracted with dichloromethane (3 × 30 mL), and washed with saturated aq. NaHCO₃ (30 mL), brine (30 mL) and water (30 mL). The organic layer was dried over anhydrous Na₂SO₄ and evaporated under reduced pressure. The crude product was purified by column chromatography (silica gel, DCM/petroleum ether, 50:50) to furnish the pure products TAPP 2/3, respectively.

TAPP 2 was synthesized by following the general procedure using TAPP 13 (105 mg, 0.16 mmol) and K₂CO₃ (68 mg, 0.5 mmol). Pure TAPP 2 was obtained as a yellow solid. Yield: 67 mg (80%); R_f = 0.64 (DCM/hexane, 40:60, v/v); ¹H NMR (500 MHz, CDCl₃): δ 3.08 (s, 2H), 3.84 (s, 6H), 6.36 (s, 2H), 6.91 (d, J = 9.0 Hz, 4H), 7.15 (d, J = 8.5 Hz, 4H), 7.20 (d, J = 9.0 Hz, 4H), 7.33 (d, J = 8.5 Hz, 4H) ppm; ¹³C NMR (125 MHz, CDCl₃): δ 55.5, 77.5, 83.8, 94.3, 114.5, 119.3, 126.6, 127.6, 131.9, 132.6, 132.9, 134.0, 135.6, 157.8 ppm; HRMS (ESI/Q-TOF) m/z: [M + H]⁺ Calcd for C₃₆H₂₇N₂O₂ 519.2067; Found 519.2076.

TAPP 3 was synthesized by following the general procedure using TAPP 14 (101 mg, 0.16 mmol) and K₂CO₃ (69 mg, 0.5 mmol). Pure TAPP 3 was obtained as a yellow solid. Yield: 73 mg (94%); R_f = 0.46 (DCM/hexane, 30:70, v/v); ¹H NMR (500 MHz, CDCl₃): δ 2.38 (s, 6H), 3.07 (s, 2H)s, 6.39 (s, 2H), 7.15–7.17 (m, 12H), 7.33 (d, J = 8.3 Hz, 4H) ppm; ¹³C NMR (125 MHz, CDCl₃): δ 21.0, 77.5, 83.8, 94.9, 119.3, 125.1, 127.6, 129.8, 131.9, 132.4, 134.0, 135.4, 135.8, 137.3 ppm; HRMS (ESI/Q-TOF) m/z: [M + H]⁺ Calcd for C₃₆H₂₇N₂ 487.2168; Found 487.2163.

A mixture of 4-iodobenzaldehyde (7) (700 mg, 3.0 mmol), an excess of pyrrole (6) (9 mL, 120.6 mmol), and trifluoroacetic acid (5 drops) was stirred at 25°C for 1 day. Excess pyrrole was distilled off, and the residue was purified by flash column chromatography (silica gel, ethyl acetate/petroleum ether, 20:80) to obtain dipyrromethane 8 (770 mg, 73%) as a cream color solid. Then, 8 (770 mg, 2.2 mmol) was dissolved in dry DCM, DDQ (753 mg, 3.3 mmol) was added into it, and the resulting mixture was stirred at 25°C for 4 h. Next, NEt₃ (1.8 mL, 13 mmol) and BF₃.OEt₂ (1.6 mL, 13 mmol) were added, and stirring was continued for another 12 h. The reaction mixture was quenched with sat. NaHCO₃ solution (50 mL), extracted with dichloromethane (100 mL), washed with water (3 × 25 mL), and dried with Na₂SO₄. The organic layer was concentrated in vacuo, and the crude product was purified by flash column chromatography (silica gel, ethyl acetate/petroleum ether, 5:95) to obtain BODIPY 9 as a dark orange solid (Betancourt-Mendiola et al., 2015). Yield: 159 mg (18%); R_f = 0.65 (ethyl acetate/hexane, 25: 75, v/v); ¹H NMR (500 MHz, CDCl₃): δ 6.56 (d, J = 4.2 Hz, 2H), 6.90 (d, J = 4.3 Hz, 2H), 7.30 (d, J = 8.3 Hz, 2H), 7.89 (d, J = 8.3 Hz, 2H), 7.95 (s, 2H) ppm; ¹³C NMR (125 MHz, CDCl₃): δ 97.5, 118.8, 131.3, 131.9, 133.1, 134.6, 137.7, 144.5, 145.9 ppm; HRMS (ESI/Q-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₁₁BF₂IN₂ 395.0022; Found 395.0021.

TAPP 2/3 (0.05 mmol), BODIPY 9 (0.10 mmol), and di-isopropyl amine (0.5 mL) were dissolved in dry THF (6 mL), and the solution was degassed properly. Pd(PPh₃)₂Cl₂ (3.5 mg, 0.005 mmol) and CuI (1 mg, 0.005 mmol) were added, and the reaction was stirred at 25°C for 24 h. Removal of the solvent in vacuo followed by column chromatography of the residue (silica gel, DCM/petroleum ether, 70:30) furnished dyads 4/5, respectively.

Dyad 4 was synthesized by following the general procedure using TAPP 2 (26 mg, 0.05 mmol) and BODIPY 1 (42 mg, 0.1 mmol). Pure dyad 4 was obtained as a dark red solid. Yield: 40 mg (80%). R_f = 0.35 (DCM/hexane, 70:30, v/v); ¹H NMR (500 MHz, CDCl₃): δ 3.86 (s, 6H), 6.42 (s, 2H), 6.57 (d, J = 7.4 Hz, 4H), 6.93–6.96 (m, 8H), 7.23–7.24 (m, 8H), 7.42 (d, J = 7.4 Hz, 4H), 7.56 (d, J = 7.7 Hz, 4H), 7.65 (d, J = 7.7 Hz, 4H), 7.96 (s, 4H) ppm; ¹³C NMR (125 MHz, CDCl₃): δ 55.5, 88.9, 92.6, 94.4, 114.5, 118.7, 119.8, 126.3, 126.7, 127.7, 130.6, 131.4, 131.5, 131.6, 132.9, 133.3, 134.7, 144.3, 146.5, 157.9 ppm; MS (MALDI-TOF): m/z [M]⁺ Calcd for C₆₆H₄₄B₂F₄N₆O₂: 1,050.4; Found 1,050.1.

Dyad 5 was synthesized by following the general procedure using TAPP 3 (25 mg, 0.05 mmol) and BODIPY 1 (42 mg, 0.10 mmol). Pure dyad 5 was obtained as a dark red solid. Yield: 24 mg (48%); R_f = 0.45 (DCM/hexane, 70:30, v/v); ¹H NMR (500 MHz, CDCl₃): δ 2.41 (s, 6H), 6.45 (s, 2H), 6.56 (d, J = 7.4 Hz, 4H), 6.95 (d, J = 7.6 Hz, 4H), 7.19–7.21 (m, 8H), 7.23 (d, J = 8.1 Hz, 4H), 7.42 (d, J = 8.1 Hz, 4H), 7.56 (d, J = 7.9 Hz, 4H), 7.65 (d, J = 7.9 Hz, 4H), 7.95 (s, 4H) ppm; ¹³C NMR (125 MHz, CDCl₃): δ 21.0, 88.9, 92.6, 94.9, 118.7, 119.8, 125.1, 126.3, 127.7, 129.9, 130.5, 131.3, 131.4, 131.5, 132.6, 133.3, 133.9, 134.7, 135.5, 135.8, 137.3, 144.2, 146.4 ppm. HRMS (ESI/Q-TOF) m/z: [M + H]⁺ Calcd for C₆₆H₄₅B₂F₄N₆ 1,019.3822; Found 1,019.3820.