w<strong>as</strong> refluxed in a Dean-Stark apparatus for 4 h. The resulting deep green solution w<strong>as</strong> chilled at 0°C overnight and the formed golden precipitate collected by suction filtration. Crystallization from EtOH gave the pure compound <strong>as</strong> a golden green powder (0.100 mg, 0.12 mmol, 18.1 %). M.p. 169-170 °C. 1 H NMR (DMSO-d 6 ) δ 8.94 (2H, J = 4.7 Hz), 8.57 (2H, d, J = 8.4 Hz), 8.34 (2H, d, J = 15.8 Hz), 8.06 (2H, d, J = 8.3 Hz), 7.98 (2H, d, J = 4.7 Hz), 7.81 (2H, d, J = 15.8 Hz), 7.79 (2H, t, J = 8.3 Hz), 7.77 (2H, d, J = 4.7 Hz), 7.69 (2H, t, J = 7.9 Hz), 7.62 (2H, d, J = 4.6 Hz), 5.12 (4H, t, J = 4.8 Hz), 3.74 (4H, t, J = 4.8 Hz), 3.47 (4H, m), 3.35 (4H, m), 3.27 (4H, m), 3.16 (4H, m), 3.05 (6H, s). Anal. Calcd for C 48 H 50 N 4 O 8 : C, 71.09; H, 6.21; N, 6.91; Found: C, 70.64; H, 6.19; N, 6.83. Preparation of squaraine 3. A suspension of 2-methylindolizine 3 (1.680 g, 12.81 mmol) and squaric acid (0.730 g, 6.40 mmol) in a 1:1 mixture of BuOH/toluene (75 ml) w<strong>as</strong> refluxed for 2h in a Dean-Stark apparatus. Upon cooling a dark green precipitate w<strong>as</strong> formed and collected by suction filtration. Crystallization from CH 3 CN afforded the pure compound <strong>as</strong> a green powder (0.101 g, 0.25 mmol, 26 %). M.p. > 300 °C (dec). 1 H NMR (CD 2 Cl 2 ) δ 10.27 (2H, d, J = 6.9 Hz), 7.26 (2H, d, J = 8.5 Hz), 7.11 (2H, t, J = 8.0 Hz), 6.81 (2H, t, J = 6.6 Hz), 6.46 (2H, s), 2.76 (6H, s); 13 C (CDCl 3 ) 176.3, 166.5,143.0, 139.2, 135.0, 127.5, 122.5, 117.7, 113.8, 111.9, 15.4. Anal. Calcd for C 22 H 16 N 2 O 2 : C, 77.63; H, 4.74; N, 8.23; Anal. Calcd for C 22 H 16 N 2 O 2 x ¼ H 2 O: C, 76.62; H, 4.82; N, 8.12; Found: C, 76.90; H, 4.72; N, 7.63. N O O H 3 C N CH 3 3 Squaraine 3 structure w<strong>as</strong> <strong>as</strong>signed on the b<strong>as</strong>is of its 1 H NMR spectrum. 2-methylindolizine 1 H NMR spectrum (DMSO-d 6 ) in fact shows two singlets in the high field aromatic region at 7.31 ppm and 6.18 ppm corresponding respectively to the H 3 and H 1 protons. Squaraine 3 1 H NMR spectrum (CD 2 Cl 2 ) only displays one singlet at 6.46 ppm. According to the electron withdrawing nature of the squaraine core, and so with the expected low field shift of the indolizine signals in the squaraine, the 6.46 ppm singlet can be unambiguously <strong>as</strong>signed to the position 1 of a 3 substituted 2- methylindolizine, hence the <strong>as</strong>signed condensation regiochemistry. Indolizine numbering: 6
7 6 8 N 4 5 1 3 2 DPBF degradations. ABS 1,2 1,0 0,8 0,6 0,4 0,2 0 min 3 min 6 min 9 min 12 min 15 min 20 min 25 min 30 min 35 min 40 min 46 min 50 min 55 min 60 min 0,0 300 400 500 600 700 800 900 Wavelength (nm) Figure S1. Time evolution of the UV-Vis spectrum of a DPBF (50 µM) and 2b (5 µM) CH 2 Cl 2 solution. 7