Fluorescence Enhancement in Crystals Tuned by a Molecular Torsion Angle: A Model to Analyze Structural Impact

Transcription

1 Electronic Supplementary Material (ESI) for Journal of Materials Chemistry C. This journal is The Royal Society of Chemistry 2016 Electronic Supplementary Information for Fluorescence Enhancement in Crystals Tuned by a Molecular Torsion Angle: A Model to Analyze Structural Impact P. Srujana, Tarun Gera and T. P. Radhakrishnan* School of Chemistry, University of Hyderabad Hyderabad , India Contents Synthesis and characterization Crystal structure details Absorption/emission spectroscopy Computational details Trends in energy transfer rates Page S2 S3 S4 S11 S12 S14 S15 S22 S23 S26

2 S2 Synthesis and Characterization Synthesis was carried out following Route B in Fig. 1 (main text) [Fig. S1 (page S3)]. 7-pyrrolidino-7,8,8-tricyanoquinodimethane (PTCNQ) The procedure reported in Ref. 14 (main text) was followed with minor modifications. Pyrrolidine (0.095 ml, mmol) was added to a warm solution of TCNQ (0.29 g, 1.4 mmol) in 35 ml acetonitrile; the solution turned dark green immediately and then changed to dark purple (CAUTION: small amount of HCN gas is formed as the byproduct of the reaction, but possibly remains dissolved in the solution). It was stirred at 65 o C for 4 h and then cooled to 4 o C. The microcrystalline solid which precipitated over 2 days was filtered, washed with acetonitrile and dried to give PTCNQ (0.23 g, 82% yield). 2-(4-dicyanomethylenecyclohexa-2,5-dieny1idene)-imidazolidine (1a) Ethylenediamine (0.054 ml, mmol) was added to a warm solution of PTCNQ (0.2 g, mmol) in 25 ml acetonitrile. The solution turned dark green immediately and a microcrystalline solid precipitated. It was stirred for 4 h at 70 o C and cooled to room temperature (~ 25 o C). The microcrystalline solid was filtered, washed with acetonitrile and dried to give 1a (0.19 g, 79% yield). The compound was dissolved in dimethylformamide and cooled to 4 o C. Crystallization was induced by diffusion of diethyl ether; crystals grown in the solution over four days were filtered out. mp: 280 o C (from DMF, dec.); FTIR (KBr) - /cm -1 : 3199(broad, NH), (CN), (CN), and (Ar ring C-C); NMR (400 MHz, DMSO-d 6 ) - δ/ppm 1 H: 9.77 (s, 2H, 2NH), (d, 2H, 2 aromatic CH), (d, 2H, 2 aromatic CH), 3.89 (s, 4H, 2CH 2 ); 13 C: , , , , , 108.6, 44.25, 35.34; HRMS (ESI) - m/z: (calculated for C 12 H 10 N 4 [M + ]: ). 2-(4-dicyanomethylenecyclohexa-2,5-dieny1idene)-1-ethylimidazolidine (1b) N-ethylethylenediamine (0.103 ml, mmol) was added to a warm solution of PTCNQ (0.2 g, mmol) in 25 ml acetonitrile. The solution turned dark green immediately and turned reddish orange in ~ 30 min. It was stirred for 4 h at 70 o C and cooled to ambient temperature (~25 o C). The microcrystalline solid which precipitated over 6 days was filtered, washed with acetonitrile and dried to give 1b (0.16 g, 82% yield). mp: o C (from MeCN, dec.); FTIR (KBr) - /cm -1 : 3545(broad, OH), (NH), (CN), (CN), and (Ar ring C-C), (CH); NMR (400 MHz, DMSO-d 6 ) - δ/ppm 1 H: 9.71 (s, 1H, 1NH) (d, 2H, 2 aromatic CH), (d, 2H, 2 aromatic CH), (t, 2H, CH 2 ), (t, 2H, CH 2 ), (q, 2H, CH 2 ), 1.25 (t, 3H, CH 3 ); 13 C: , , , , , , 49.70, 42.91, 42.20, 33.77, 13.19; HRMS (ESI) - m/z: (calculated for C 14 H 15 N 4 [M + +H]: ). 2-(4-dicyanomethylenecyclohexa-2,5-dieny1idene)-1,3-dimethylimidazolidine (1c) N,N'-dimethylethylenediamine (0.103 ml, mmol) was added to a warm solution of PTCNQ (0.2 g, mmol) in 25 ml acetonitrile. The solution turned dark green immediately and turned reddish orange in ~ 30 min. It was stirred for 4 h at 70 o C and cooled to ambient temperature (~25 o C). The crystalline solid which precipitated over 8 days was filtered, washed with acetonitrile and dried to give 1c (0.12 g, 63% yield). mp: o C (from MeCN); FTIR (KBr) - /cm -1 : (weak, Ar CH), (CN), (CN), and (Ar ring

3 S3 C-C), (CH); NMR (400 MHz, DMSO-d 6 ) - δ/ppm 1 H: (d, 2H, 2 aromatic CH), (d, 2H, 2 aromatic CH), 3.92 (s, 4H, 2CH 2 ), 2.98 (s, 6H, 2CH 3 ); 13 C: , , , , , , 50.20, 35.46, 32.96; HRMS (ESI) - m/z: (calculated for C 14 H 15 N 4 [M + +H]: ). Possible routes for the synthesis of 1a from TCNQ are shown in Fig. S1. In the original paper that reported 1a (Ref. 14), synthesis was carried out using Route A, and the only characterizations provided were elemental analysis and ir spectrum. These would be consistent with either the monomer or oligomer structure shown along Route A. It was also mentioned in Ref. 14, that the compound did not melt till 405 o C; this could be indicative of a polymeric structure. 1a that we have synthesized and structurally characterized is found to decompose irreversibly at 280 o C. Figure S1. Scheme for the synthesis of 1a from TCNQ following two routes; possible formation of polymeric products is indicated. NC CN Route A NH NH 2 CN HN δ + NH HN δ + NH H 2 N NH 2 + HCN HCN NC CN TCNQ NC X A CN NC δ - 1a CN NC δ - CN n Route B N H HCN NH 2 NC N NH N HN δ + NH H 2 N NH 2 NC PTCNQ CN HCN NC X B CN The reduced probability of polymeric product formation in Route B may be attributed to the following. The C atom bearing the cyano group in the intermediate X A is relatively more electron poor than the corresponding C in X B bearing a pyrrolidine group; hence the efficiency of nucleophilic attack of the amine on the C atom is enhanced in the former. Even though this would make the intramolecular cyclization more efficient in X A, it will also facilitate intermolecular reactions as the entropy cost is partly offset by the high enthalpy gain. In the case of X B, the intramolecular reaction is possibly slower, but at the same time, the intermolecular reaction becomes highly unlikely. Thus polymerization is possible along Route A, but not along Route B. N H NC δ - 1a CN

4 S4 Crystal structure Details Single crystal X-ray diffraction studies were carried out on a Bruker SMART APEX CCD area detector system equipped with a graphite monochromator and a MoK α fine-focus sealed tube (λ = Å) operated at 1200 W (40 kv, 30 ma). Data was collected at 100 K, and the reduction was performed using Bruker SAINT software; the structure was solved and refined using the Bruker SHELXTL (Version 6.14) Software. The molecular structure and unit cell packing from crystal structure determination, as well as the basic crystallographic information for 1a-c are provided in the following pages. CCDC deposition number for the three crystals: 1a : b : c :

5 S5 Figure S2. Molecular structure showing 99% thermal ellipsoids and the unit cell of 1a C15 C16 N9 C1 C7 N10 b C2 C6 C3 C5 C11 C8 C4 C12 a o c N13 N14 Table S1. Crystallographic details of 1a Common name 2-(4-dicyanomethylenecyclohexa-2,5- dieny1idene)-imidazolidine. DMF Empirical formula C 15 H 17 N 5 O Crystal system Triclinic Space group P-1 a / Å (5) b / Å (10) c / Å (10) α / deg (10) β / deg (10) γ /deg (10) V / Å (11) Z 2 ρ calc. / g cm μ / cm Temperature / K 100(2) λ / Å No. of reflections 2813 No. of parameters 258 Max., Min. transmission 0.964, GOF R [for I 2σ I ] wr Largest difference peak and hole / eå , 0.048

6 S6 Table S2. Bond lengths and bond angles for 1a Atoms Bond length (Å) Atoms Bond angle ( o ) Atoms Bond angle ( o ) O(17)-C(18) (14) C(7)-N(10)-C(16) (9) N(9)-C(15)-C(16) (8) N(10)-C(7) (13) C(7)-N(10)-H(10) 124.4(9) N(9)-C(15)-H(15A) 109.3(8) N(10)-C(16) (13) C(16)-N(10)-H(10) 122.7(9) C(16)-C(15)-H(15A) 111.0(8) N(10)-H(10) 0.909(16) C(18)-N(20)-C(21) (9) N(9)-C(15)-H(15B) 109.6(8) N(20)-C(18) (14) C(18)-N(20)-C(22) (9) C(16)-C(15)-H(15B) 112.7(8) N(20)-C(21) (14) C(21)-N(20)-C(22) (9) H(15A)-C(15)-H(15B) 111.3(11) N(20)-C(22) (14) C(7)-N(9)-C(15) (9) N(9)-C(7)-N(10) (9) N(9)-C(7) (13) C(7)-N(9)-H(9) 126.2(10) N(9)-C(7)-C(1) (9) N(9)-C(15) (13) C(15)-N(9)-H(9) 121.3(10) N(10)-C(7)-C(1) (9) N(9)-H(9) 0.944(17) C(2)-C(3)-C(4) (9) C(6)-C(5)-C(4) (10) C(3)-C(2) (14) C(2)-C(3)-H(3) 120.0(8) C(6)-C(5)-H(5) 120.4(8) C(3)-C(4) (14) C(4)-C(3)-H(3) 118.8(8) C(4)-C(5)-H(5) 118.5(8) C(3)-H(3) 0.976(13) N(14)-C(12)-C(8) (11) N(20)-C(22)-H(22A) 109.2(8) N(14)-C(12) (14) C(5)-C(6)-C(1) (10) N(20)-C(22)-H(22B) 109.1(8) C(12)-C(8) (15) C(5)-C(6)-H(6) 118.2(8) H(22A)-C(22)-H(22B) 110.4(11) N(13)-C(11) (14) C(1)-C(6)-H(6) 120.6(8) N(20)-C(22)-H(22C) 109.4(8) C(6)-C(5) (15) N(13)-C(11)-C(8) (11) H(22A)-C(22)-H(22C) 109.3(11) C(6)-C(1) (14) C(3)-C(2)-C(1) (9) H(22B)-C(22)-H(22C) 109.4(11) C(6)-H(6) 0.974(14) C(3)-C(2)-H(2) 119.4(8) N(10)-C(16)-C(15) (8) C(11)-C(8) (14) C(1)-C(2)-H(2) 119.9(8) N(10)-C(16)-H(16A) 110.0(8) C(2)-C(1) (14) C(6)-C(1)-C(2) (9) C(15)-C(16)-H(16A) 111.2(8) C(2)-H(2) 0.940(15) C(6)-C(1)-C(7) (9) N(10)-C(16)-H(16B) 109.9(7) C(1)-C(7) (14) C(2)- C(1)-C(7) (9) C(15)-C(16)-H(16B) 112.5(7) C(8)-C(4) (14) C(12)-C(8)-C(11) (9) H(16A)-C(16)-H(16B) 110.6(10) C(18)-H(18) 0.999(14) C(12)-C(8)-C(4) (9) C(21)-H(21A) 0.968(14) C(11)-C(8)-C(4) (9) C(21)-H(21C) 0.962(14) O(17)-C(18)-N(20) (10) C(21)-H(21B) 0.964(16) O(17)-C(18)-H(18) 122.4(8) C(4)-C(5) (14) N(20)-C(18)-H(18) 112.7(8) C(15)-C(16) (15) N(20)-C(21)-H(21A) 108.8(8) C(15)-H(15A) 0.980(13) N(20)-C(21)-H(21C) 110.4(8) C(15)-H(15B) 0.988(14) H(21A)-C(21)-H(21C) 109.1(11) C(5)-H(5) 0.945(14) N(20)-C(21)-H(21B) 110.1(9) C(22)-H(22A) 0.976(15) H(21A)-C(21)-H(21B) 107.9(12) C(22)-H(22B) 0.979(14) H(21C)-C(21)-H(21B) 110.4(12) C(22)-H(22C) 1.001(14) C(3)-C(4)-C(5) (9) C(16)-H(16A) 0.961(13) C(3)-C(4)-C(8) (9) C(16)-H(16B) 0.960(12) C(5)-C(4)-C(8) (9)

7 S7 Figure S3. Molecular structure showing 99% thermal ellipsoids and the unit cell of 1b N9 C15 C16 N10 C18 c C1 C7 C17 C2 C6 C3 C4 C5 C11 N13 C8 C12 N14 b o a Table S3. Crystallographic details of 1b Common name 2-(4-dicyanomethylenecyclohexa-2,5- dieny1idene)-1-ethylimidazolidine. H 2 O Empirical formula C 28 H 30 N 8 O Crystal system Monoclinic Space group P2 1 /c a / Å (12) b / Å (6) c / Å (2) α / deg β / deg (10) γ /deg V / Å (4) Z 4 ρ calc. / g cm μ / cm Temperature / K 100(2) λ / Å No. of reflections 4506 No. of parameters 352 Max., Min. transmission 0.990, GOF R [for I 2σ I ] wr Largest difference peak and hole / eå , 0.048

8 S8 Table S4. Bond lengths and bond angles for 1b Atoms Bond length (Å) Atoms Bond length (Å) Atoms Bond length (Å) N(9)-C(3) 1.317(3) C(9)-C(12) 1.406(3) C(19)-C(23) 1.408(3) N(9)-C(8) 1.458(3) C(10)-H(10A) C(19)-H(19) N(9)-H(41) 0.91(2) C(10)-H(10B) C(20)-H(20) N(5)-C(3) 1.324(3) C(11)-H(11) C(21)-H(21) N(5)-C(14) 1.461(3) C(13)-C(14) 1.509(3) C(22)-C(24) 1.402(3) N(5)-C(10) 1.470(3) C(13)-H(13A) C(22)-C(23) 1.441(3) C(1)-C(6) 1.407(3) C(13)-H(13B) C(24)-N(3) 1.156(3) C(1)-C(11) 1.411(3) C(13)-H(13C) C(25)-C(26) 1.525(3) C(1)-C(9) 1.440(3) C(14)-H(14A) C(25)-H(25A) C(2)-C(5) 1.398(3) C(14)-H(14B) C(25)-H(25B) C(2)-C(7) 1.404(3) N(10)-C(16) 1.326(3) C(26)-H(26A) C(2)-C(3) 1.462(3) N(10)-C(25) 1.465(3) C(26)-H(26B) C(4)-N(2) 1.157(3) N(10)-H(40) 0.90(3) C(27)-C(28) 1.512(3) C(4)-C(9) 1.405(3) C(15)-C(21) 1.396(3) C(27)-H(27A) C(5)-C(11) 1.366(3) C(15)-C(20) 1.398(3) C(27)-H(27B) C(5)-H(5) C(15)-C(16) 1.460(3) C(28)-H(28A) C(6)-C(7) 1.377(3) N(7)-C(16) 1.321(3) C(28)-H(28B) C(6)-H(6) N(7)-C(27) 1.465(3) C(28)-H(28C) C(7)-H(7) N(7)-C(26) 1.472(3) O(1)-H(43) 0.90(3) C(17)-C(21) 1.371(3) O(1)-H(42) 0.93(4) C(17)-C(23) 1.404(3) N(1)-C(12) 1.155(3) C(17)-H(17) C(8)-C(10) 1.529(3) C(18)-N(4) 1.158(3) C(8)-H(8A) C(18)-C(22) 1.404(3) C(8)-H(8B) C(19)-C(20) 1.376(3)

9 S9 Atoms Bond angle ( o ) Atoms Bond angle ( o ) Atoms Bond angle ( o ) C(3)-N(9)-C(8) (19) N(5)-C(10)-H(10B) C(19)-C(20)-C(15) 120.8(2) C(3)-N(9)-H(41) 125.3(14) C(8)-C(10)-H(10B) C(19)-C(20)-H(20) C(8)-N(9)-H(41) 122.4(14) H(10A)-C(10)-H(10B) C(15)-C(20)-H(20) C(3)-N(5)-C(14) (19) C(5)-C(11)-C(1) 121.8(2) C(17)-C(21)-C(15) 121.2(2) C(3)-N(5)-C(10) (18) C(5)-C(11)-H(11) C(17)-C(21)-H(21) C(14)-N(5)-C(10) (18) C(1)-C(11)-H(11) C(15)-C(21)-H(21) C(6)-C(1)-C(11) (19) N(1)-C(12)-C(9) 177.1(2) C(24)-C(22)-C(18) (19) C(6)-C(1)-C(9) (19) C(14)-C(13)-H(13A) C(24)-C(22)-C(23) (19) C(11)-C(1)-C(9) (19) C(14)-C(13)-H(13B) C(18)-C(22)-C(23) (19) C(5)-C(2)-C(7) (19) H(13A)-C(13)-H(13B) C(17)-C(23)-C(19) (19) C(5)-C(2)-C(3) (19) C(14)-C(13)-H(13C) C(17)-C(23)-C(22) (19) C(7)-C(2)-C(3) (19) H(13A)-C(13)-H(13C) C(19)-C(23)-C(22) (19) N(9)-C(3)-N(5) (19) H(13B)-C(13)-H(13C) N(3)-C(24)-C(22) 178.2(2) N(9)-C(3)-C(2) 122.3(2) N(5)-C(14)-C(13) 113.4(2) N(10)-C(25)-C(26) (17) N(5)-C(3)-C(2) 126.0(2) N(5)-C(14)-H(14A) N(10)-C(25)-H(25A) N(2)-C(4)-C(9) 179.0(3) C(13)-C(14)-H(14A) C(26)-C(25)-H(25A) C(11)-C(5)-C(2) 120.8(2) N(5)-C(14)-H(14B) N(10)-C(25)-H(25B) C(11)-C(5)-H(5) C(13)-C(14)-H(14B) C(26)-C(25)-H(25B) C(2)-C(5)-H(5) H(14A)-C(14)-H(14B) H(25A)-C(25)-H(25B) C(7)-C(6)-C(1) 121.3(2) C(16)-N(10)-C(25) (18) N(7)-C(26)-C(25) (17) C(7)-C(6)-H(6) C(16)-N(10)-H(40) 123.6(16) N(7)-C(26)-H(26A) C(1)-C(6)-H(6) C(25)-N(10)-H(40) 122.8(16) C(25)-C(26)-H(26A) C(6)-C(7)-C(2) 120.8(2) C(21)-C(15)-C(20) (19) N(7)-C(26)-H(26B) C(6)-C(7)-H(7) C(21)-C(15)-C(16) (19) C(25)-C(26)-H(26B) C(2)-C(7)-H(7) C(20)-C(15)-C(16) (19) H(26A)-C(26)-H(26B) H(43)-O(1)-H(42) 103(3) C(16)-N(7)-C(27) (18) N(7)-C(27)-C(28) (18) N(9)-C(8)-C(10) (17) C(16)-N(7)-C(26) (17) N(7)-C(27)-H(27A) N(9)-C(8)-H(8A) C(27)-N(7)-C(26) (17) C(28)-C(27)-H(27A) C(10)-C(8)-H(8A) N(7)-C(16)-N(10) (19) N(7)-C(27)-H(27B) N(9)-C(8)-H(8B) N(7)-C(16)-C(15) (19) C(28)-C(27)-H(27B) C(10)-C(8)-H(8B) N(10)-C(16)-C(15) (19) H(27A)-C(27)-H(27B) H(8A)-C(8)-H(8B) C(21)-C(17)-C(23) 121.5(2) C(27)-C(28)-H(28A) C(4)-C(9)-C(12) (19) C(21)-C(17)-H(17) C(27)-C(28)-H(28B) C(4)-C(9)-C(1) (19) C(23)-C(17)-H(17) H(28A)-C(28)-H(28B) C(12)-C(9)-C(1) (19) N(4)-C(18)-C(22) 176.2(2) C(27)-C(28)-H(28C) N(5)-C(10)-C(8) (17) C(20)-C(19)-C(23) 121.5(2) H(28A)-C(28)-H(28C) N(5)-C(10)-H(10A) C(20)-C(19)-H(19) H(28B)-C(28)-H(28C) C(8)-C(10)-H(10A) C(23)-C(19)-H(19) 119.3

10 S10 Figure S4. Molecular structure showing 99% thermal ellipsoids and the unit cell of 1c C15 C16 C17 N9 C1 C7 N10 C18 b C2 C3 C6 C5 C4 N13 C11 C8 C12 N14 a o c Table S5. Crystallographic details of 1c Common name 2-(4-dicyanomethylenecyclohexa-2,5- dieny1idene)-1,3-dimethylimidazolidine Empirical formula C 14 H 14 N 4 Crystal system Monoclinic Space group P2 1 /c a / Å (6) b / Å (7) c / Å (7) α / deg β / deg (10) γ /deg V / Å (13) Z 4 ρ calc. / g cm μ / cm Temperature / K 100(2) λ / Å No. of reflections 2165 No. of parameters 165 Max., Min. transmission 0.978, GOF 1.05 R [for I 2σ I ] wr Largest difference peak and hole / eå , 0.041

11 S11 Table S6. Bond lengths and bond angles for 1c Atoms Bond length (Å) Atoms Bond angle ( o ) Atoms Bond angle ( o ) N(2)-C(12) (15) C(12)-N(2)-C(14) (10) N(4)-C(8)-C(1) (13) N(2)-C(14) (16) C(12)-N(2)-C(9) (10) N(2)-C(9)-C(11) (9) N(2)-C(9) (15) C(14)-N(2)-C(9) (10) C(5)-C(10)-C(2) (11) N(1)-C(12) (16) C(12)-N(1)-C(13) (10) N(1)-C(11)-C(9) (9) N(1)-C(13) (16) C(12)-N(1)-C(11) (10) N(2)-C(12)-N(1) (11) N(1)-C(11) (15) C(13)-N(1)-C(11) (10) N(2)-C(12)-C(6) (11) C(1)-C(8) (17) C(8)-C(1)-C(3) (11) N(1)-C(12)-C(6) (11) C(1)-C(3) (17) C(8)-C(1)-C(2) (11) N(4)-C(8)-C(1) (13) C(1)-C(2) (17) C(3)-C(1)-C(2) (11) C(2)-C(10) (17) C(10)-C(2)-C(4) (11) C(2)-C(4) (17) C(10)-C(2)-C(1) (11) C(3)-N(3) (16) C(4)-C(2)-C(1) (11) C(4)-C(7) (17) N(3)-C(3)-C(1) (13) N(4)-C(8) (16) C(7)-C(4)-C(2) (11) C(5)-C(10) (17) C(10)-C(5)-C(6) (11) C(5)-C(6) (17) C(7)-C(6)-C(5) (11) C(6)-C(7) (17) C(7)-C(6)-C(12) (11) C(6)-C(12) (16) C(5)-C(6)-C(12) (11) C(9)-C(11) (17) C(4)-C(7)-C(6) (11)

12 S12 Absorption and Emission Spectroscopy Electronic absorption spectra of solutions (in transmission mode) and solid (in diffuse reflectance mode) were recorded on a Varian model Cary 100 UV-Vis spectrometer; the reflectance spectra were converted to absorption spectra using the Kubelka-Munk function. Steady-state fluorescence emission and excitation spectra were recorded on a Horiba Jobin Yvon model FL3-22 Fluorolog spectrofluorimeter, in right angle geometry. The microcrystalline solid in the form of KBr pellet was mounted on a solid sample holder. Optical density of the samples was maintained below 0.2 in order to avoid inner-filter effects. Fluorescence quantum yield of solution was determined using quinine sulfate in 1N H 2 SO 4 (Φ = 0.546) as the standard; absolute values of the quantum yield of solid samples were determined using an integrating sphere and the PLQY Calculator v.3 software (Jobin Yvon). Figure S5. Photographs of solid samples (KBr pellet ) of 1a-c under ambient and UV (365 nm) light; solutions, even with OD ~ 0.1 show no visible fluorescence. 1a 1b 1c 1a 1b 1c

13 S13 Figure S6. Fluorescence excitation spectra of solution and solid 1a-c a Solution [λ em = 464 nm] Solid [λ em = 500 nm] Relative Intensity (x10 4 ) 4.0 1b Solution [λ em = 490 nm] Solid [λ em = 450 nm] Relative Intensity (x10 6 ) Solution [λ 1c em = 494 nm] Solid [λ em = 447 nm] Wavelength (nm) 0

14 S14 Solvatochromism data Absorption spectra of 1a-c were recorded in methanol (Reichardt parameter, E T N = 0.762), acetonitrile (E T N = 0.460), acetone (E T N = 0.355) and ethyl acetate (E T N = 0.228) solvents to study the solvatochromic effect. The spectra show clear blue shift with increasing solvent polarity. Figure S7. Absorption spectra of 1a-c in solvents of varying polarity. Normalized absorbance 1.0 1a Methanol Acetonitrile Acetone Ethyl acetate Normalized absorbance 1.0 1b Methanol Acetonitrile Acetone Ethyl acetate Wavelength (nm) Wavelength (nm) Normalized absorbance c Methanol Acetonitrile Acetone Ethyl acetate Wavelength (nm)

15 S15 Computational Details Methods Gaussian 09 (Revision C.01)* was used for all the geometry optimization studies. Computations were carried out at the B3LYP/6-31G* level; TD-DFT method was used for the excited states and SCRF to model the environment effect. Full as well as partial optimization computations were carried out as described in the main text. The full optimization used the molecular structure from the crystal structure analysis for the initial geometry. Partial optimization calculations for the ground state started with the molecular structure from the crystal structure analysis as the initial geometry with the torsion angle, ϕ frozen; in 1b, the optimized geometry corresponded to the θ = 30 o point, whereas in 1a and 1c these were at θ = 7 o and 45 o respectively. θ was fixed at 10 o intervals (starting with 10 o, 30 o and 40 o respectively in 1a, 1b and 1c), each optimization carried out using the previous point geometry as the initial one. These calculations, together with single point TD-DFT calculation at the partially optimized geometry, generated the S 0 (G) and S 1 (G) profiles. The partially optimized geometry corresponding to the minimum of the S 0 (G) profile was used to optimize the excited state geometry (keyword ROOT=1), again freezing the ϕ angle. Following the protocol as in the case of the ground state geometry, the S 0 (E) and S 1 (E) profiles were computed. Default thresholds (0.0003) were used for all geometry optimizations. However, the partial optimization of the excited state structures of 1a with θ = 70 o, 80 o and 90 o (on the S 1 (E) profile in Fig. 5a) failed to converge, possibly because these are far from minimum energy geometries. The optimizations were attempted with lower thresholds and were successful with threshold values of 0.003, for the 70 o and 80 o points; these are shown for 1a in Fig. 5a. Even with this lowered optimization threshold, the θ = 90 o structure could not be optimized, and hence was abandoned and not shown in the figure. * M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian 09, Revision C.01, Gaussian, Inc., Wallingford CT, 2010.

16 S16 Fully optimized ground and excited states Figure S8. Fully optimized [B3LYP/6-31G*; SCRF (acetonitrile)] (a) ground state and (b) excited state geometries of molecules 1a-c (a) 1a 1b 1c (b) 1a 1b 1c Table S7. Computed [B3LYP/6-31G*; SCRF (acetonitrile)] absorption energy (wavelength maximum, ) and oscillator strength (f) for the first three excited states of the fully optimized ground state structures of 1a-c (shown in Fig. S8a). Molecule S 0 S 1 S 0 S 2 S 0 S 3 (nm) f (nm) f (nm) f 1a b c

17 S17 Table S8. Computed [B3LYP/6-31G*; SCRF (acetonitrile)] emission energy (wavelength maximum, ) and oscillator strength (f) for the first three excited states of the fully optimized excited state structures of 1a-c (shown in Fig. S8b). Molecule S 1 S 0 S 2 S 0 S 3 S 0 (nm) f (nm) f (nm) f 1a b c Table S9. Computed [B3LYP/6-31G*; SCRF (acetonitrile)] emission energy (wavelength maximum, ) and oscillator strength (f) for the first three excited states of the partially optimized (with θ and φ fixed at the values of fully optimized ground state geometry) excited state structures of 1a-c. Molecule S 1 S 0 S 2 S 0 S 3 S 0 (nm) f (nm) f (nm) f 1a b c Computed transition dipole moments Figure S9. Calculated [B3LYP/6-31G*, TD-DFT (ROOT=1)] transition dipole moments of the molecules 1a-c with the geometry taken from the crystal structure (1a in vacuum, 1b in propanonitrile and 1c in acetonitrile environments). 1a 1b 1c

18 S18 Frequencies calculated for the fully optimized structures Table S10. Computed frequencies of the fully optimized structure of 1a in vacuum and acetonitrile environments (all positive, indicating that these are genuine energy minimum structures) No. Frequency (cm -1 ) No. Frequency (cm -1 ) No. Frequency (cm -1 ) Acetonitrile Vacuum Acetonitrile Vacuum Acetonitrile Vacuum

19 S19 Table S11. Computed frequencies of the fully optimized structure of 1b in propanonitrile and acetonitrile environments (all positive, indicating that these are genuine energy minimum structures) No. Frequency (cm -1 ) No. Frequency (cm -1 ) No. Frequency (cm -1 ) Acetonitrile Propanonitrile Acetonitrile Propanonitrile Acetonitrile Propanonitrile

20 S20 Table S12. Computed frequencies of the fully optimized structure of 1c in acetonitrile environment (all positive, indicating that this is a genuine energy minimum structure) No. Frequency (cm -1 ) No. Frequency (cm -1 ) No. Frequency (cm -1 ) No. Frequency (cm -1 ) Acetonitrile Acetonitrile Acetonitrile Acetonitrile

21 S21 Computed oscillator strengths Figure S10. Oscillator strength computed for the S 0 S 1, S 0 S 2, S 0 S 3 transitions of the excited state geometries (partially optimized, fixing the angle, ϕ at the value in the crystal structure and with different values of θ) of 1a-c a b Oscillator strength S 1 S 2 S c Wavelength (nm)

22 S22 Lattice energy calculations Materials Studio (v 6.0.0, Accelrys Software Inc.) with GULP module* and Dreiding force field was used for all the computations. A cluster of molecules (54 in 1a, 61 in 1b, 55 in 1c) excised from the crystal lattice was used with torsional angle rotations imposed on one molecule deeply embedded in the middle of the cluster. *J. D. Gale, A. L. Rohl, The General Utility Lattice Program (GULP), Molecular Simulation, 2003, 29, Figure S11. Variation of the lattice energy as a function of the twist angle (a) ϕ and (b) θ (see Fig. 2 in main text) for the crystals 1a-c. (a) ΔE (ev) a 1b 1c Twist angle, φ ( ο ) (b) ΔE (ev) a 1b 1c Twist angle, θ ( o )

23 S23 Trends in Energy Transfer Rates Förster resonance energy transfer As shown on page S17, the computed transition dipole moment in each of the molecules 1a-c coincides closely with the molecular dipole axis along the diaminomethylene carbon dicyanomethylene carbon axis (C7-C8 axis in Figs. S2-S4). The computed transition dipole was used in the following calculations. In view of the similar overlap between the absorption and emission spectra across the series, dependence of the rate of energy transfer on the relative orientation of the transition dipoles is given by, ö 3 where α 12 is the angle between the transition dipoles of molecules 1 and 2, α 1 and α 2 are the angles between the transition dipoles and the vector connecting the two, and R is the distance between the transition dipoles (Fig. S12). Figure S12. Schematic diagram showing the transition dipoles and the relevant geometric parameters. α 12 α 2 α 1 R For each crystal, the neighboring molecules with transition dipoles within a distance (R) of 10 Å, around that of a selected molecule in the crystal lattice were considered, to estimate the values of listed in Table S13. The sum for each crystal and the relative values across the series are shown.

24 S24 Table S13. The relevant geometric parameters (for molecules within a distance (R) of 10 Å from a reference molecule) and the relative rates of Förster energy transfer in crystals of 1a-c. Crystal R (Å) α 12 ( o ) α 1 ( o ) α 2 ( o ) κ (10-6 ) a Total b Total c Total Relative rate

25 S25 Dexter energy transfer In order to assess the impact of distances and relative orientations of molecules in the crystals on the Dexter energy transfer rate, a common protocol to define the relevant fluorophores is required. As a reasonable approximation for a uniform model across the series of molecules, the moiety consisting of the benzenoid ring and the diaminomethylene (C7) and dicyanomethylene (C8) carbon atoms (Figs. S2- S4) was chosen to define the molecular plane and the centroid of this moiety to measure the distances. In view of the similar overlap between the absorption and emission spectra across the series, dependence of the rate of energy transfer on the relative molecular orientations is given by, δ... where r' is the perpendicular distance between the mean planes of the molecules, r is the distance between the centroids, α is the angle between the normal to the mean plane of the reference molecule and the vector connecting the centroids of the two molecules, δ is the angle between the mean planes of molecules 1 and 2 (Fig. S13). Overlap of the relevant wave functions on the two molecules that leads to the Dexter energy transfer decreases exponentially with the distance between the molecular planes (r'). The overlap would be a maximum when the planes are parallel and exactly on top of each other, in a sandwich configuration; slip of the molecule (α increasing from 0 o ) and tilt of the planes away from parallel (δ increasing from 0 o ) decrease the overlap and can be modeled by the functions cosα and cosδ respectively. Figure S13. Schematic diagram showing the molecular mean planes and the relevant geometric parameters. δ r' r α

26 S26 For each crystal, the neighboring molecules with r.sinα < 10 Å [r.sinα = displacement of the centroid of one molecule with respect to the centroid of the other; 10 Å = the approximate length of the diaminodicyanoquinodimethane chromophore; if r.sinα > 10 Å, there is likely to be negligible overlap between the two molecules] and r' in the range 2.5 to 4.5 Å [meaningful π- stacking distance range] around a selected molecule in the crystal lattice were considered, to estimate the values of listed in Table S14. The sum for each crystal and the relative values across the series are shown. Table S14. The relevant geometric parameters (for molecules around a reference molecule, satisfying the conditions, 2.5 < r' < 4.5 Å and r.sinα < 10 Å, and the relative rates of Dexter energy transfer in crystals of 1a-c. Crystal r (Å) r' (Å) α ( o ) δ ( o ) r.sinα (Å) S. Relative rate a Total b Total c Total The relative rates of the two modes of energy transfer in crystals of 1a-c are shown graphically in Fig. 6c of the main text.