Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S160053680705009X/bh2134sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S160053680705009X/bh2134Isup2.hkl |
CCDC reference: 667375
Key indicators
- Single-crystal X-ray study
- T = 293 K
- Mean (C-C) = 0.003 Å
- Disorder in main residue
- R factor = 0.057
- wR factor = 0.169
- Data-to-parameter ratio = 14.8
checkCIF/PLATON results
No syntax errors found
Alert level B PLAT220_ALERT_2_B Large Non-Solvent C Ueq(max)/Ueq(min) ... 3.60 Ratio
Alert level C ABSTM02_ALERT_3_C The ratio of expected to reported Tmax/Tmin(RR') is < 0.90 Tmin and Tmax reported: 0.820 0.990 Tmin(prime) and Tmax expected: 0.975 0.993 RR(prime) = 0.844 Please check that your absorption correction is appropriate. PLAT061_ALERT_3_C Tmax/Tmin Range Test RR' too Large ............. 0.84 PLAT222_ALERT_3_C Large Non-Solvent H Ueq(max)/Ueq(min) ... 3.61 Ratio PLAT242_ALERT_2_C Check Low Ueq as Compared to Neighbors for C11 PLAT301_ALERT_3_C Main Residue Disorder ......................... 18.00 Perc.
Alert level G PLAT199_ALERT_1_G Check the Reported _cell_measurement_temperature 293 K PLAT200_ALERT_1_G Check the Reported _diffrn_ambient_temperature . 293 K PLAT860_ALERT_3_G Note: Number of Least-Squares Restraints ....... 4
0 ALERT level A = In general: serious problem 1 ALERT level B = Potentially serious problem 5 ALERT level C = Check and explain 3 ALERT level G = General alerts; check 2 ALERT type 1 CIF construction/syntax error, inconsistent or missing data 2 ALERT type 2 Indicator that the structure model may be wrong or deficient 5 ALERT type 3 Indicator that the structure quality may be low 0 ALERT type 4 Improvement, methodology, query or suggestion 0 ALERT type 5 Informative message, check
For the structure of isoindigo, see: von Eller-Pandraud (1960). For the properties of isoindigo derivatives, see: Sassatelli et al. (2004).
1-Butyl-1H-indole-2,3-dione (1.5 g) and 1-butyl-1H-indole-2-one (1.5 g) were mixed with polyphosphoric acid (15 g), reacted at 333–338 K for 30 min. under N2, and then heated to 433–443 K with stirring. After 3 h, the mixture was poured into ice water and stirred for 1 h. The solution was extracted in chloroform and dried over Na2SO4. After removing the solvent, the crude product was purified by column chromatography on silica gel, eluting with petrol ether, affording the title compound (1.4 g, 47.1%). The compound was dissolved in THF and purple plate crystals of (I) formed on slow evaporatiion, at room temperature, over one week.
Atoms C10, C11 and C12 were found to be disordered over two positions. Site occupation factors converged to 0.540 (8) [C10/C11/C12] and 0.460 (8) [C10'/C11'/C12']. Bond lengths C10—C11, C11—C12, C10'—C11', and C11'—C12' were restrained to 1.54 (1) Å. H atoms were positioned geometrically and allowed to ride on their carrier atom. The C—H bond lengths for aromatic, methyl and methylene groups were set to 0.93, 0.96 and 0.97 Å, respectively.
Isoindigo, which contains a bis-indole framework, can be obtained from various natural sources. Its derivatives are usually known as useful medicines (Sassatelli et al., 2004). Isoindigo may also be used as a precursor for the synthesis of organic two-photon absorption (TPA) compounds, because of its perfect planar π-conjugated structure. When exploring new TPA compounds, we obtained an intermediate compound 1,1'-dibutyl-isoindigo (I), for which we now report the synthesis and structure,
The molecule is centrosymmetric with its inversion centre placed at the midpoint of the C2═C2i bond [symmetry code (i): 1 - x, 1 - y, -z], which thus presents an E conformation. The asymmetric unit thus contains one-half molecule. The two indole-2-one moieties are coplanar: the maximum displacement from the least-squares plane defined by 20 atoms of the isoindigo core is 0.036 (2) Å for O1 atom.
Both indole-2-one heterocycles are connected by the central C═C double bond, which has a bond length of 1.369 (4) Å, longer than typical C(sp2)═C(sp2) double bonds. On the other hand, The C2—C3 bond length, 1.476 (3) Å, is shorter than typical C(sp3)—C(sp3) single bonds. This means that the bonding in the fragment C3—C2═C2i—C3i is conjugated, as observed in stilbene. Considering the excellent planarity of (I), the π-conjugation of the title molecule should be better than that of stilbene, which is known as a TPA active compound. The geometry, conformation, and bond characters of (I) are very similar to those of isoindigo (von Eller-Pandraud, 1960). In (I), n-butyl groups bonded to the isoindigo core are disordered over two positions (Fig. 1).
There are no significant hydrogen bonds in the crystal structure (Fig. 2). However, the weak intramolecular interaction C4—H4···O1i helps the isoindigo core of the molecule to keep a perfect planar conformation (Fig. 1). This intramolecular contact is characterized by a separation H4···O1i = 2.05 Å and a C4—H4···O1i angle of 137°.
For the structure of isoindigo, see: von Eller-Pandraud (1960). For the properties of isoindigo derivatives, see: Sassatelli et al. (2004).
Data collection: APEX2 (Bruker, 2005); cell refinement: APEX2 (Bruker, 2005); data reduction: APEX2 (Bruker, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).
Fig. 1. The molecular structure of (I). Displacement ellipsoids are drawn at the 30% probability level. | |
Fig. 2. The molecular packing of crystal (I). All H atoms have been omitted for clarity. |
C24H26N2O2 | F(000) = 400 |
Mr = 374.47 | Dx = 1.237 Mg m−3 |
Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2yn | Cell parameters from 977 reflections |
a = 9.0301 (2) Å | θ = 2.8–20.6° |
b = 12.0559 (3) Å | µ = 0.08 mm−1 |
c = 9.8618 (2) Å | T = 293 K |
β = 110.546 (2)° | Plate, purple |
V = 1005.32 (4) Å3 | 0.32 × 0.15 × 0.09 mm |
Z = 2 |
Bruker APEXII CCD area-detector diffractometer | 2321 independent reflections |
Radiation source: fine-focus sealed tube | 1203 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.035 |
Detector resolution: 10.00 pixels mm-1 | θmax = 27.6°, θmin = 2.8° |
φ and ω scans | h = −10→11 |
Absorption correction: multi-scan (APEX2; Bruker, 2005) | k = −15→13 |
Tmin = 0.82, Tmax = 0.99 | l = −12→12 |
7181 measured reflections |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.057 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.170 | H-atom parameters constrained |
S = 1.00 | w = 1/[σ2(Fo2) + (0.0743P)2 + 0.1566P] where P = (Fo2 + 2Fc2)/3 |
2321 reflections | (Δ/σ)max < 0.001 |
157 parameters | Δρmax = 0.31 e Å−3 |
4 restraints | Δρmin = −0.16 e Å−3 |
C24H26N2O2 | V = 1005.32 (4) Å3 |
Mr = 374.47 | Z = 2 |
Monoclinic, P21/n | Mo Kα radiation |
a = 9.0301 (2) Å | µ = 0.08 mm−1 |
b = 12.0559 (3) Å | T = 293 K |
c = 9.8618 (2) Å | 0.32 × 0.15 × 0.09 mm |
β = 110.546 (2)° |
Bruker APEXII CCD area-detector diffractometer | 2321 independent reflections |
Absorption correction: multi-scan (APEX2; Bruker, 2005) | 1203 reflections with I > 2σ(I) |
Tmin = 0.82, Tmax = 0.99 | Rint = 0.035 |
7181 measured reflections |
R[F2 > 2σ(F2)] = 0.057 | 4 restraints |
wR(F2) = 0.170 | H-atom parameters constrained |
S = 1.00 | Δρmax = 0.31 e Å−3 |
2321 reflections | Δρmin = −0.16 e Å−3 |
157 parameters |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
N1 | 0.5019 (2) | 0.30724 (15) | 0.18760 (19) | 0.0628 (5) | |
C3 | 0.6752 (2) | 0.39265 (16) | 0.0975 (2) | 0.0512 (5) | |
C2 | 0.5247 (2) | 0.45466 (16) | 0.04392 (19) | 0.0508 (5) | |
C8 | 0.6545 (3) | 0.30456 (18) | 0.1826 (2) | 0.0570 (6) | |
O1 | 0.2828 (2) | 0.41384 (15) | 0.0981 (2) | 0.0947 (7) | |
C1 | 0.4184 (3) | 0.39394 (19) | 0.1093 (2) | 0.0599 (6) | |
C6 | 0.9168 (3) | 0.2428 (2) | 0.2345 (3) | 0.0756 (7) | |
H6 | 0.9986 | 0.1936 | 0.2799 | 0.091* | |
C7 | 0.7717 (3) | 0.2290 (2) | 0.2498 (3) | 0.0709 (7) | |
H7 | 0.7537 | 0.1707 | 0.3037 | 0.085* | |
C5 | 0.9414 (3) | 0.3278 (2) | 0.1536 (3) | 0.0728 (7) | |
H5 | 1.0400 | 0.3355 | 0.1446 | 0.087* | |
C4 | 0.8229 (3) | 0.40266 (19) | 0.0847 (2) | 0.0636 (6) | |
H4 | 0.8422 | 0.4598 | 0.0298 | 0.076* | |
C9 | 0.4387 (3) | 0.2311 (2) | 0.2693 (3) | 0.0799 (8) | |
H9A | 0.3428 | 0.2628 | 0.2761 | 0.096* | 0.540 (8) |
H9B | 0.5148 | 0.2246 | 0.3670 | 0.096* | 0.540 (8) |
H9C | 0.3906 | 0.2761 | 0.3243 | 0.096* | 0.460 (8) |
H9D | 0.5255 | 0.1906 | 0.3384 | 0.096* | 0.460 (8) |
C10 | 0.4018 (9) | 0.1163 (7) | 0.2045 (8) | 0.088 (2) | 0.540 (8) |
H10A | 0.4922 | 0.0888 | 0.1833 | 0.106* | 0.540 (8) |
H10B | 0.3842 | 0.0669 | 0.2750 | 0.106* | 0.540 (8) |
C11 | 0.2597 (10) | 0.1148 (7) | 0.0693 (8) | 0.112 (3) | 0.540 (8) |
H11A | 0.2804 | 0.1587 | −0.0046 | 0.134* | 0.540 (8) |
H11B | 0.1709 | 0.1478 | 0.0881 | 0.134* | 0.540 (8) |
C12 | 0.217 (3) | −0.0054 (14) | 0.014 (2) | 0.183 (8) | 0.540 (8) |
H12A | 0.1180 | −0.0055 | −0.0654 | 0.274* | 0.540 (8) |
H12B | 0.2095 | −0.0510 | 0.0906 | 0.274* | 0.540 (8) |
H12C | 0.2984 | −0.0341 | −0.0188 | 0.274* | 0.540 (8) |
C10' | 0.3178 (15) | 0.1506 (8) | 0.1785 (13) | 0.101 (3) | 0.460 (8) |
H10C | 0.2780 | 0.1067 | 0.2408 | 0.121* | 0.460 (8) |
H10D | 0.2296 | 0.1914 | 0.1119 | 0.121* | 0.460 (8) |
C11' | 0.3837 (11) | 0.0751 (6) | 0.0947 (11) | 0.105 (3) | 0.460 (8) |
H11C | 0.4727 | 0.0344 | 0.1605 | 0.125* | 0.460 (8) |
H11D | 0.4211 | 0.1182 | 0.0301 | 0.125* | 0.460 (8) |
C12' | 0.254 (3) | −0.0077 (13) | 0.0052 (18) | 0.126 (6) | 0.460 (8) |
H12D | 0.2979 | −0.0570 | −0.0473 | 0.190* | 0.460 (8) |
H12E | 0.1675 | 0.0327 | −0.0617 | 0.190* | 0.460 (8) |
H12F | 0.2169 | −0.0499 | 0.0694 | 0.190* | 0.460 (8) |
U11 | U22 | U33 | U12 | U13 | U23 | |
N1 | 0.0559 (12) | 0.0657 (12) | 0.0699 (11) | 0.0018 (10) | 0.0258 (9) | 0.0148 (10) |
C3 | 0.0481 (12) | 0.0534 (12) | 0.0515 (11) | −0.0018 (10) | 0.0168 (9) | −0.0030 (9) |
C2 | 0.0484 (13) | 0.0523 (12) | 0.0547 (12) | −0.0042 (10) | 0.0217 (10) | −0.0047 (9) |
C8 | 0.0519 (14) | 0.0619 (14) | 0.0559 (12) | −0.0017 (11) | 0.0173 (10) | −0.0004 (10) |
O1 | 0.0688 (12) | 0.0964 (14) | 0.1388 (16) | 0.0190 (10) | 0.0611 (11) | 0.0470 (12) |
C1 | 0.0544 (14) | 0.0634 (14) | 0.0666 (13) | 0.0014 (12) | 0.0270 (11) | 0.0055 (11) |
C6 | 0.0585 (16) | 0.0811 (17) | 0.0828 (16) | 0.0139 (13) | 0.0192 (13) | 0.0164 (14) |
C7 | 0.0652 (16) | 0.0703 (16) | 0.0761 (15) | 0.0069 (13) | 0.0236 (12) | 0.0185 (13) |
C5 | 0.0507 (14) | 0.0808 (17) | 0.0896 (17) | 0.0049 (13) | 0.0279 (13) | 0.0079 (14) |
C4 | 0.0561 (14) | 0.0657 (15) | 0.0713 (14) | 0.0005 (12) | 0.0253 (12) | 0.0058 (11) |
C9 | 0.0768 (19) | 0.0812 (18) | 0.0902 (17) | −0.0015 (15) | 0.0400 (15) | 0.0249 (15) |
C10 | 0.076 (5) | 0.078 (5) | 0.114 (6) | −0.001 (4) | 0.038 (4) | 0.024 (4) |
C11 | 0.133 (7) | 0.100 (6) | 0.095 (5) | 0.017 (5) | 0.030 (5) | −0.005 (4) |
C12 | 0.169 (13) | 0.141 (15) | 0.241 (18) | 0.001 (10) | 0.074 (11) | 0.012 (11) |
C10' | 0.090 (8) | 0.101 (7) | 0.119 (9) | 0.004 (6) | 0.044 (7) | 0.031 (6) |
C11' | 0.106 (7) | 0.108 (6) | 0.106 (7) | 0.001 (5) | 0.045 (5) | 0.007 (5) |
C12' | 0.174 (15) | 0.098 (10) | 0.098 (7) | −0.067 (9) | 0.037 (8) | −0.033 (7) |
N1—C1 | 1.360 (3) | C9—H9C | 0.9700 |
N1—C8 | 1.397 (3) | C9—H9D | 0.9701 |
N1—C9 | 1.463 (3) | C10—C11 | 1.492 (8) |
C3—C4 | 1.389 (3) | C10—H10A | 0.9700 |
C3—C8 | 1.406 (3) | C10—H10B | 0.9700 |
C3—C2 | 1.476 (3) | C11—C12 | 1.549 (9) |
C2—C2i | 1.369 (4) | C11—H11A | 0.9700 |
C2—C1 | 1.519 (3) | C11—H11B | 0.9700 |
C8—C7 | 1.377 (3) | C12—H12A | 0.9600 |
O1—C1 | 1.214 (2) | C12—H12B | 0.9600 |
C6—C5 | 1.364 (3) | C12—H12C | 0.9600 |
C6—C7 | 1.380 (3) | C10'—C11' | 1.486 (9) |
C6—H6 | 0.9300 | C10'—H10C | 0.9700 |
C7—H7 | 0.9300 | C10'—H10D | 0.9700 |
C5—C4 | 1.382 (3) | C11'—C12' | 1.556 (9) |
C5—H5 | 0.9300 | C11'—H11C | 0.9700 |
C4—H4 | 0.9300 | C11'—H11D | 0.9700 |
C9—C10' | 1.500 (12) | C12'—H12D | 0.9600 |
C9—C10 | 1.511 (9) | C12'—H12E | 0.9600 |
C9—H9A | 0.9700 | C12'—H12F | 0.9600 |
C9—H9B | 0.9700 | ||
C1—N1—C8 | 110.78 (17) | N1—C9—H9D | 109.1 |
C1—N1—C9 | 123.35 (18) | C10'—C9—H9D | 109.5 |
C8—N1—C9 | 125.83 (19) | C10—C9—H9D | 80.8 |
C4—C3—C8 | 116.91 (19) | H9A—C9—H9D | 132.2 |
C4—C3—C2 | 135.73 (19) | H9C—C9—H9D | 107.4 |
C8—C3—C2 | 107.34 (17) | C11—C10—C9 | 112.5 (7) |
C2i—C2—C3 | 133.3 (2) | C11—C10—H10A | 109.1 |
C2i—C2—C1 | 122.6 (2) | C9—C10—H10A | 109.1 |
C3—C2—C1 | 104.10 (17) | C11—C10—H10B | 109.1 |
C7—C8—N1 | 126.8 (2) | C9—C10—H10B | 109.1 |
C7—C8—C3 | 123.1 (2) | H10A—C10—H10B | 107.8 |
N1—C8—C3 | 110.06 (18) | C10—C11—C12 | 110.9 (11) |
O1—C1—N1 | 122.95 (19) | C10—C11—H11A | 109.5 |
O1—C1—C2 | 129.3 (2) | C12—C11—H11A | 109.5 |
N1—C1—C2 | 107.71 (18) | C10—C11—H11B | 109.5 |
C5—C6—C7 | 120.6 (2) | C12—C11—H11B | 109.5 |
C5—C6—H6 | 119.7 | H11A—C11—H11B | 108.1 |
C7—C6—H6 | 119.7 | C11'—C10'—C9 | 112.2 (9) |
C8—C7—C6 | 117.8 (2) | C11'—C10'—H10C | 109.2 |
C8—C7—H7 | 121.1 | C9—C10'—H10C | 109.2 |
C6—C7—H7 | 121.1 | C11'—C10'—H10D | 109.2 |
C6—C5—C4 | 121.4 (2) | C9—C10'—H10D | 109.2 |
C6—C5—H5 | 119.3 | H10C—C10'—H10D | 107.9 |
C4—C5—H5 | 119.3 | C10'—C11'—C12' | 109.9 (11) |
C5—C4—C3 | 120.1 (2) | C10'—C11'—H11C | 109.7 |
C5—C4—H4 | 120.0 | C12'—C11'—H11C | 109.7 |
C3—C4—H4 | 120.0 | C10'—C11'—H11D | 109.7 |
N1—C9—C10' | 114.8 (4) | C12'—C11'—H11D | 109.7 |
N1—C9—C10 | 114.3 (3) | H11C—C11'—H11D | 108.2 |
N1—C9—H9A | 108.7 | C11'—C12'—H12D | 109.5 |
C10—C9—H9A | 108.7 | C11'—C12'—H12E | 109.5 |
N1—C9—H9B | 108.7 | H12D—C12'—H12E | 109.5 |
C10—C9—H9B | 108.7 | C11'—C12'—H12F | 109.5 |
H9A—C9—H9B | 107.6 | H12D—C12'—H12F | 109.5 |
N1—C9—H9C | 107.1 | H12E—C12'—H12F | 109.5 |
C10'—C9—H9C | 108.7 | ||
C4—C3—C2—C2i | 2.3 (5) | C3—C2—C1—N1 | −1.0 (2) |
C8—C3—C2—C2i | −178.7 (3) | N1—C8—C7—C6 | −178.2 (2) |
C4—C3—C2—C1 | −178.0 (2) | C3—C8—C7—C6 | 1.4 (4) |
C8—C3—C2—C1 | 0.9 (2) | C5—C6—C7—C8 | −0.9 (4) |
C1—N1—C8—C7 | 179.5 (2) | C7—C6—C5—C4 | 0.1 (4) |
C9—N1—C8—C7 | 1.7 (4) | C6—C5—C4—C3 | 0.3 (3) |
C1—N1—C8—C3 | −0.1 (2) | C8—C3—C4—C5 | 0.2 (3) |
C9—N1—C8—C3 | −177.9 (2) | C2—C3—C4—C5 | 179.0 (2) |
C4—C3—C8—C7 | −1.0 (3) | C1—N1—C9—C10' | 73.5 (5) |
C2—C3—C8—C7 | 179.8 (2) | C8—N1—C9—C10' | −108.9 (5) |
C4—C3—C8—N1 | 178.60 (18) | C1—N1—C9—C10 | 108.4 (4) |
C2—C3—C8—N1 | −0.6 (2) | C8—N1—C9—C10 | −74.0 (4) |
C8—N1—C1—O1 | −179.6 (2) | N1—C9—C10—C11 | −71.8 (8) |
C9—N1—C1—O1 | −1.7 (4) | C10'—C9—C10—C11 | 26.5 (8) |
C8—N1—C1—C2 | 0.7 (2) | C9—C10—C11—C12 | −175.1 (12) |
C9—N1—C1—C2 | 178.54 (19) | N1—C9—C10'—C11' | 62.3 (10) |
C2i—C2—C1—O1 | −1.0 (4) | C10—C9—C10'—C11' | −34.3 (7) |
C3—C2—C1—O1 | 179.3 (2) | C9—C10'—C11'—C12' | 178.9 (10) |
C2i—C2—C1—N1 | 178.7 (2) |
Symmetry code: (i) −x+1, −y+1, −z. |
Experimental details
Crystal data | |
Chemical formula | C24H26N2O2 |
Mr | 374.47 |
Crystal system, space group | Monoclinic, P21/n |
Temperature (K) | 293 |
a, b, c (Å) | 9.0301 (2), 12.0559 (3), 9.8618 (2) |
β (°) | 110.546 (2) |
V (Å3) | 1005.32 (4) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 0.08 |
Crystal size (mm) | 0.32 × 0.15 × 0.09 |
Data collection | |
Diffractometer | Bruker APEXII CCD area-detector |
Absorption correction | Multi-scan (APEX2; Bruker, 2005) |
Tmin, Tmax | 0.82, 0.99 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 7181, 2321, 1203 |
Rint | 0.035 |
(sin θ/λ)max (Å−1) | 0.651 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.057, 0.170, 1.00 |
No. of reflections | 2321 |
No. of parameters | 157 |
No. of restraints | 4 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.31, −0.16 |
Computer programs: APEX2 (Bruker, 2005), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), SHELXTL (Bruker, 1997), WinGX (Farrugia, 1999).
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Isoindigo, which contains a bis-indole framework, can be obtained from various natural sources. Its derivatives are usually known as useful medicines (Sassatelli et al., 2004). Isoindigo may also be used as a precursor for the synthesis of organic two-photon absorption (TPA) compounds, because of its perfect planar π-conjugated structure. When exploring new TPA compounds, we obtained an intermediate compound 1,1'-dibutyl-isoindigo (I), for which we now report the synthesis and structure,
The molecule is centrosymmetric with its inversion centre placed at the midpoint of the C2═C2i bond [symmetry code (i): 1 - x, 1 - y, -z], which thus presents an E conformation. The asymmetric unit thus contains one-half molecule. The two indole-2-one moieties are coplanar: the maximum displacement from the least-squares plane defined by 20 atoms of the isoindigo core is 0.036 (2) Å for O1 atom.
Both indole-2-one heterocycles are connected by the central C═C double bond, which has a bond length of 1.369 (4) Å, longer than typical C(sp2)═C(sp2) double bonds. On the other hand, The C2—C3 bond length, 1.476 (3) Å, is shorter than typical C(sp3)—C(sp3) single bonds. This means that the bonding in the fragment C3—C2═C2i—C3i is conjugated, as observed in stilbene. Considering the excellent planarity of (I), the π-conjugation of the title molecule should be better than that of stilbene, which is known as a TPA active compound. The geometry, conformation, and bond characters of (I) are very similar to those of isoindigo (von Eller-Pandraud, 1960). In (I), n-butyl groups bonded to the isoindigo core are disordered over two positions (Fig. 1).
There are no significant hydrogen bonds in the crystal structure (Fig. 2). However, the weak intramolecular interaction C4—H4···O1i helps the isoindigo core of the molecule to keep a perfect planar conformation (Fig. 1). This intramolecular contact is characterized by a separation H4···O1i = 2.05 Å and a C4—H4···O1i angle of 137°.