Crystal structure and solvent-dependent behaviours of 3-amino-1,6-diethyl-2,5,7-trimethyl-4,4-diphenyl-3a,4a-diaza-4-bora-s-indacene

3-Amino-1,6-diethyl-2,5,7-trimethyl-4,4-diphenyl-4-bora-3a,4a-diaza-s-indacene displays solvent-dependent behaviour in both NMR and fluorescence spectroscopy.

In the title compound (3-amino-4,4-diphenyl-BODIPY), C 28 H 32 BN 3 , the central six-membered ring has a flattened sofa conformation, with one of the N atoms deviating by 0.142 (4) Å from the mean plane of the other five atoms, which have an r.m.s. deviation of 0.015 Å . The dihedral angle between the two essentially planar outer five-membered rings is 8.0 (2) . In the crystal, molecules are linked via weak N-HÁ Á Á interactions, forming chains along [010]. The compound displays solvent-dependent behaviours in both NMR and fluorescence spectroscopy. In the 1 H NMR spectra, the aliphatic resonance signals virtually coalesce in solvents such as chloroform, dichloromethane and dibromoethane; however, they are fully resolved in solvents such as dimethyl sulfoxide (DMSO), methanol and toluene. The excitation and fluorescence intensities in chloroform decreased significantly over time, while in DMSO the decrease is not so profound. In toluene, the excitation and fluorescent intensities are not time-dependent. This behaviour is presumably attributed to the assembly of 3-amino-4,4-diphenyl-BODIPY in solution that leads to the formation of noncovalent structures, while in polar or aromatic solvents, the formation of these assemblies is disrupted, leading to resolution of signals in the NMR spectra.

Solvent-dependent behaviour of BODIPY 2b observed by NMR spectroscopy
The characterization of 2b by 1 H NMR spectroscopy yielded intriguing results. While the proton signals in 1 H NMR spectra are fully resolved in DMSO-d 6 (as in Fig. 2f), the aliphatic protons are completely coalesced in CDCl 3 . It is also observed that gradual addition of CDCl 3 to a solution of 2b in DMSO-d 6 led to a loss of resolution of the aliphatic protons (Figs. 2b-e).
In deuterated dichloromethane and 1,2-dibromoethane, the 1 H NMR spectra are similarly coalesced (data not shown). On the other hand, spectra are resolved in deuterated methanol and toluene (data not shown), despite the poor solubility of 2b in methanol. These observations prompted us to further investigate the absorption and fluorescent emission behaviour of BODIPY 2b in solution.
1.3. Solvent-dependent behavior of BODIPY 2b observed by fluorescence spectroscopy Fig. 3(a) suggests that the fluorescence spectra of 2b in chloroform, and to some extend in DMSO as well, shows timedependent fluorescent intensities. In contrast, most solvatochromic BODIPY fluorophores that have been reported in the literature often show different maximal emission wavelengths (Baruah et al., 2006;Clemens et al., 2008;Filarowski et al., 2010Filarowski et al., , 2015de Rezende et al., 2014), however, those solvatochromic BODIPY dyes do not display a time-dependent change in fluorescent intensity.
On the other hand, time-dependent spectroscopic changes, in emission intensity, shift of maximal emission wavelength, or absorbance, have been observed for compounds that undergo self-assembly in solution (Gassensmith et al., 2007;Miyatake et al., 2005). Taken together, these observations suggest that BODIPY 2b shows a tendency to form assembled structures in The molecular structure of the title compound, with displacement ellipsoids drawn at the 30% probabilty level. H atoms are not shown. Table 1 Hydrogen-bond geometry (Å , ).
Cg1 and Cg2 are the centroids of the C17-C22 and N2/C6-C9 rings, respectively. (3) 3.223 (2) 150 (2) chloroform, not as significantly in DMSO, and particularly not in toluene. It can be seen from the crystal structure of BODIPY 2b that the molecules are linked along the BODIPY plane by interactions between one of the amino H atoms and the BODIPY ring (N-HÁ Á Á ring; Table 1 and Fig. 4). It is conceivable that in solutions such as in dichloromethane, chloroform and dibromoethane, compound 2b could maintain similar intermolecular assemblies. As a consequence of the reduced mobility of the BODIPY molecules in these assembled structures, the alkyl signals are broadened to the extent that they become invisible in the NMR spectra (Celis et al., 2013;Brand et al., 2008;Chen et al., 2015). Motion of the phenyl rings, however, is not affected in the assembly, and thus the phenyl aromatic protons are visible in these solvents. In polar solvents such as DMSO and methanol, it is possible that solvation of the BODIPY NH 2 group abolishes the ability for such assemblies to occur. On the other hand, in toluene, strong interactions of the aromatic benzene ring with the BODIPY co-plane could also diminish the assemblies. The emission profiles of BODIPY 2b in DMSO, chloroform and toluene also corroborate this model.

Spectroscopy and experimental
Bruker Avance 300 and 600 Digital NMR spectrometers with a 14.1 and 7.05 Tesla Ultrashield magnet, respectively, were used to obtain 1 H and 11 B NMR spectra. 1 H NMR spectra were measured at 300 or 600 MHz, and 11 B at 96 MHz. Chemical shifts and coupling constants (J values) are given in ppm () and Hz, respectively. Deuterated solvents were purchased from C/D/N Isotopes Inc. Fluorescence spectro- scopy was recorded using a QuantaMaster model QM-2001-4 cuvette-based L-format scanning spectrofluorometer from Photon Technology International (PTI), interfaced with FeliX32 software. UV-Vis spectra were obtained using a Thermospectronic/Unicam UV/Vis spectrometer configured to the Vision32 software.
Anhydrous dichloromethane, triethylamine and toluene were generated by first heating under reflux in the presence of phosphorus pentoxide, calcium hydride and sodium metal, respectively, followed by distillation under an atmosphere of nitrogen. All other chemicals and reagents were purchased from Sigma-Aldrich or TCI without further purification prior to use.

Synthesis and crystallization
For the preparation of 2b, a solution of sodium nitrite (80 mg, 1.2 mmol) in water (1.0 ml) was added dropwise to another solution of 3-ethyl-2,4-dimethylpyrrole (0.25 ml, 1.85 mmol) in acetic acid (7.5 ml) and acetic anhydride (7.5 ml). The mixture was then heated at 373 K for 4 h. The solvents were removed   under reduced pressure. The resulting products were diluted with dichloromethane (20 ml) and washed with a saturated aqueous sodium bicarbonate solution (2 Â 15 ml). The organic phase was dried (MgSO 4 ) and evaporated to dryness under reduced pressure. The residue was co-evaporated with dry toluene (10 ml) and then redissolved in dry dichloromethane (10 ml), followed by addition of dry triethylamine (1.0 ml, 7.1 mmol). After stirring for 30 min, boron-diphenylbromide (Noth & Vahrenkamp, 1968) (1.5 ml, 8.2 mmol) was added. Stirring was continued for 20 h and the products were washed with water (3 Â 30 ml), dried (MgSO 4 ) and evaporated under reduced pressure. The residue was purified by column chromatography on silica gel. The appropriate fractions, eluted with dichloromethane-hexane (1:9 v/v), were pooled and concentrated under reduced pressure to give the title compound as an orange solid (yield 18 mg, 4%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms bonded to C atoms were included in calculated positions, with C-H = 0.95-0.99 Å , and were allowed to refine in a riding-motion approximation, with U iso (H) = 1.2U eq (C) or 1.5U eq (C methyl ). The amino H atoms were refined independently with isotropic displacement parameters.