Substituent enhanced fluorescence properties of star α-cyanostilbenes and their application in bioimaging.

A series of star α-cyanostilbenes with D-π-A structures comprising triphenylamine as the donor, the cyanovinyl group as the acceptor and different substituents on the terminal phenyl rings were synthesized and characterized. The influence of the substituents on the photophysical, electrochemical and thermal properties of the star molecules was investigated in detail. A strongly electron withdrawing nitro substituent on the phenyl ring (NTBTNPA) increased the absorption and emission maxima, the Stokes shift, and the difference between the ground and excited state dipole moments and decreased the fluorescence quantum yield, fluorescence lifetime and band gap energy between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). This molecule (i.e., NTBTNPA) also showed aggregation-induced enhanced emission; it showed emission at 608 nm in pure DMF and displayed red-shifted emission at 625 nm and new emission at 706 nm in a DMF/water (50 : 50) binary mixture. Aggregation of this molecule in different DMF/water mixtures was confirmed by the dynamic light scattering method. The HOMO, LUMO and band gap energy values of all the star molecules calculated theoretically using density functional theory (DFT) were in good agreement with the experimentally determined values. The biocompatibility of NTBTNPA was tested with two Gram positive and two Gram negative bacteria and also with a fungus. Based on the photophysical properties, NTBTNPA was used as a fluorophore for bio-imaging application of a fungus, Rhizoctonia solani, as a model and the results obtained were excellent.

Crystal structure of phenyl N-(3,5-di-methyl-phenyl)carbamate.

The asymmetric unit of the title compound, C15H15NO2, contains two independent mol-ecules (A and B). The di-methyl-phenyl ring, the phenyl ring and the central carbamate N-C(=O)-O group are not coplanar. In mol-ecule A, the di-methyl-phenyl and phenyl rings are inclined to the carbamate group mean plane by 27.71 (13) and 71.70 (4)°, respectively, and to one another by 84.53 (13)°. The corresponding dihedral angles in mol-ecule B are 34.33 (11), 66.32 (13) and 85.48 (12)°, respectively. In the crystal, the A and B mol-ecules are arranged alternately linked through N-H⋯O(carbon-yl) hydrogen bonds, forming -A-B-A-B- chains, which extend along [100]. Within the chains and linking neighbouring chains there are C-H⋯π inter-actions present, forming columns along the a-axis direction. The columns are linked by offset π-π stacking inter-actions, forming a three-dimensional network [shortest centroid-centroid distance = 3.606 (1) Å].

Crystal structures of 4-chloro-phenyl N-(3,5-di-nitro-phen-yl)carbamate and phenyl N-(3,5-di-nitro-phen-yl)carbamate.

The title compounds, C13H8ClN3O6, (I), and C13H9N3O6, (II), differ in the orientation of the two aromatic rings. In (I), they are essentially coplanar, making a dihedral angle of 8.2 (1)°, while in (II), they are inclined to one another by 76.2 (1)°. The two nitro groups are essentially coplanar with the attached benzene rings, as indicated by the dihedral angles of 1.4 (2) and 2.3 (2)° in (I), and 4.96 (18) and 5.4 (2)° in (II). The carbamate group is twisted slightly from the attached benzene ring, with a C-N-C-O torsion angle of -170.17 (15)° for (I) and 168.91 (13)° for (II). In the crystals of of both compounds, mol-ecules are linked via N-H⋯O hydrogen bonds, forming chains propagating along [010]. In (I), C-H⋯O hydrogen bonds also link mol-ecules within the chains. The crystal packing in (I) also features a very weak π-π inter-action [centroid-centroid distance = 3.7519 (9) Å].

Crystal structure of phenyl N-(4-nitro-phen-yl)carbamate.

The asymmetric unit of the title compound, C13H10N2O4, contains two independent mol-ecules (A and B). The dihedral angle between the aromatic rings is 48.18 (14)° in mol-ecule A and 45.81 (14)° in mol-ecule B. The mean plane of the carbamate N-C(=O)-O group is twisted slightly from the attached benzene and phenyl rings, making respective dihedral angles of 12.97 (13) and 60.93 (14)° in A, and 23.11 (14) and 59.10 (14)° in B. In the crystal, A and B mol-ecules are arranged alternately through N-H⋯O hydrogen bonds and C-H⋯π inter-actions, forming chains along the a axis. The chains are further linked by C-H⋯O hydrogen bonds into a double-chain structure.