Fluorescence Quenching of Two Coumarin-3-carboxylic Acids by Trivalent Lanthanide Ions.

The effects of various trivalent lanthanide ions (acetates of Ce3+, Er3+, Eu3+, Nd3+) on the electronic absorption and fluorescence spectra of un-substituted coumarin-3-carboxylic acid (CCA) and 7-N,N-diethylamino-coumarin-3-carboxylic acid (DECCA) have been investigated in dimethylsulfoxide (DMSO) at room temperature. Depending on the lanthanide ion nature and concentration, significant spectral changes of absorption bands occurred for both coumarin derivatives. These spectral changes were attributed to the formation of ground-state complexes between the coumarin carboxylate derivatives and lanthanide ions. The fluorescence quenching of CCA and DECCA upon increasing the lanthanide ion concentration was studied. Different quantitative treatments, including the Stern-Volmer equation, the Perrin equation and a polynomial equation, were applied and compared in order to determine the nature of the quenching mechanisms for both coumarin derivatives. The results suggested the contribution of both dynamic and static quenching. Significant differences of CCA and DECCA fluorescence quenching efficiency were also observed, depending on the lanthanide ion. DECCA fluorescence lifetime measurements, performed in the absence and in the presence of Ln3+, confirmed a contribution of static quenching.

Crystal structure of 2-oxo-2H-chromen-3-yl propano-ate.

In the title compound, C12H10O4, the dihedral angle between the coumarin ring system [maximum deviation = 0.033 (8) Å] and the propionate side chain is 78.48 (8)°. In the crystal, weak C-H⋯O hydrogen bonds generate inversion dimers and and C-H⋯π and π-π inter-actions link the dimers into a three-dimensional network. A quantum chemical calculation is in good agreement with the observed structure.

2-Oxo-2H-chromen-4-yl 4-methyl-benzoate.

The asymmetric unit of the title compound, C17H12O4, consists of two independent mol-ecules. The chromen-2-one ring and the 4-methyl-benzoate side chain are inclined to one another at a dihedral angle of 64.79 (10)° in one mol-ecule and 88.3 (1)° in the other. In the crystal, mol-ecules form R 2 (2)(8) centrosymmetric dimers via C-H⋯O hydrogen bonds. These dimers are stacked by C-H⋯O hydrogen bonds, resulting in R 2 (2)(18) and R 3 (2)(16) ring motifs. π-π stacking inter-actions between two parallel chromen-2-one rings, with centroid-centroid distances of 3.743 (1) and 3.771 (1) Å, are also present.

2-Oxo-2H-chromen-4-yl 4-tert-butyl-benzoate.

In the title mol-ecule, C(20)H(18)O(4), the three methyl groups of the tert-butyl substituent show rotational disorder. Each methyl group is split over three positions, with refined site-occupation factors of 0.711 (4), 0.146 (3) and 0.144 (4). The benzene ring of the benzoate group is oriented at a dihedral angle of 60.70 (7)° with respect to the planar chromene ring [maximum deviation = 0.046 (2) Å]. The crystal structure features centrosymmetric R(2) (2)(8) dimers formed via C-H⋯O inter-actions, and these dimeric aggregates are connected by C-H⋯π inter-actions.

4-[(Hy-droxy)(4-methyl-phen-yl)methyl-idene]isochroman-1,3-dione.

In the title compound, C(17)H(12)O(4), the six-membered heterocyclic ring adopts a distorted screw-boat conformation. The mol-ecular structure exhibits an S(6) ring motif, owing to an intra-molecular O-H⋯O hydrogen bond. In the crystal, weak C-H⋯O contacts generate an infinite chain along the c axis. There are also π-π stacking inter-actions between neighbouring isochromanedione benzene rings, with a centroid-centroid distance of 3.755 (1) Å, and C-O⋯π inter-actions with an O⋯centroid distance of 3.964 (2) Å.

2-Oxo-2H-chromen-4-yl 4-meth-oxy-benzoate.

In the title mol-ecule, C17H12O5, the chromen-2-one ring and the 4-meth-oxy-benzoate side chain are inclined to one another at a dihedral angle of 69.82 (9)°. The crystal structure features parallel sheets of centrosymmetric R2(2)(6) dimers joined by a C(7) chain, resulting in centrosymetric tetra-mers of hydrogen-bonded mol-ecules with graph-set motif R4(4)(40). These centrosymetric tetra-mers are connected by a pair of hydrogen bonds described by an R2(2)(8) ring motif and a C(7) chain via C-H⋯O inter-actions. In the structure, there are also π-π stacking inter-actions between chromene benzene and the six-membered heterocyclic rings [centroid-centroid distance = 3.691 (2) Å] and weak C=O⋯π inter-actions [O⋯(ring centroid) distance = 3.357 (3) Å].

4-[(4-Chloro-phen-yl)(hy-droxy)methyl-idene]isochromane-1,3-dione.

In the title compound, C(16)H(9)ClO(4), the six-membered heterocyclic ring adopts a screw-boat conformation. The benzene rings are oriented to each other at a dihedral angle of 59.26 (9)°. The mol-ecular structure exhibits a ring motif, viz. S(6), owing to an intra-molecular O-H⋯O hydrogen bond. The presence of C-H⋯O contacts generates an infinite chain along [001]. Also present are π-π stacking inter-actions between neighbouring isochromanedione benzene rings [centroid-centroid distance = 3.746 (1) Å], and C-O⋯π inter-actions [O⋯centroid = 3.934 (2) Å].

2-Oxochromen-4-yl 4-(dimethyl-amino)-benzoate.

In the title mol-ecule, C(18)H(15)NO(4), the benzoate ring is oriented at a dihedral angle of 43.43 (6)° with respect to the planar [maximum deviation = 0.038 (2) Å] chromene ring. The crystal structure features R(2) (2)(12) centrosymetric dimers formed via C-H⋯O inter-actions and these dimeric aggregates are connected by C-H⋯π inter-actions.

Revisiting the photophysical properties and excited singlet-state dipole moments of several coumarin derivatives.

The solvent effects on the electronic absorption and fluorescence emission spectra of several coumarins derivatives, containing amino, N,N-dimethyl-amino, N,N-diethyl-amino, hydroxyl, methyl, carboxyl, or halogen substituents at the positions 7, 4, or 3, were investigated in eight solvents with various polarities. The first excited singlet-state dipole moments of these coumarins were determined by various solvatochromic methods, using the theoretical ground-state dipole moments which were calculated by the AM1 method. The first excited singlet-state dipole moment values were obtained by the Bakhshiev, Kawski-Chamma-Viallet, Lippert-Mataga, and Reichardt-Dimroth equations, and were compared to the ground-state dipole moments. In all cases, the dipole moments were found to be higher in the excited singlet-state than in the ground state because of the different electron densities in both states. The red-shifts of the absorption and fluorescence emission bands, observed for most compounds upon increasing the solvent polarity, indicated that the electronic transitions were of π-π* nature.

Mass Spectrometry Study of Coumarins: Correlation Between Charges of Atoms and Fragmentation Processes.

The mass spectrometry of a number of 6-substituted coumarins was studied in the context of correlating fragmentation pathways and electronic charges of atoms performed by AM1 semiempirical method. The atomic charges of atoms are found to be good predictors of the fragmentation pathways.