The absorbance and fluorescence properties of three 2,3-diarylindenes substituted with donor and acceptor functions were studied and compared with the corresponding donor/acceptor-substituted stilbenes. The parent systems, 2-phenyl-3-(4-methoxyphenyl)-6-methoxyindene (1c) and 4-methoxystilbene (2c), show absorption and emission spectra typical for stilbene systems. Relative to the stilbene 2c, absorbance of indene 1c shows a small bathochromic shift (1500 cm-1) and a somewhat larger Stokes shift (7000 versus 5000 cm-1); neither absorbance nor emission shows much solvent sensitivity. The meta-substituted systems 2-(3-nitrophenyl)-3-(4-methoxyphenyl)-6-methoxyindene (1b) and 4′-methoxy-3-nitrostilbene (2b) both show absorbances having little solvent dependence that are shifted ca. 1200 cm-1 relative to the unsubstituted analogues (1c and 2c); however, their fluorescence spectra are now very solvent sensitive, with Stokes shifts for both systems ranging from ca. 11 000 cm-1 in cyclohexane to ca. 16 000 cm-1 in chloroform. The very large Stokes shift of 1b and 2b can best be explained by the emission arising from a twisted intramolecular charge-transfer (TICT) state, in which the two π systems are mutually perpendicular, with complete charge transfer from donor to acceptor. In addition to this TICT band, the indene system 1b also shows a shorter wavelength emission that is ascribed to a locally excited state (LE). The relative intensities of the dual-emission (LE and TICT) bands of this are very solvent dependent. The absence of an LE band in the stilbene 2b suggests that it can access the twisted conformation required for the TICT state more readily than can the diarlyindene 1b. The para-substituted systems 2-(4-nitrophenyl)-3-(4-methoxyphenyl)-6-methoxyindene (1a) and 4′-methoxy-4-nitrostilbene (2a) show absorbance bands shifted bathochromically by 5000-6000 cm-1 relative to the unsubstituted systems 1c and 2c. The Stokes shift is again very solvent dependent but is smaller than for the para-substituted systems, in the range 5500-9000 cm-1 for 1a and 8000-12000 cm-1 for 2a, consistent with emission from a simple intramolecular charge-transfer state (ICT) involving a more planar system having only partial charge separation. The greater Stokes shift of the stilbene 2a relative to the indene 1a may be due to the greater planarity of the stilbene system. The relative emission intensities of the indene systems varies p-NO2 > H > m-NO2, that of stilbenes H > p-NO2 > m-NO2, consistent with the forbidden nature of the TICT, with the indenes 1a and 1b being far more intense than the corresponding stilbenes. The donor/acceptor-substituted 2,3-diarylindene systems provide interesting structure-dependent fluorescence characteristics that may be exploitable in biological applications.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry