Electronic spectroscopy of highly-polar aromatics.* II. Luminescence of nitroanilines

Abstract

The luminescences of isomeric nitroanilines and their N-methylated derivatives, as observed in glassy media at 77°K, are reported. The o- and m-derivatives fluoresce only. The p-derivatives may fluoresce only, phosphoresce only, or show both a fluorescence and a phosphorescence whose relative intensities are dependent on the polarity of the medium and the exciting wavelength. These p-derivatives which possess the largest ground-state dipole moments exhibit the largest phosphorescence yields; they also exhibit a dependence of φp/φf on excitation energies which lie within the wavelength compass of the lowest-energy absorption band, a ``red-edge'' effect, and different fluorescence and phosphorescence excitation spectra. Some suggestions as to the interpretation of these effects are made.

Dipolar Triplet States ofp-Nitroaniline andN-Alkyl Derivatives with One-, Two-, and Three-Fold Symmetry

“…4,6,8,9,11 Evidence of slight shoulders on the long-wavelength side of the absorption spectra of DMPNA and 1-PNA are apparent and are probably due to an increased separation between these lowest lying excited singlet levels. Despite this, the overall half-widths are less than for PNA itself.…”

“…This is demonstrated by the large yields of phosphorescence which are found in low-temperature rigid matrices. 4,9 In the case of PNA itself, only phosphorescent emission has in fact ever been observed and the quantum yield for triplet state formation is most probably close to unity. For N-alkyl substituted derivatives both fluorescence and phosphorescence are found, 4,9 indicating that internal conversion of S 1 to S 0 can now compete with intersystem crossing to T 1 .…”

“…4,9 In the case of PNA itself, only phosphorescent emission has in fact ever been observed and the quantum yield for triplet state formation is most probably close to unity. For N-alkyl substituted derivatives both fluorescence and phosphorescence are found, 4,9 indicating that internal conversion of S 1 to S 0 can now compete with intersystem crossing to T 1 . Phosphorescence is however still found to be the dominant process at low temperatures, even for DMPNA for which the phosphorescence to fluorescence ratio has been determined to be approximately 2:1.…”

“…Studies of the excitation-wavelength dependence of the fluorescence to the phosphorescence ratio and the relative polarization of the emissions for N-methyl substituted PNA indicate the presence of two singlet state transitions within the absorption envelope. [7][8][9][10][11] On the basis of electrochromic effects on the absorption of PNA and its high efficiency in second harmonic generation (SHG), it can be concluded that vertical excitation within the first absorption band leads to a highly dipolar excited singlet state. The dipole moment has been estimated to be approximately 15 D. 2,3,12,13 Intersystem crossing is clearly an important radiationless decay channel for the S 1 state of PNA derivatives.…”

“…[1][2][3][4][5][6][7][8][9][10][11] A common feature found is an extremely short S 1 lifetime and very weak, if any, fluorescence. For PNA itself only phosphorescent emission has ever been observed.…”