Syn- And anti-[3.3](3,9)carbazolophanes, which are suitable model compounds for sandwich and partial-overlap excimers, respectively, have been synthesized and characterized; the structures of both singlet and triplet carbazole excimer have been described.

The photophysical properties of carbazole excimers were investigated using syn-and anti- [3.3](3,9)carbazolophanes: the syn-carbazolophane is a model compound for a fully overlapped carbazole excimer and the anti-carbazolophane for a partially overlapped carbazole excimer the structures of which have been determined by X-ray analysis. 1 H NMR spectra showed that two carbazole moieties in the syn-and anti-[3.3](3,9)carbazolophanes did not flip even in solution owing to short [3.3](3,9)linkage: geometrical alignments of two carbazole moieties remained the same as those determined by the X-ray analysis. Absorption bands of the carbazolophanes were broadened and blue-or red-shifted compared with those of 1,3-bis(3-methyl-Ncarbazolyl)propane used as a reference compound. These spectral features were explained by Kasha's molecular exciton theory. Fluorescence spectra of both carbazolophanes were red-shifted, broad, and structureless bands. These broad emissions were ascribed to intramolecular excimer fluorescence of the carbazolophanes, which was insensitive to solvent polarity. The dipole moment in the excited state was estimated to be ∼0 D for the anti-carbazolophane and to be 4.8 D for the syn-carbazolophane from Lippert-Mataga plots. The radiative rate of excimer emission was 3.4 × 10 6 s -1 for the anti-carbazolophane and 7.2 × 10 5 s -1 for the syncarbazolophane. Our [3.3](3,9)linkage carbazolophanes clearly showed that these different photophysical properties of carbazole excimers result from the geometries of their carbazole moieties. † Part of the special issue "Noboru Mataga Festschrift".

The photophysical properties of a series of triply bridged [3.3.n](3,6,9)carbazolophanes ([3.3.n]Cz, n = 3, 4, 5, 6) were studied as a model compound for a fully overlapped carbazole excimer. In these [3.3.n]Cz molecules, a plane angle of the two carbazole moieties changed systematically from nearly parallel to oblique, with increases in the length of the methylene chain n bridging at the 9-position of each carbazole ring. Absorption bands of [3.3.n]Cz showed the blue-shift and the splitting for (1)L(a) <-- (1)A and (1)L(b) <-- (1)A transition bands of the reference carbazole monomer, respectively. These spectral changes in [3.3.n]Cz were explained by Kasha's molecular exciton theory with the distance r and dihedral angle theta between the carbazole moieties in the ground state. In both liquid and glass matrixes, [3.3.n]Cz showed intramolecular excimer emission. The emission peak wavelength changed from 409 nm (n = 6) to 480 nm (n = 3) depending on r in the ground state. The dependence of the peak wavelength on r clearly showed that relative configurations of carbazole moieties in the ground state were preserved even in the excimer states. The smaller radiative rate of the excimer emission than the reference monomer was explained by the dimer symmetry of [3.3.n]Cz.

Intermoiety electronic interactions in the singlet and triplet excimer states of triply bridged [3.3.n](3,6,9)carbazolophanes ([3.3.n]Cz, n=3-6) were studied by emission and transient absorption measurements. In these [3.3.n]Cz molecules, the dihedral angle and the separation distance r between fully overlapped two carbazole rings change systematically from nearly parallel (n=3, r=3.35 A) to oblique (n=6, r=4.03 A). In rigid glass at 77 K, [3.3.n]Cz (n=3, 4) (r<4 A) exhibited red-shifted and structureless excimer fluorescence and phosphorescence while [3.3.n]Cz (n=5, 6) (r>4 A) exhibited monomer-like vibrational fluorescence and phosphorescence. In solution at 130 K, all [3.3.n]Cz molecules exhibited an excimeric fluorescence band while [3.3.5]Cz still exhibited monomer-like phosphorescence. Transient absorption spectra measured at 294 K exhibited local excitation and charge-transfer bands for all [3.3.n]Cz molecules in the excited singlet and triplet states, suggesting that not only singlet but also triplet excimers of carbazole are formed at room temperature. Furthermore, the singlet-triplet energy gap decreased with the decrease in n, suggesting that electrons are effectively delocalized over the two carbazole moieties. These findings showed that both singlet and triplet excimers of carbazole are formed with a separation distance shorter than about 4 A and are most stable in the parallel-sandwich structure and that the configurational mixing between exciton resonance and charge resonance states plays an essential role in the formation of singlet and triplet excimers of carbazole.

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