Photoluminescence Studies of Phenanthrene−Azomethyne Conjugated−Nonconjugated Multiblock Copolymer

Abstract

A Schiff base type polymer containing phenanthrene and an aliphatic spacer in the main chain was prepared, and its photophysical behavior in diluted and concentrated solutions as well as in film form was studied by steady-state fluorescence spectroscopy. These data indicated that ground-state fluorescent phenanthrene aggregates were present in the films. Time-resolved measurements in diluted conditions showed biexponential decay implying in different microenvironments around the chromophore. Theoretical simulations predicted aggregation in π-stack arrangement, and calculation of optical properties for small aggregates shows the introduction of new states at lower energies, in good agreement with the experimental results. Theoretical studies also forecast the possibility of domains with highly ordered morphology, which was confirmed by DSC measurements providing further support for the presence of the emitting aggregated form.

Photophysical properties and quantum chemical studies of poly(2,7-9,9'-dihexylfluorene-dyil)

“…[1][2][3][4][5][6][7][8] Several of them are also electroluminescent materials. 1,[4][5][6][7][8][9] The backbone of fluorene-based polymers contains a rigidly planar biphenyl structure which can be functionalized in the C-9 position to enhance the polymer solubility.…”

“…28 Several reports indicated that this is a convenient approach since this methodology combines good data quality with reasonable computational effort. 3,29 The theoretically simulated spectra of some fluorene oligomers were compared with the experimental UV-Vis spectra of the poly(2,7-9,9'-dihexylfluorenedyil) (PHF, Scheme 1) in dilute solutions. The number of repeating units in the oligomers varied from n = 1 to n = 8 and for each one of these we obtained the optimized geometry, the dipole moment, the electronic energy levels and the correspondent simulated electronic absorption spectra.…”

“…For example, we observed that for conjugated-nonconjugated alternated copolymers, the emission band is defined by the conjugated group and small spectral shifts were observed when compared with the isolated lumophores. [1][2][3]9,18,23 The critical conjugation length may be determined by theoretical quantum mechanical calculations based on the red-shift of the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for larger oligomers. In general it has been observed that the critical conjugation length involves between 7 and 12 mers, depending on the polymer.…”

“…[2][3][4]22,23 The reason for such differences is the occurrence of aggregation process that influences not only the spectral profile but also the emission intensity. In particular for conjugated polymers which undergo photo and electroluminescence, [1][2][3][4][5][6][7][8][9] it has been observed in several cases that both emissions are very similar, and thus the knowledge of the solid state emission is important. Here we compare the spectral profile of PHF in solid state and in dilute solutions in an attempt to identify the lumophores responsible by the emission.…”

“…The other possibility is the aggregation processes of the polymer in solid state, which may be responsible by the presence of excimer emission, emission from ground state dimmers or red-edge emission band. [2][3][4][13][14][15]20,38 Here we are not observing excimer emission since the excitation and the absorption profiles are very similar, no red-edge emission band has been detected and consequently the presence of dimmers can be excluded. The coil conformation is probably inhibiting the spatial coupling between fluorenyl units and imposing restraints to the dimmer or excimer formations.…”