Theoretical Study of Vibrational and Optical Spectra of Methylene-Bridged Oligofluorenes

Abstract

We report a systematic characterization of methylene-bridged fluorene oligomers constructed of two, four, six, and eight aromatic rings using time-dependent density functional theory (TDDFT), the ab initio approximate coupled-cluster singles and doubles (CC2) method, and semiempirical spectroscopic Zerner's intermediate neglect of differential overlap method (ZINDO/S). Geometry optimizations have been performed for the ground state and for the first electronically excited state. Vertical excitations and the fluorescence transition from the lowest excited state have been calculated. Computed ground-state geometries and infrared spectra for fluorene are in good agreement with experimental results. The RI-CC2 and ZINDO/S absorption and fluorescence spectra agree very well with the available experimental data for studied fluorene oligomers and for para oligophenylenes films. On the other hand, TDDFT calculations underestimate excitation and fluorescence energies systematically for larger systems (N > 4) in comparison with the above-mentioned results. The effective conjugation length was estimated to 13-14 repeat units. The computed radiative lifetimes for the fluorene molecule show good agreement with experiment within realistic expectations. The decrease of the radiatiave fluorescence lifetime with the increase in the conjugation length has been discussed also.

Excited state properties, fluorescence energies, and lifetime of a poly(fluorene‐pyridine) copolymer, based on TD‐DFT investigation

“…The extrapolated vertical excitation energy is somewhat lower than the experimental values 18 (3.20 eV in film and 3.26 eV in THF solution). Previous experience with TD-DFT/B3LYP calculations on methylenebridged oligofluorenes 47 showed that this approach underestimated the extrapolated excitation energies. Another reason might be that the effective conjugation length is smaller because of defects in the polymer chains.…”

“…Thus, major progress in the accuracy of the structural description of excited states is expected beyond the CIS approach, which had to be used for that purpose so far. This work also relies on experience with previous investigations comparing TD-DFT with the approximate coupled cluster singles and doubles method (CC2) 44 in combination with the resolution of the identity (RI) method 45,46 for oligo-fluorenes, 47 oligo-paraphenylenes (Lukeš, V.; Aquino, A.; Kauffmann, H. F.; Lischka, H. J Chem Phys, to be submitted) and on excited-state proton transfer systems. 48 In our approach the properties (UV, fluorescence spectra, and radiative lifetime) of a series of oligomers with increasing chain length is computed following by extrapolation to infinite chain length.…”

Facile preparation and fluorescence properties of a soluble oligopyrrole derivative

“…The average lifetime (t) of OMDPP solution in THF was calculated to be 5.567 ns, which was higher than that of PMDPP (3.543 ns) and lower than that of the monomer MDPP (6.006 ns). It indicates that the longer chain length tended to decrease the average lifetime, which was probably caused by a rapid increase in nonradiative decay as the conjugation length increased [43,44]. Additionally, the comparison of the average fluorescence lifetimes in different solvents showed that the fluorescence lifetime of OMDPP was not significantly dependent on the solvent polarity.…”

A theoretical investigation of the two-photon absorption and fluorescent properties of coumarin-based derivatives for Pd2+ detection

“…The emission transitions of all the studied product compounds are assigned to S 1 / S 0 , composed of L / H. During the chemical modications based on the coumarin core in this work, the changing trends of the emission spectra are in accordance with the results of the OPA spectra. The uorescence lifetime (s) and uorescence radiation rate (K r ) for spontaneous emission are calculated by employing the Einstein transition probabilities according to the formulae in (4) and (5), 55,56 and are also shown in Table 3:…”