Photophysical Propetries Of Fluorene-based Linear Conjugated Oligomers

Organic photoelectric materials have received much attention due to theirpotential applications in many extensive aspects，such as light-emitting diodes,organic solar cells, organic field-effect transistors, nonlinear optical and so on. Amongthe organic photoelectric materials, fluorene and phenothiazine have been a majorconcern for their finer photoelectric properties. Both fluorene and phenothiazine havelarge rigid structure and π conjugated system. Fluorene shows high thermal stability,light stability and large band gap energy level. Further more, fluorene acting ascommon electron-donor group is extensively used in synthesis of oligomers, whichhas low ionization potential. Phenothiazine is a conjugated heterocyclic compoundbearing electron-rich sulfur and nitrogen heteroatoms, and it has low ionizationpotential too. Further more, phenothiazine owns special spatial structure, which isbeneficial for the delocalization of π electron in oligomers, so that is able to restrainthe aggregation owing to the π-π interaction. Based on this, people synthesize twolinear oligomers （P-F-P and F-P-F-P-F）, in which the fluorene unit acts as core andthe phenothiazine groups acts as “peripheral arm” in company with fluorene unit.In this paper, we report the results of linear and nonlinear optical measurementsof F-P-F-P-F and P-F-P. The F-P-F-P-F has much more two fluorene units incomparison with P-F-P and exhibit large extent of π-extension. Their linear andnonlinear optical properties have been probed by combining various experimentalapproaches, involving UV-vis spectra, one photon fluorescence, two-photonfluorescence, z-scan, and transient absorption （TA） technique. Meanwhile, theelectronic transition and the absorption spectra of oligomers are also analyzed throughquantum chemical calculation. This paper would focus on the effect of π-extension onthe oligomers’ optical properties, and the conclusions we got are described below.The UV-Vis absorption spectrum of P-F-P and F-P-F-P-F shows similarstructure, and both of them own one main absorption band. The maximum absorbentwavelength of P-F-P is413.5nm, and that of F-P-F-P-F is425nm. What’s more, it is noted that the molar absorption coefficients of F-P-F-P-F is1.04×10⁵cm^–1M^–1andlarger than that (8.63×10⁴cm^–1M^–1) of P-F-P. One-photon fluorescence （OPF）spectra shows that the maximum emission wavelength of P-F-P is498.5nm, and thatof F-P-F-P-F is520nm. Both of the absorption band and the emission band ofF-P-F-P-F shows a obvious red-shift as to P-F-P and the bandgap of F-P-F-P-Fdecreases.Transient absorption study of P-F-P and F-P-F-P-F shows that the enhancementof conjugated system would modulate the IC dynamic and influence the competitionbetween relaxation channels. Transient absorption measurement indicates that there isan intermediate state in the relaxation process of excited state. It is interesting foundthat the fluorene unit would prolong the internal conversion time from local excitonstate to the intermediate state. In addition, a fast thermal relaxation appears afterphoto-excitation and may decrease the yield of fluorescence. The molecule structuresof P-F-P and F-P-F-P-F don’t twist after photo-excitation, and the twist ICT statemay not be responsible for the I-S. Therefore, the origin of I-S need to furtherdiscussed in the future.In the quantum chemistry calculations, the molecular frontier orbitals of P-F-Pand F-P-F-P-F are optimized by B3LYP/6-31G. The calculations show that theintroduction of fluorene units mainly influences the position of HOMO in comparisonwith that of LUMO. The calculated dipole moment enhances from1.77（P-F-P） to5.41D （F-P-F-P-F）, meanwhile the calculated bandgap of oligomer narrows from3.01to2.95eV, which has the same tendency as the experimental data. The simulatedabsorption spectrum （SAS） of P-F-P and F-P-F-P-F can be achieved withTD-DFT/6-31G（d）, and we try to analyze the mechanism of the electron transition.Two-photon fluorescence（TPF） shows that the TPF yield of F-P-F-P-F is higherthan that of P-F-P. The open aperture z-scan traces of P-F-P and F-P-F-P-F can offerthe two-photon absorption （TPA） coefficient （β）. Apparently, through adding fluorenegroups, the σ（2）value of F-P-F-P-F increases about two times in comparison with thatof P-F-P, and the results approve again that the π-conjugated system could effectivelyenhance the TPA cross-section.