Investigation On The Photophysical Propetries Of Organic Linear Oligomers By Using Time-resolved Spectroscopy

In recent years, donor–acceptor (D–A) type π-conjugated organic oligomers haveattracted much attention, because of their strong nonlinear susceptibility, ultrafastoptical response and high optical damage threshold, which can be used in the fieldsof two-photon fluorescence microscopy, three-dimensional fabrication, opticallimiting, photodynamic therapy and solar cells, etc. Among the variety ofmolecular structures based on D–A architecture, the symmetrical D–π–A–π–Dstructure is highlighted due to the facilitation of charge transfer between the donorsubstituents and the acceptor center, which leads to excellent optical and electricalproperties. Mono-dispersed π-conjugated oligomer is one of the important modelmolecules that can be used to understand the structure–property relationship oforganic materials, and the optical properties of materials could be affected by manyfactors, such as donor–acceptor strength, π-bridge groups, molecular structure,conjugation length and the intramolecular charge transfer (ICT) character.Therefore, the fine-tuning of the electron donor, acceptor or π-spacer unit makes itpossible to achieve considerable optical and electrical characteristics.In this paper, we systemically investigated the photophysical properties of aseries of D–π–A–π–D type fluorenone-based organic conjugated oligomers withdifferent donors. The main results are as follows:1. We compare the photophysical properties of two oligomers (D–π–A–π–D typefluorenone-based conjugated oligomers, where fluorene and triphenylamine actas the donors, respectively) and discuss the effect of donor units on thephotophysical properties of oligomers. The triphenylamine chromophores withstrong electron-donating ability could lead to red-shifts of spectral features,shorten the fluorescence lifetime, and enhance the two-photon fluorescence (TPF) efficiency and two-photon absorption (TPA) cross-section of materials.An intramolecular charge transfer (ICT) state exists in the relaxation process ofboth oligomers after photoexcitation, and the transfer rate from the initialexcited state to the ICT state is dependent on the electron-donating ability ofdonor units. Quantum chemical calculations provide information regarding themolecular structures and the frontier orbitals and confirm the existence of theintermediate ICT state. We simulate the absorption spectra of oligomers anddetailedly give the mechanisms of electronic transitions. The calculation resultsare in agreement with the experimental data.2. We compare the effects of fluorene-ethylene (FE) spacers on the photo-physicalproperties of fluorenone-based conjugated oligomers. The FE spacer couldmodulate the steady-state spectral feature, increase the TPF efficiency andenhance the TPA cross-section due to the extended π-system. Meanwhile, theintroduction of FE spacer is able to narrow the difference in optical propertiesbetween oligomers with different donors. Transient absorption (TA)measurements confirm an ICT state exists in the relaxation process of excitedoligomers, and the rate of ICT process (corresponding to the rise process in thedynamic curves) could be accelerated with the introduction of FE spacer.Meanwhile, the FE spacer could weaken the effect of donor groups on thedifference of transfer rate from the initial excited state to the ICT state. Thequantum chemical calculations exhibit that the electron transition mechanismmodulated by FE unit is variable with the donor of oligomers. The FE π-spacermainly rises the HOMO and facilitates the electron delocalization in thefluorene end-capped oligomer, while it lowers the LUMO and enhances thedonor effect in the triphenylamine end-capped one. The simulation ofabsorption spectra detailedly gives the mechanisms of electronic transitions.3. Effect of fluorene-ethylene (FE) on the optical properties of two D–π–A–π–Dtype fluorenone-based linear conjugated oligomers is investigated, where fluorenone and phenothiazine units act as the electron acceptor and donormoieties, respectively. Because of the extension of π-conjugated system, the FEspacer could modulate the steady-state spectra, improve the TPF efficiency andenhance the TPA cross-section, meanwhile accelerate the ICT process.Quantum chemical calculations offer complementary information regarding themolecular structures and frontier orbitals, which further confirm that the ICTtransition exists in the excited state.Therefore, increasing the electron-donating ability of electron donor orintroducing an effective π-conjugated spacer can both improve the photophysicalproperties of oligomers. The study in this paper could provide a useful guidelinefor the design and synthesis of novel organic π-conjugated oligomers withexcellent nonlinear optical properties and intramolecular charge transfer character.