Han-zhuang Zhang

Self-assembly of a chiral lipid gelator controlled by solvent and speed of gelation.

Glutamine derivative 1 with two-photon absorbing units has been synthesized and was found to show gelation ability in some solvents. Its self-assembly in the gel phase could be controlled by the solvent and speed of gelation. For example, in DMSO the organogelator self-assembled into H-aggregates with weak exciton coupling between the aromatic moieties. On the other hand, in DMSO/diphenyl ether (1:9, v/v) the molecules formed 1D aggregates, but with strong exciton coupling due to the small distance between the chromophores. Moreover, the formation of these two kinds of aggregates could be adjusted by the ratio of DMSO to diphenyl ether. In DMSO/toluene, DMSO/butanol, DMSO/butyl acetate, and DMSO/acetic acid systems similar results were observed. Therefore, conversion of the packing model occurs irrespective of the nature of the solvent. Notably, a unique sign inversion in the CD spectra could be realized by controlling the speed of gelation in the DMSO/diphenyl ether (1:9, v/v) system. It was found that a low speed of gelation induces the gelator to adopt a packing model with strong pi-pi interactions between the aromatic units. Moreover, the gels, when excited at 800 nm, emit strong green fluorescence and the quantum chemical calculations suggest that intramolecular charge transfer leads to two-photon absorption of the gelator molecule.

Synthesis of star-shaped monodisperse oligo(9,9-di-n-octylfluorene-2,7-vinylene)s functionalized truxenes with two-photon absorption properties

A series of new star-shaped monodisperse conjugated truxene derivatives bearing oligo(fluorene-vinylene) arms (Tr-OFVn, n = 1, 2, 3, 4) have been synthesized. It is found that the conjugation of the oligomers can be extended with prolonging the arms. Notably, the branched oligomers Tr-OFVn without strong donor and acceptor units exhibit two-photon absorption properties, and the two-photon absorption cross sections (δ(max)) increase with increasing the number of fluorene-vinylene units in the arms. The maximum value of δ(max) reaches 8073 GM for compound Tr-OFV4, which made it one of the most competitive compounds with enhanced TPA cross section. It provides a new platform for exploiting strong TPA compounds, in which the extended π-conjugated systems are involved in the absence of strong donor and acceptor units.

Linear and Nonlinear Optical Properties of Novel Multi-branched Oligomers

We investigate the fluorene-vinylene unit dependent photo-physical properties of multi-branched truxene based oligomers (Tr-OFVn, n=1–4) employing steady-state absorption and emission spectroscopy, transient absorption spectroscopy, two-photon fluorescence, and z-scan technique. The results show that the increasing of fluorene-vinylene unit leads to a red-shift in the spectra of absorption and fluorescence, and shortens the excited state lifetime. Meanwhile, two-photon fluorescence efficiency and two-photon absorption cross section of truxene based oligomers gradually enhance in company with the extension of π-conjugated length. In addition, the values of two-photon absorption cross section modeled on the sum-over-state approach agree well with the experimental ones. The results indicate multi-branched truxene based oligomers bearing oligo(fluorene-vinylene) arms are promising organic materials for two-photon applications.

Investigation on Excited-State Photophysical Characteristics of Low Bandgap Polymer APFO3

The excited state photophysics of low bandgap polymer APFO3 has been investigated in detail. The chemical calculations confirm that the intrachain charge transfer (ICT) may occur after photo-excitation and is mainly responsible for the first absorption band. The transient absorption results confirm that ICT indeed exists and competes with the vibrational relaxation at the same time, when APFO3 is in a monodisperse system. This ICT process would disappear due to the influence of interchain interaction when APFO3 is in the condensed phase, where the exciton decay would be dominant in the relaxation process after photoexcitation. The photoexcitation dynamics of APFO3 film blending with PC61 BM are presented, which shows that the exciton may be dissociated completely as the percentage of PC61 BM reaches ∼50%. Meanwhile, the photovoltaic performance based on blend heterojunction shows that the increase of photocurrent is little if the percentage of PC61 BM exceeds ∼50%. Overall, the present study has covered s...

Studying the emission complexity of conjugated molecules by manipulating the molecular aggregate state

We prepared quantum dots (QDs) based on a conjugated molecule (coumarin 6, C-6), which is considered to be a mesoscopic aggregate, to build a bridge linking the monodisperse state (solution state) and macroscopic aggregates (powder state). Their spectral features, including steady spectral data, dynamic features and fluorescence quantum yields, have been compared in detail. All dynamic data were modelled using the continuous rate distribution function. The final results showed that the fluorescence quantum yield of organic quantum dots based on C-6 gradually decreases as the size of the QDs increases, which may be attributed to enhancement of the rate of non-radiative relaxation, which originates from the aggregation. This is beneficial for further understanding the optical characteristics of conjugated molecules.

π-Conjugated Unit-Dependent Optical Properties of Linear Conjugated Oligomers

The optical properties of three linear conjugated oligomers (F-P, F-P-F, and P-F-P-F-P), where phenothiazine (P) and fluorene (F) groups arrange alternately, are investigated. With the enhancement of the π-conjugated system, their absorption and emission bands both gradually red shift, and their two-photon properties are also improved. Meanwhile, their fluorescence dynamic traces are analyzed with continuous rate distribution model, exhibiting that their decay rates gradually accelerate and the rate distribution width become narrower. The quantum chemical calculation offers their molecular structures and transition mechanism, showing that the enhancement of π-conjugated system should be responsible for the improvement of two-photon properties.

Optical Properties of Oligo(Fluorene-Vinylene) Functionalized Anthracene Linear Oligomers: Effect of π-Extension

The photo-physical properties of oligo(fluorene-vinylene) functionalized anthracene linear oligomers (An-OFVn (n=1-4)) have been systemically investigated through experimental and theoretical methods. The steady-state spectral measurement shows that the increasing of fluorene-vinylene (FV) group could lead to the red shift of absorption spectra and restrain the excimer formation between oligomers. Quantum chemical calculations exhibit that the energy levels of HOMO, LUMO, and the band gap gradually converge to a constant in accompany with the increasing of FV unit. Meanwhile, the electronic cloud which distributes on the branch arms, also gradually enhances and makes the absorption spectral shape of oligomers become similar to that of branch arms step by step. The time-resolved fluorescence tests exhibits that the lifetime of excimer emission would be almost invariable after the number of FV group in oligomer is >= 2. In nonlinear optical test, the two-photon photoluminescence efficiency and two-photon absorption cross-section will both gradually enhance and be close to an extremum after the number of FV unit is equal to 4. These results will provide a guideline for the design of novel photo-electronic materials.

Theoretical and Experimental Study of Photophysical Characteristics between Poly(9,9‐dioctylfluorene) and Poly(9,9‐dioctylfluorene‐cobenzothiadiazole)

The photo‐physical characteristics of semiconductor polymer are systematically studied through comparing poly (9,9‐dioctylfluorene) (PFO) and poly (9,9‐dioctylfluorene‐cobenzothiadiazole) (F8BT). The quantum chemical calculation shows that the introduction of benzothiadiazole unit facilitates the intrachain charge transfer (ICT) and modulates the electronic transition mechanism of polymer. The transient absorption measurement exhibits that intrachain exciton relaxation is dominant in the decay of excited PFO in a monodisperse system and intrachain exciton interaction could appear at high excitation intensity. In F8BT solution, the ICT state exists and participates in the relaxation of excited state. The relaxation processes of PFO and F8BT in the condensed phase both accelerate and show obvious exciton‐exciton annihilation behavior at high excitation intensity. At the same excitation intensity, the mean lifetime of F8BT is longer than that of PFO, which may be assigned to the excellent delocalization of charge.