Ai Xi-Cheng

Mechanism of intramolecular charge transfer in DNA helix as probed by the use of the fluorescent 2-aminopurine

As a structural analogue of adenine, 2-aminopurine (2Ap) is often used as a fluorescent probe to study the intramolecular charge transfer reaction in DNA. We have designed and synthesized a series of model DNA helix with the variation in the distance between the 2Ap probe and the GGG sequence, and have investigated, by means of picosecond time-resolved fluorescence spectroscopy, the effect of the length of the bridge (consisting of a number of inosines, I) separating the electron donor (...GGG...) and the acceptor (2Ap) on the charge transfer dynamics. The fluorescence dynamics of 2Ap exhibited three exponential decay components, the one with a time constant of a few hundred picoseconds is assigned to the intramolecular charge transfer from GGG to 2Ap. Within 2.4 nm of the donor-acceptor separation, the rate of charge transfer decreased exponentially upon increasing the separation, from which the decay factorβ is determined to be 1.3 nm−1. Beyond 2.4 nm, however, the rate started to increase, this abnormal behavior of charge transfer is interpreted in terms of the match of electronic energies between the l-bridge and the donor/acceptor couple.

Effects of different aggregation forms of LHC II from Bryopsis corticulans on the excited state properties of Chl a

Steady-state and time-resolved fluorescence spectroscopies have been used to study the excited state properties of Chl a in different aggregation forms of light-harvesting complex II (LHC II) from an intertidal green alga, Bryopsis corticulans, i.e. LHC II monomer, trimer and oligomer. When either Chl a or Chl b was selectively excited, the observed decrease in Chl a fluorescence in the oligomer is proved to be caused mainly by the fast fluorescence quenching among Chl a molecules, rather than by the decrease in Chl b-to-Chl a singlet excitation transfer efficiency. Analyses of the picosecond time-resolved fluorescence kinetics identified two exponential decay components in all of the three forms of LHC II: a longer-lived component (4.1–4.7 ns) originating from fluorescence emission of Chl a, and a shorter-lived one (135–540 ps) from the rapid equilibration of singlet excitation among Chl a molecules. The time constant of excitation equilibration is 135 ps in oligomer, 520 ps in trimer and 540 ps in monomer. These results imply that LHC II in oligomer form is inherently able to quench Chl a excitation, a mechanism which may be related to the photoprotection of PS II via changing the degree of LHC II aggregation in Bryopsis corticulans.

Influence of Donor and Acceptor Mass Ratios on P3HT:PCBM Film Structure and Device Performance

Organic bulk heterojunction photovoltaic devices based on poly(3-hexylthiophene)(P3HT,donor) and [6,6]-phenyl-C61-butyric acid methyl ester(PCBM,acceptor) were fabricated using solvent annealing treatment.The nanoscale morphology and structure of the P3HT:PCBM blend films were characterized by UV-Vis absorption spectroscopy(UV-Vis),atomic force microscopy(AFM),and X-ray diffraction(XRD) analyses.In addition,the AFM images were processed using the entropyfilt method.The performances of the P3HT:PCBM devices with different mass ratios were measured,having a structure of indium tin oxide(ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)/P3HT:PCBM/aluminium(Al).The results showed that the crystallinity of the P3HT polymer can be disturbed by the relative amount of PCBM molecules.The 1:1(mass ratio) blend film possessed the greatest absorption width by UV-Vis absorption,as well as good phase separation and a high level of crystallinity,providing the best device performance(2.77%).This study indicates that the donor and acceptor mass ratios do have an influence on the nanoscale morphology and structure of the blend films,which can in turn affect the device performance.

Excited-State Dynamics of a Novel Bisimide–C60 Complex

The excited-state dynamics of a bisimide derivative (Z1) and a novel Z1–C60 complex (PSF) has been investigated by means of steady state optical spectroscopies and femtosecond time-resolved absorption spectroscopies. The lifetime of the singlet excited state of Z1 shortens drastically from 2.4 ns to 1.7 ps upon going from Z1 to PSF, a significant kinetics change is in coincidence with the complete fluorescence quench caused by the C60 attachment. Highly efficient Z1-to-C60 excitation energy transfer which is governed by the mechanism of Z1–C60 electron exchange has been verified for the PSF complex.