Theoretical Study On The Packing Structures And Optical Properties Of Oligofluorenol Aggregates In Condensed Phase

The rapid development of organic light-emitting materials,especially those with novel electronic structures and optical properties,greatly promotes the progress of the various electronic devices.The exploration of aggregation effects in condensed phase with various different environmental factors(such as solvent,temperature)on the optical properties is a challenge in both experimental and theoretical studies.With the advances in molecular simulation techniques and electronic structure methods,theoretical computations have become a useful tool to understand the structure and properties of low-lying excited states.In this thesis,by using both quantum chemical calculations and molecular dynamics simulations,we present a systematically theoretical study on the correlations among electronic structures of ground states and low-lying excited states,packing structures of aggregates,and absorption/emission spectra of π-conjugated oligomers in solutions,films,and crystals.Furthermore,the packing structures,and hence,photophysical properties are modulated by chemical modification and annealing treatment.We also study the electronic structure of low-lying excited states and the catalytic mechanisms of the transition metal(Ir and Ru)-contained photosensitizers in photo catalytic reactions.Our results are summarized as follows:1.The increase of planar configurations induced the red shift of absorption spectra in oligofluorenols aggregatesThe aggregation effects on electronic structure,and the UV/Vis absorption spectra of oligofluorenols,with and without OC8H17 side chains,were studied experimentally and theoretically in crystal,amorphous solids and solutions,respectively.For an isolated molecule in vacuum,the steric repulsion between two adjacent fluorenol units renders the PFOH oligomers twisted in a helix conformation,while the molecular aggregation favors the appearance of planar π-conjugated structures.The red shift in absorption spectra was observed in a systematic experimental study of unsubstituted and substituted oligofluorenols with the increasing concentration both in toluene and chloroform solutions.Time-dependent density functional theory(TDDFT)calculations revealed that the increase in the content of planar π-conjugated conformations induced the red shift in the absorption spectra upon the increasing the solution concentrations.In addition,the hydroxyl(OH)groups in oligofluorenols facilitate the formation of hydrogen bonding networks in the condensed phase.Through the chemical modification and annealing treatment on oligofluorenol PFOH,we study the modulation effects of spiro-ring steric hindrance and supramolecular functionalization on the intermolecular vdW,hydrogen-bonding(HB)and π-π stacked interactions and the packing pattern.Aggregation effects of the H-typed π-π aggregation and the pseudo-1D aligned packing on the UV/Vis absorption spectra and the NMR spectra were revealed.The theoretical results demonstrated that the introduction of the spiro-ring and the alkoxyl side-chains induced the increase in ratio of vdW forces in non-bonded interaction of oligofluorenols and the interchain distances between the adjacent molecules.The percentage of the planar conformations of β phase is raised to some extent.The red-shift of the experimental UV/vis absorption spectra verified the increase of the planar conformations.With the introduction of spiro-ring steric hindrance,the π-πstacking between two spiro-rings in two neighboring oligofluorenol chains may also exist,increasing the torsion angle between fluorenol units and depressing the population of the planar conformations.The qusi-1D-HB packing with head-to-tail hydrogen bonding contact was also formed between two adjacent molecules.Such supramolecular HB interactions were reflected by the down-shift of H chemical shifts of hydroxyl OH group in NMR.2.The electronic structures of low-lying excited states of oligofluorenols in solidsThe emission spectra of polyfluorenes,one of the important blue-emission materials,often exhibited a long wavelength green band(also called g-band),which severely affected the color purity of the materials and the stability of the spectra.But the origin of g-band is still an open question.By employing QM/MM method,we study the electronic structure of the low-lying excited states,the radiative and nonradiative rates and the absorption and emission spectra of oligofluorenols.For the studied oligomers,MFOH(n=1),DFOH(n=2),PFOH1(n=3),the planarity of the first singlet excited states(Si)is better than that of the ground state(So).The torsional relaxations at 4’ and 6’ position in DFOH in solid were effectively suppressed.It was indicated that the torsion between fluoreneol units was very important in controlling the photophysical properties.Compared to the gas phase,in the solid the Huang Rhys factor and reorganization energy became smaller.The low-frequency modes formed by aggregation were suppressed.The nonradiative rate was reduced by 5-orders of magnitude,indicating that the aggregation in solid phase suppressed the non-radiative rate.The main channels for nonradiatively dissipating the electronic excited-state energy are found to be the stretching vibrations of phenyl carbon-carbon bonds and the out of plane bending vibrations between fluorenol repeat units.This implies that the enhancement of the light-emitting efficiency could be achieved through suppressing those out of plane bending motions.3.The electronic structures of low-lying excited states of the transition metal complexesUsing density functional theory calculations,we have studied the electronic structures of the low-lysing excited states and the molecular orbitals of four transition metal complexes:fac-Ir(ppy)3,Ru(bpy)32+,Ir(ppy)2(dtbbpy)+and Ir{dF(CF3)2ppy)}2(dtbbpy)+.When the four compounds were excited to the lowest triplet states,Ti,the ligand moved closely to the metal center.The electron transfer from metal to ligand(MLCT)and ligand to ligand(LLCT)were presented in the transition from the highest occupied molecular orbital(HOMO)to the lowest unoccupied molecular orbital(LUMO).The Ir-contained complex,Ir(PPy)2(dtbbpy)+,has a relatively small energy difference between HOMO and LUMO,indicating a weaker kinetic stability and stronger reactivity than the other catalysts.The calculated results are helpful for understanding the applications of organometallic photosensitizers in visible light photoredox catalysis.