Theoretical Study On Electronic Structure And Photophysical Properties Of Spiro-type Organic/organometallic Complexes

Organic light-emitting diodes (OLEDs), having excellent properties of low driving voltageand bright emission, have been extensively studied due to their possible applications for flatpanel color displays. Recently, Organic/organometallic complexes with spiro-type structurehave attracted a considerable attention. With research deepening, some questions concerningmaterial optimum and emission mechanism, such as molecular architecture, charge transport,relation between electronic and photophysical properties, become increasing important. In the present work, the relations between electronic structure and spectroscopic featuresof spiro-type organic/organometallic complexes have been investigated by quantum chemistrycalculations. The results suggest new theoretical basis and direction for design of novel organicmaterials. Our work will focus on four aspects: 1. Electronic structure and photophysical properties of oligothiophene-(9,9'-spirofluorene)-oligothiophene with spiro-linked structure characters have been studiessystematically. The dependence relations between thiophene chain length and energies gaps,excitation energies and singlet-triplet splitting have been elucidated. The conjugated bondrelaxation of singly oxidation state and triplet shows that these complexes have obvious trap-barrier-trap form, a kind of new polaron distribution form. HOMO-LUMO gaps decreased withincreasing chain length. The TDDFT and ZINDO results indicated that with the increase of thenumber of thiophene rings, the maximum absorption wavelength is redshifted and the oscillatorstrength increases. The lowest excitation transition localized a single chain and mainly comefrom fluorine and thiophene above spiro-structure. This excited state does not possess anycharge-transfer character. Notely, there is an obvious characteristic absorption at 4.03 eV for allsystems, independent of the chain length. This state origin mainly from charge transfer betweentwo spiro fluorene fragment intramolecule. The calculated result of energy difference betweensinglet and triplet indicates that the energy of the S1 excited state is less affected by change inthe conjugation length than the T1 excited state. 2. The effect of heteroatom on geometry of singlet and triplet, energy gaps andphotopysicals properties for series of 9,9′-spirofluorene heterocyclic oligomers (oligothiophene,oligofunan and oligopyrrole) have been investigated comparably by DFT and localized densitymatrix(LDM) method. Calculated results indicate that heteroatom significantly affect thegeometry of conjugated chain. All the complexes have obvious trap- barrier-trap form. As thechain repeated unit increased, HOMO-LUMO gaps decreased and the effective conjugatedlength (ECL) decreased for oligothiophene, oligofunan and oligopyrrole species, respectively.The excitation energy calculated by TDDFT and ZINDO results indicated that the maximumabsorption wavelength is different among the three kinds systems, the oligothiophene specieshas maximal absorption wavelength compared with other heterocyclic complexes with samerepeated unit. The lowest excitation transition mainly localized on the long single chain whichdistributing to fluorene and thiophene above spiro-structure. There is an obvious characteristicabsorption at 4.03 eV for all systems, independent of the chain length. This state origins mainlyfrom charge transfer between two spiro fluorene fragment intramolecule and independent ofheteroatoms substituent. 3. The effect of substituent (cyclopentyl, fluorenyl, and fluorene core bis-oligothienyl) withdifferent size of series of 9,9′-spirofluorene thiophene oligomers on geometry of singlet andtriplet, energy gaps, ionization potential(IP), electron affinity(EA) and electronic spectroscopieshave been investigated systematically. Calculated results indicate that different substituent at9,9′-fluorene position has more effect on bond length of central fluorene moieties, but little onthat of oligothiophene moieties in the ground state. There is same rule for he conjugated bondrelaxation of singly o...