Quantum Theoretical Study Toward Rational Designing Of Four Kinds Of High Mobility Materials For OLEDs And OFETs

Organic light emitting diodes (OLEDs) and organic field-effect transistors (OFETs) based on organic molecules have received significant attentions because of their flexibility and low cost processing. The research activities deal with the structural, electronic, optical and interfacial properties of novel organic materials with promising characteristics in the field of electronics, photonics, and information technology. The study of charge-carrier mobilities in organic molecular crystals has continued for more than 40 years. Developing new organic semiconductors with improved performance in specific devices is an important goal for materials chemistry. As tris(8-hydroxyquinolinato)aluminum (mer-Alq3) is very good OLED material, thus some novel emitting materials with tunable color have been designed toward high stability and high charge mobility in this thesis. The fairly large charge-carrier mobility attracts significant attention with regard to using Metallophthalocyanines (MPcs) as organic semiconductors in molecular electronic devices. In this issue we have shed light how mobility of tin phthalocyanine (SnPc) can be enhanced. We have also designed new anthracene and ?,??-bis(dithieno[3,2-b:2?,3?- d]thiophene) (BDT) derivatives toward high mobility and stability considering the packing effects. The key points of our research are following:1. Twenty three mono-substituted derivatives of mer-Alq3 have been designed, by substituting electron donating groups (-CH3, -OCH3) and electron withdrawing groups (-F and -CN) at different positions on the ligands. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) play important role in the charge transfer properties. Thus we have explained the distribution of HOMO and LUMO on different individual ligands based on energy decomposition analysis. On the basis of reorganization energy, we have explained the charge transfer properties as well, i.e., the substitution of mono-fluorine and methoxy has no confirmed effect to enhance the intrinsic charge mobility as compared to mer-Alq3. The electron reorganization energies of cyano derivatives indicate that electron mobility for these derivatives would be comparable/better than mer-Alq3.2. To design innovative and novel optical materials with high mobility, eighteen disubstituted derivatives for mer-Alq3 with push-pull (X-Y) substituents have been designed. The structures of tris(4-X-6-Y-8-hydroxyquinolinato)aluminum, tris(4-X-5-Y-8-hydroxyquin- olinato)aluminum, tris(4-Y-6-X-8-hydroxyquinolinato)aluminum and tris(4-Y-6-X-8- hydroxyquinolinato)aluminum in the ground (S0) and first excited state (S1) have been optimized at the B3LYP/6-31G* and CIS/6-31G* level of theories, respectively. We have also explained the push-pull effect on the charge transfer and optical properties, if only one of the ligand among three ligands of mer-Alq3 has been substituted. Various colors can be tuned by the substituents. Blue emitting materials were predicted for tris(4-methyl-6- chloro-8-hydroxyquinolinato)aluminum (1), tris(4-methyl-6-cyano-8-hydroxyquinolinato)- aluminum (2), tris(4-amino-6-chloro-8-hydroxylquinoli-nato)aluminum (3), tris(4-methyl- 5-cyano-8-hydroxyquinolinato)aluminum (10) and 4-methyl-6-cyano-(8-hydroxyquino)- bis(8-hydroxyquinolinato) aluminum (16). Tris(4-amino-6-cyano-8-hydroxyquinolinato)- aluminum (4) might be violet. Eight green emitting materials were predicted for tris(6-methyl-4-chloro-8-hydroxyquinolinato)aluminum (5), tris(4-methyl-5-chloro-8- hydroxyquinolinato)aluminum (9), tris(4-methyl-5-fluoro-8-hydroxyquinolinato)aluminum (11), tris(4-chloro-5-methyl-8-hydroxyquinolinato)aluminum (12), and tris(4-fluoro-5-methyl- 8-hydroxyquinolinato)aluminum (14), 4-methyl-6-chloro-(8-hydroxyquino)bis(8-hydroxy- quinolinato)aluminum (15), 4-amino-6-chloro-(8-hydroxylquino)bis(8-hydroxyquino- linato)aluminum (17), and 4-amino-6-cyano-(8-hydroxyquino)bis(8-hydroxyquinolinato)- aluminum (18). While three red light emitting materials were simulated for tris(6-methyl-4-cyano-8-hydroxyquinolinato)aluminum (6), tris(6-amino-4-cyano-8- hydroxyquinolinato)aluminum (8), tris(4-cyano-5-methyl-8-hydroxyquinolinato)aluminum (13). The newly developed derivatives 1, 2, 5, 6, 9, 10, 11, 12, 13, 15, 16, 18 might have comparable/better charge carrier mobility as mer-Alq3. The tris(4-methyl-5-fluoro-8-hydroxyquinolinato)aluminum (11), tris(4-cyano-5-methyl-8-hydr- oxyquinolinato)aluminum (13) and tris(4-fluoro-5-methyl-8-hydroxyquinolinato)aluminum (14), 4-methyl-6-cyano-(8-hydroxyquino)bis(8-hydro-xyquinolinato)aluminum (16) and 4-amino-6-cyano-(8-hydroxyquino)bis(8-hydroxyquinolinato)aluminum (18) would make oxidation more difficult and improve their stability.3. By employing a diabatic model and a first-principle direct method, we have investigated the carrier transport properties of dichlorotitanium phthalocyanine (TiCl2Pc) and tin phthalocyanine (SnPc), 9,10-bis(methylthio)anthracene, 9,10-bis(trifluoromethyl- selleno)anthracene, 9,10-bis(methylselleno)- anthracene, 9,10-bis(trifluoromethylthio)- anthracene and its newly designed derivatives. The intermolecular electronic couplings for a wide variety of nearest-neighbor charge transfer pathways have been obtained by directly evaluating the dimer Fock matrix with unperturbed monomer's molecular orbits at the DFT/pw91pw91/6-31G* (Lanl2dz was applied for metals) level. The reorganization energies have been computed at the DFT (B3LYP/6-31G*) level. The calculated reorganization energies, transfer integrals, and mobilities showed that TiCl2Pc and SnPc are hole trasfer materials which is in good agreement with experimental observations. The hole mobility of SnPc may boost by minimizing the polarization. The structure of 9,10-bis(methylthio)anthracene has been simulated and compared with experimental parameters, then by applying the same methodology crystal structures of their derivatives have been simulated. The reorganization energies and transfer integrals showed that 9,10-bis(methylthio)anthracene, 9,10-bis(methylselleno)anthracene and 9,10-bis(trifluoro- methylthio)anthracene would be good both for hole and electron transport while 9,10-bis(trifluoromethylselleno)anthracene might be hole transfer material. The 9,10-bis(trifluoromethylselleno)anthracene and 9,10-bis(trifluoromethylthio)anthracene would possess high stability.4. We have investigated the packing dependence for transfer integrals and mobility. It has been explained that how mobility can be boost up by changing the space group. More precisely, the packing effect for ?,??-bis(dithieno[3,2-b:2?,3?-d]thiophene) (BDT) and simulated crystals was investigated by various space groups. The designed derivatives where sulfur was substituted by NH (NHBDT) and by oxygen (OBDT) are blue shifted while BH substituted one (BHBDT) is red shifted. The vertical ionization potentials of NHBDT and OBDT are smaller than BDT revealed that injection barrier for hole would be small. The vertical electronic affinity of BHBDT is 3.32 eV while its vertical ionization potential is 6.53 eV indicating it to be good ambipolar material. The calculated mobility showed that BDT is hole transfer material which is in good agreement with experiment. The BDT, NHBDT, and OBDT are predicted to be hole transfer materials in C2/c space group. The hole mobility of BHBDT is seven times while electron mobility is twenty times higher than the BDT. Generally mobility increases in BDT and its derivatives by changing the packing from space group C2/c to space groups P1 or . In the designed ambipolar material BHBDT hole mobility has been predicted 0.774 cm2/Vs and 3.460 cm2/Vs which is 10 times and 48 times higher than BDT in space groups P1 and , respectively.