Theoretical Investigations Of Donor-Acceptor Based Organic Thermally Activated Delayed Fluorescence (TADF) Materials

One of the important goals of materials chemistry is to develop new organic electroluminescent materials with improved performance.The thermally activated delayed fluorescence?TADF?materials are evolving as one of the promising next-generation organic electroluminescent materials which are being utilized in organic light-emitting diodes?OLEDs?and also in other optoelectronic devices,recently.TADF-OLEDs have been used in illumination and display technologies along with sensing applications and fluorescence microscopy.They represent an active field of current research in the electroluminescent materials because they generate light by harvesting singlet as well as triplet excitons via the effective reversible intersystem crossing?RISC?process.A small energy splitting between the lowest excited singlet?S₁?and triplet?T₁?states(?E_ST)is the most essential criterion for realizing high TADF efficiency and potentially increases the internal quantum efficiency?IQE?of the device.There have been tremendous advances and booming developments in molecular design architecture since Adachi et al.described TADF-OLEDs with the high-performance.Regardless of the pronounced achievements in designing the TADF materials having high photoluminescence quantum yield?PLQY or??and verifying their aptness in OLEDs with higher external quantum efficiency?EQE?,it must be stated that the theoretical comprehension of organic-TADF materials needs to be upgraded employing density functional theory?DFT?and time-dependent density functional theory?TD-DFT?.To date,no perfect design rules exist to account for the large photophysical differences concerning marginally different core/principal structures.The main theme or scope of our thesis was to build and design novel donor-acceptor-based organic TADF emitters by adopting different structural morphologies to be used in OLEDs and other optoelectronic devices.This research deals with improving the electronic,structural,optical,charge transfer,excitation,and material properties of unique TADF emitters with auspicious characteristics in the field of information technology,photonics,and electronics.The structure of the thesis includes the first chapter describing detailed introduction,background,and description about TADF and OLEDs,design principles of TADF emitters and future challenges.The second chapter includes computational methodology and fundamental theories of the organic electroluminescent materials based on quantum chemical calculations.In the third part of the thesis,we have designed a series of derivatives based on 10,10-dimethyl-5,10-dihydro-pyrido[4,3-b][1,6]naphthyridine-diphenylsulphone?DMDHNP-DPS?named 1a with CH/N&H/CN substitution at the DPS acceptor unit to obtain the blue TADF materials.The parent molecule 1a was chosen from our previous report after a CH/N substitution at DMAC donor fragment.The highest occupied molecular orbitals?HOMOs?and the lowest unoccupied molecular orbitals?LUMOs?were largely distributed over the DMDHNP donor and DPS acceptor units,respectively,resulting in a slight overlap between the HOMO-LUMO and hence?E_ST.Steric hindrance caused a large dihedral angle??82°-89°?between the plane of the electron-donating DMDHNP unit and the electron-accepting DPS unit in the substituted derivatives.Calculated results indicated that the?E_ST values of H/CN substituted derivatives were smaller than those of corresponding CH/N derivatives which were favorable for the RISC process from the lowest excited triplet states?T₁?to the lowest excited singlet?S1?states and ultimately to the ground state?S0?causing delayed emission.The emission wavelengths(?_em)of all the designed molecules were found in the range of 397-497 nm.The incorporation of–N=atom or–CN group at the ortho and meta position at DPA reduced the transition energies from LUMO?HOMO in the S₁ states,ensuing in red-shift.Moreover,the?_em values displayed more substantial bathochromic-shift as the number of–N=atoms or–CN groups increased.The two of the designed molecules?1h and 1i?showed sky-blue emission?494 nm and 497 nm?,and the four of the investigated compounds?1c,1d,1f,and 1g?displayed blue emission?416 nm,447 nm,437 nm,and 432 nm,respectively?indicating that these investigated derivatives were efficient sky-blue to blue TADF candidates.Among all the investigated derivatives,the smaller?E_ST values for designed system 1f?0.03eV?and 1g?0.02 eV?and proper?_em values of 437 nm and 432 nm make them as the excellent candidates for blue TADF materials.Our theoretical investigation might offer hints for the construction of efficient blue TADF-based OLEDs in the future.In the fourth section,via structural modification of donor fragment,we have designed a series of three-dimensional donor-acceptor triptycences using TPA-QNX?CN?2 as the parent molecule to tune the emitting color.All the designed molecules have the same dicyanoquinoxaline?QNX?CN?2?acceptor unit,but different donor units including the N-phenylcarbazole?PhCz?,N-phenylphenoxazine?PPXZ?and 9,9-dimethyl-10-phenylacridine?DMPA?.The influences of different donor fragments with various electron-donating strengths on?E_ST and?_em are extensively investigated by using the quantum chemical approach.The calculated results imply that,as the electron-donating strength of the donor fragment increases,the energy splitting between the lowest singlet?S₁?and the triplet?T₁?excited state(?E_S1-T1)decreases and?_emm is red-shifted for the molecules using the same acceptor units.Analogously,the ¹CT-³CT state splitting(?E_ST?CT?)is also decreased by enlarging the twist angle??₁ and?₂?between the phenyl ring and the alternative donor fragment.Therefore,efficient color tuning within a broad emission range?433-609 nm?as well as small?E_ST?CT?values?0.01-0.05 eV?has been accomplished by the structural modification of the Donor moiety.Finally,in the fifth chapter,we have examined the effect of introducing the linker between Donor?D?and Acceptor?A?parts of traditional D-A-type TADF scaffolds on the electronic,charge transport and photophysical properties using DFT and TD-DFT approaches.We have demonstrated the unique molecular design architecture for efficient TADF materials based on donor-linker-acceptor-linker-donor?D-L-A-L-D?framework which can be employed as predecessors of OLED materials.Different from traditional D-A-type TADF scaffolds,the D-L-A-L-D structural design avoids direct coupling between the D and A fragments allowing HOMO and the LUMO to be spatially separated.It results in a reduced overlap between HOMOs and LUMOs,thus realizing fairly a small?E_ST and high?.We revealed that manipulating a linker between D and A fragment in intramolecular-charge-transfer?ICT?compounds is an auspicious approach for realizing small?E_ST.Herein,we report a group of metal-free organic electroluminescent D-L-A-L-D-type molecules with different electron-donor and electron-accepter fragments.Two types of linkers,the?-conjugated phenylene?-C₆H₄-?and aliphatic alkyl chains or?-spacer?-CH₂-and-2CH₂-?,were exploited between D and A fragments.In principle,the conjugation in D-?-A-?-D-type molecules and hyperconjugation in D-?-A-?-D type molecules favor the spatial separation of HOMOs and LUMOs,thus resulting in reduced?E_ST.All the designed molecules show blue-shift in the?em over the directly linked parent molecules except DPA-DPS-C₆H₄ and BTPA-DPS-C₆H₄ which show red-shift.Violet-blue to green-yellow?376-566 nm?TADF was observed from all of the investigated molecules.Other Important properties which affect the efficacy of emission quantum yield such as electron excitation,stoke-shift,exciton binding energies,vertical and adiabatic electron affinities,and ionization potentials,as well as reorganization energies for the hole??_h?and electron??_e?of the designed molecules were also inspected using DFT and TD-DFT calculations.We believe that our work will successfully provide a simple and unique strategy towards designing highly efficient TADF-OLEDs and will offer a new understanding of their structural design.