유기발광소재(OLED) 후보물질의 지연형광(TADF) 성질에 대한 이론적 연구

Theoretical Study for Thermally Activated Delayed Fluorescence (TADF) Property in Organic Light-Emitting Diode (OLED) Candidates

Abstract

본 연구는 밀도 범함수 이론(DFT) 가운데 하나인 B3LYP 방법을 $6-31G^{**}$, cc-pVDZ, cc-pVTZ의 바탕함수 집합(basis set)과 함께 사용하여 전자 공여성 분자(D)로 카바졸(carbazol) 그리고 전자 구인성 분자(A)로 dicyanobenzene, diphenyl sulfone, benzonitrile 등의 조합으로 이루어진 열 활성화 지연형광(TADF) 후보 물질에 대하여 분자구조를 최적화하고 진동주파수를 계산하였다. 또한 최적화된 분자 구조에 대하여 HOMO와 LUMO 에너지 차이를 계산하였으며, 나아가 시간 의존 밀도 범함수 이론(TD-DFT)을 사용하여 분자의 최대 흡수 및 방출 파장(${\lambda}_{max}$) 그리고 단일항과 삼중항 들뜬 상태의 에너지 차이(${\Delta}E_{ST}$) 등을 계산하여 열 활성 지연형광(TADF) 소재로서의 가능성을 예측하였다.

The TADF properties for carbazol-dicyanobenzene, carbazol-diphenyl sulfone, carbazol-benzonitrile derivatives as OLED candidate materials are theoretically investigated using density functional theory (DFT) with $6-31G^{**}$, cc-pVDZ, and cc-pVTZ basis sets. The optimized geometries, harmonic vibrational frequencies, and HOMO-LUMO energy separations are predicted at the B3LYP/$6-31G^{**}$ level of theory. The harmonic vibrational frequencies of the molecules considered in this study show all real numbers implying true minima. The time dependent density functional theory (TD-DFT) calculations have been also applied to investigate the absorption and emission wavelength (${\lambda}_{max}$), energy differences (${\Delta}E_{ST}$) between excited singlet ($S_1$) and triplet ($T_1$) states of candidate materials.

Figure 1. The schematic representation and molecular orbitals (HOMO, LUMO) of 2CzTPN derivatives (C₅₆H₃₂N₆) at the B3LYP/6-31G** level of theory.

Figure 2. The schematic representation and molecular orbitals (HOMO, LUMO) of para-A, B and meta-A, B (C₃₆H₂₄N₂S₁O₂) at the B3LYP/6-31G** level of theory.

Figure 3. The schematic representation and molecular orbitals (HOMO, LUMO) of CzBN derivatives (I-VI) at the B3LYP/cc-pVTZlevel of theory.

Table 1. HOMO, LUMO, HOMO-LUMO gap(eV) and predicted electronic spectra of C₅₆H₃₂N₆ and C₃₆H₂₄N₂S₁O₂ at the B3LYP and TDB3LYP methods with 6-31G** basis set.

Table 2. HOMO, LUMO, HOMO-LUMO gap(ΔE_g, eV) and predicted electronic spectra of C₁₉H₁₂N₂ ~ C₃₁H₂₀N₂(I-VI) at the B3LYP and TD-B3LYP methods with cc-pVTZ basis set.

Table 3. The energy difference between the first excited singlet and triplet state(ΔE_ST) of C₅₆H₃₂N₆ and C₃₆H₂₄N₂S₁O₂ at the TD-B3LYP/ 6-31G** level of theory.

Table 4. The energy difference between the first excited singlet and triplet state(ΔE_ST) of C₁₉H₁₂N₂ ~ C₃₁H₂₀N₂ (I-VI) at the TD-B3LYP/ cc-pVTZ level of theory.