Photoemitters in organic light emitting diodes (OLEDs)
''Push–pull'' organic chromophores are molecules consisting of electron donor (D) and electron acceptor (A) moieties typically bridged together by a ð-conjugated organic backbone. Due to this particularly special electronic framework, low-lying excited electronic states of D+A- character appear as a result of the facilitated D→A intramolecular charge transfer (CT). This gives rise to solvatochromic properties which are exploited in organic light-emitting diodes (OLEDs) for screen displays and solid-state lighting, where the organic molecule is used as a photoemitter. The emitted colors are tuned accordingly by embedding the molecules inside specifically tailored polymer matrices.
We have investigated the absorption and emission spectra of a neutral and protonated red (4-dimethylamino-4'-nitrostilbene, or ''DANS'') and green/blue organic emitter {1-[4-(dimethylamino)phenyl]-6-phenylhexa-1,3,5-triene, or ''DMA-DPH''} in different solvents resembling the polymeric environment with Time-Dependent Density Functional Theory (TDDFT) as well as coupled cluster (RI-CC2) methods. These chromophores have been previously dispersed in the lab within a single polymer layer along with a photoacid generator (triphenylsulfonium trifluoromethane sulfonate, SPh3+CF3SO3-) and produced on demand the three primary colors (red, green and blue).
It is found that the polyene bridge is primarily responsible for the absorption/emission intensities. The lowest excited state of DANS, as indicated by the electron density plots of the frontier orbitals, is characterized by a transfer of electronic charge from the dimethylamino group toward the nitrostilbene, which is more pronounced at the twisted secondary minimum energy geometry of the excited state. The observed solvatochromism in the emission spectra of neutral DMA-DPH comes mostly from the electronic stabilization of the excited state in solvent environments, and can to some extent be attributed to solvent-dependent dipole moment changes upon de-excitation, which in part appear to stem from electronic effects mediated by the pð orbitals of the dimethylamino group (which are unavailable in DMA-DPH_p, and of course, methyl-DPH) in conjunction with the strongly delocalized polyene excited state.
Fig. 1: Gas-phase optimized ground (black color) and 1st excited state (red color) geometry for neutral DMA-DPH at the RI-CC2/TZVPP level of theory.
Fig. 2: Electron density difference plots for the S5,S4,S3,S2,S1)←S0 absorptions and S1→S0 emission in neutral and protonated DMA-DPH respectively, at the TD-CAM-B3LYP/6-311G(d,p) level of theory.
Fig. 3: UV-vis spectra of DANS and its protonated form, based on the calculated transition energies and oscillator strengths of the lowest five excited singlet states. For clarity, different horizontal and vertical scales are used in the different plots.
1. I. D. Petsalakis, D. G. Georgiadou, M. Vasilopoulou, G. Pistolis, D. Dimotikali, P. Argitis, and G. Theodorakopoulos, Theoretical Investigation on the Effect of Protonation on the Absorption and Emission Spectra of Two Amine-Group-Bearing, Red ''Push-Pull'' Emitters, 4-Dimethylamino-4'-nitrostilbene and 4-(dicyanomethylene)-2-methyl-6-p-(dimethylamino) styryl-4H-pyran, by DFT and TDDFT Calculations. J. Phys. Chem. A 114, 5580 (2010).
2. I. S. K. Kerkines, I. D. Petsalakis, P. A. Argitis and G. Theodorakopoulos, Fluorescence properties of organic dyes: quantum chemical studies on the green/blue neutral and protonated DMA-DPH emitters in polymer matrices. Phys. Chem. Chem. Phys. 13, 21273 (2011).
