2,7-Carbazole-Based Conjugated Polymers for Blue, Green, and Red Light Emission

Tris-Cyclometalated Iridium(III) Complexes of Carbazole(fluorenyl)pyridine Ligands: Synthesis, Redox and Photophysical Properties, and Electrophosphorescent Light-Emitting Diodes

Using ligands synthesized by Suzuki cross-coupling methodology, new phosphorescent homoleptic tris-cyclometalated complexes have been obtained, namely fac-[Ir(Cz-2-Fl(n)Py)(3)] (1 d-f) and fac-[Ir(Cz-3-Fl(n)Py)(3)] (2 d-f), which are solution-processible triplet emitters (Cz denotes N-hexylcarbazole, n is the number of 9,9'-dihexylfluorene (Fl) units (n=0,1,2) and Py is pyridine). In all cases, Py and Fl are substituted at the 2- and 2,7-positions, respectively, and Cz moieties are substituted by either Py or Fl at the 2- or 3-positions, in series 1 and 2, respectively. The oxidation potential of 1 d studied by cyclic voltammetry ({E{{{\rm ox}\hfill \atop 1/2\hfill}}}=0.14 V, versus Ag/AgNO(3), CH(2)Cl(2)) is less positive (i.e. raised HOMO level) compared to that of the isomer 2 d ({E{{{\rm ox}\hfill \atop 1/2\hfill}}}=0.30 V), where the Cz-nitrogen is meta to the Ir center. Ligand-centered oxidations occur at more positive potentials, leading to 7+ oxidation states with good chemical reversibility and electrochemical quasi-reversibility, for example, for 2 f {E{{{\rm ox}\hfill \atop {\rm pa}\hfill}}} =0.45 (1e), 0.95 (3e), 1.24 V (3e). Striking differences are seen in the solution-state photophysical data between complexes [Ir(Cz-2-Py)(3)] (1 d) and [Ir(Cz-3-Py)(3)] (2 d), in which the Cz moiety is bonded directly to the metal center: for the latter there is an 85 nm blue-shift in emission, a decrease in the luminescence lifetime and an increase in the PLQY value. Organic light emitting devices were made by spin-coating using polyspirobifluorene:bis(triphenyl)diamine (PSBF:TAD) copolymer as host and the complexes 1 d or 2 d as dopants. Turn-on voltages are low (3-4 V). With 1 d orange light is emitted at lambda(max)=590 nm with an EQE of 1.3 % (at 7.5 mA cm(-2)) and an emission intensity (luminance) of 4354 cd m(-2) (at 267 mA m(-2)). The green emission from 2 d devices (lambda(max)=500 nm) is due to the reduced electron-donating ability of the carbazole unit in 2 d. Recording the EL spectra of the 1 d device at 6 V (current density, 100 mA cm(-2)) established that the time to half brightness was about 9 h under continuous operation with no change in the spectral profile, confirming the high chemical stability of the complex.

A Theoretical, Spectroscopic, and Photophysical Study of 2,7-Carbazolenevinylene-Based Conjugated Derivatives

A combined theoretical and experimental study of the structure, optical, and photophysical properties of four 2,7-carbazolenevinylene-based derivatives in solution is presented. Geometry optimizations of the ground states of PCP, PCP-CN, TCT, and TCT-CN were carried out using the density functional theory (DFT/B3LYP/6-31G*). It is found that PCP and TCT are nearly planar in their ground electronic states (S0), whereas the cyano derivatives are more twisted. The nature and the energy of the first singlet-singlet electronic transitions have been obtained from time-dependent density functional theory (TDDFT) calculations performed on the optimized geometries. For all the compounds, excitation to the S1 state corresponds mainly to the promotion of one electron from the highest-occupied molecular orbital to the lowest-unoccupied molecular orbital, and the S1 <-- S0 electronic transition is strongly allowed and polarized along the long axis of the molecular frame. The optimization (relaxation) of the first singlet excited electronic state (S1) has been done using the restricted configuration interaction (singles) (RCIS/6-31G*) approach. It is observed that all four investigated compounds become more planar in their S1 relaxed excited state. Electronic transition energies from the relaxed excited states have been obtained from TDDFT calculations performed on the S1-optimized geometries. The absorption and fluorescence spectra of the carbazolenevinylenes have been recorded in chloroform. A good agreement is obtained between TDDFT vertical transitions energies and the (0,0) absorption and fluorescence bands. The change from phenylene to thiophene rings as well as the incorporation of cyano substituents induce bathochromic shifts in the absorption and fluorescence spectra. From the analysis of the energy of the frontier molecular orbitals, it is believed that thiophene rings and CN substituents induce some charge-transfer character to the first electronic transition, which is responsible for the red shifts observed. Finally, the fluorescence quantum yield and the lifetime of the compounds in chloroform have been obtained. In sharp contrast with many oligothiophenes, it is observed that TCT possesses a high fluorescence quantum yield. On the other hand, the CN-containing derivatives exhibit much lower fluorescence quantum yields, probably due to the combined influence of steric effects and charge-transfer interactions caused by the cyano groups.

Spectroscopic and photophysical properties of carbazole-based triads

A study of the spectroscopy and photophysics of four carbazole-based triads, namely, 1,4-bis(N-octylcarbazol-2-yl)phenylene (CPC), 2,5-bis(N-octylcarbazol-2-yl)thiophene (CTC), 2,7-bis(N-octylcarbazol-2-yl)-9,9-dihexylfluorene (CFC), and 2,7-bis(N-octylcarbazol-2-yl)-N-octylcarbazole (CCC) are reported. From ZINDO/S calculations performed on the optimized ground state geometries (HF/6-31G*), the S1 ← S0 and S2 ← S0 electronic transitions of CPC, CFC, and CCC are weakly allowed, whereas the S3 ← S0 electronic transition is dipole-allowed (x axis) and possesses a high value of the oscillator strength. On the other hand, for CTC, the dipole-allowed electronic transition is S1 ← S0. For the four derivatives, the dipole-allowed transition corresponds mainly to the promotion of an electron from the HOMO to the LUMO. The first absorption band of each derivative can be assigned to the dipole-allowed transition. The geometry optimization of the excited states has been done with RCIS/6-31G* ab initio calculations. For CPC, CFC, and CCC, S3 is much more stabilized than S1 and S2, causing a crossing of the singlet excited states (S3 becomes lower in energy than S1 and S2). Emission energies from the first relaxed excited states (S1 → S0) have been obtained from ZINDO/S calculations performed on the optimized S1 geometries (RCIS/6-31G*). It is found that these energies are in very good agreement with those determined from the fluorescence spectra. Going from CPC to CTC, the replacement of a phenylene ring by a thiophene unit induces a red shift in the absorption and fluorescence spectra. This behavior is interpreted in terms of the electron donor properties of the thiophene ring. The increase of the oligomer chain (CFC and CCC compared to CPC) also induces bathochromic shifts in the spectra because of a longer electronic delocalization along the molecular frame. From fluorescence data, it is observed that a more planar conformation is favored in the relaxed excited states. RCIS/6-31G* ab initio calculations performed on the S1 relaxed excited states confirm this statement. Finally, the photophysical properties of CTC are mainly governed by nonradiative processes (knr), whereas the variation in the photophysics of CPC, CFC, and CCC mainly involves radiative processes (kF).Key words: electronic spectroscopy, fluorescence, photophysics, ab initio calculations, excited states.