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A fluorene-bridged squaraine dimer (SD-FLU-SD) was designed with the purpose of combining various chromophores in one molecule and enhancing its two-photon absorption properties using intra-and interchromophore transitions. Linear and nonlinear absorption properties of SD-FLU-SD were investigated with the goals of understanding the nature of one-and two-photon absorption spectra, determining the molecular optical parameters, and performing modeling of the photophysical processes. The optical behavior of this new SD-FLU-SD "hybrid" molecule was compared with its separate squaraine constituent moiety. Linear spectroscopic characterization includes absorption, fluorescence, excitation and emission anisotropy, and quantum yield measurements in solvents of different polarity and viscosity. Spectral positions of the absorption−fluorescence peaks and quantum yields of SD-FLU-SD and its separate squaraine moiety exhibited complex and nontrivial behavior as a function of solvent polarity. Comprehensive study of this unusual solvatochromism was conducted and interpreted using various models. Nonlinear spectroscopic studies included two-photon absorption measurements using the femtosecond Z-scan technique. The two-photon absorption spectrum of SD-FLU-SD was broad, covering the spectral range from 800 to 1400 nm with a maximum two-photon absorption cross section of 2 750 GM (1 GM = 1 × 10 −50 cm 4 s/photon). Quantum chemical analysis, based on time-dependent density functional theory, agreed with the experimental data and revealed details on the energy-level structure and origin of the linear and nonlinear absorption behavior of this novel SD-FLU-SD compound. These investigations advance the understanding of the nature of electronic transitions and the structure−property relations in long conjugated molecules, which are important for the rational design of new organic optical materials.

A new series of unsymmetrical diphenylaminofluorene-based chromophores with various strong π-electron acceptors were synthesized and fully characterized. The systematic alteration of the structural design facilitated the investigation of effects such as molecular symmetry and strength of electron-donating and/or withdrawing termini have on optical nonlinearity. In order to determine the electronic and geometrical properties of the novel compounds, a thorough investigation was carried out by a combination of linear and nonlinear spectroscopic techniques, single crystal X-ray diffraction, and quantum chemical calculations. Finally, on the basis of two-photon absorption (2PA) cross sections, the general trend for π -electron accepting ability, i.e., ability to accept charge transfer from diphenylamine was: 2-pyran-4-ylidene malononitrile (pyranone) > dicyanovinyl > bis(dicyanomethylidene)indane > 1-(thiophen-2-yl)propenone > dicyanoethylenyl > 3-(thiophen-2-yl)propenone. An analog with the 2-pyran-4-ylidene malononitrile acceptor group exhibited a nearly three-fold enhancement of the 2PA< δ (1650 GM at 840 nm), relative to other members of the series.

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Carbene–metal–amides (CMAs) are an emerging class of photoemitters based on a linear donor–linker–acceptor arrangement. They exhibit high flexibility about the carbene–metal and metal–amide bonds, leading to a conformational freedom which has a strong influence on their photophysical properties. Herein we report CMA complexes with (1) nearly coplanar, (2) twisted, (3) tilted, and (4) tilt-twisted orientations between donor and acceptor ligands and illustrate the influence of preferred ground-state conformations on both the luminescence quantum yields and excited-state lifetimes. The performance is found to be optimum for structures with partially twisted and/or tilted conformations, resulting in radiative rates exceeding 1 × 10 6 s –1 . Although the metal atoms make only small contributions to HOMOs and LUMOs, they provide sufficient spin–orbit coupling between the low-lying excited states to reduce the excited-state lifetimes down to 500 ns. At the same time, high photoluminescence quantum yields are maintained for a strongly tilted emitter in a host matrix. Proof-of-concept organic light-emitting diodes (OLEDs) based on these new emitter designs were fabricated, with a maximum external quantum efficiency (EQE) of 19.1% with low device roll-off efficiency. Transient electroluminescence studies indicate that molecular design concepts for new CMA emitters can be successfully translated into the OLED device.