X-ray Generated Recombination Exciplexes of Substituted Diphenylacetylenes with Tertiary Amines: A Versatile Experimental Vehicle for Targeted Creation of Deep-Blue Electroluminescent Systems

Abstract

Customizable and technology-friendly functional materials are one of the mainstays of emerging organic electronics and optoelectronics. We show that recombination exciplexes of simple substituted diphenylacetylenes with tertiary amines can be a convenient source of tunable deep-blue emission with possible applications in organic electroluminescent systems. The optically inaccessible exciplexes were produced via recombination of radiation-generated radical ion pairs in alkane solution, which mimics charge transport and recombination in the active layer of practical organic light-emitting diodes in a simple solution-based experiment. Despite varying and rather poor intrinsic emission properties, diphenylacetylene and its prototypical methoxy (donor) or trifluoromethyl (acceptor) monosubstituted derivatives readily form recombination exciplexes with N,N-dimethylaniline and other tertiary amines that produce emission with maxima ranging from 385 to 435 nm. The position of emission band maximum linearly correlates with readily calculated gas-phase electron affinity of the corresponding diphenylacetylene, which can be used for fast computational prescreening of the candidate molecules, and various substituted diphenylacetylenes can be synthesized via relatively simple and universal cross-coupling reactions of Sonogashira and Castro. Together, the simple solution-based experiment, computationally cheap prescreening method, and universal synthetic strategy may open a very broad and chemically convenient class of compounds to obtain OLEDs and OLED-based multifunctional devices with tunable emission spectrum and high conversion efficiency that has yet not been seriously considered for these purposes.

Magnetic Field-Sensitive Radical Pair Dynamics in Polymethylene Ether-Bridged Donor–Acceptor Systems

“…Exciplexes are excited-state charge-transfer complexes that can be formed in photo-induced electron-transfer (PET) reactions in moderately polar solutions. Magnetic field effects (MFEs) on the emission of exciplexes have been extensively studied for many years. − The MFEs result from the magnetic field-dependent intersystem crossing within the fully charge-separated radical ion pairs (RIPs) in equilibrium with the exciplexes (see Figure ). By systematically studying the MFE of the exciplex emission, details of the dynamics of the spin-correlated RIP can be revealed indirectly. ,,− The MFEs are theoretically accounted for in the framework of the so-called radical pair mechanism . ,,,− It relies on the quantum-coherent mixing, by the hyperfine interaction (HFI) and its interplay with the Zeeman interaction, of the singlet (S) and three triplet sublevels (T ± , T 0 ) describing the spin configurations of the two uncompensated electron spins on the radical ions (hyperfine mechanism).…”

Determination of Hyperfine Coupling Constants of Fluorinated Diphenylacetylene Radical Anions by Magnetic Field-Affected Reaction Yield Spectroscopy

“…Similar to the mixture of DMPFPEA with C 6 D 6 , the MARY spectrum of this system consists of the strong zero-field line, smooth MFE curve, and four additional MARY lines at ±3.5 mT and ±6.2 mT. In this case p TPd 14 molecule was used as the radical cation of the pair and luminophore, because PFPEB , similar to some other substituted diphenylacetylenes, − has low Φ L (0.01 ± 0.002 in CHX ) and short τ F (<200 ps in CHX ). The main recombining pair was PFPEB –• / p TPd 14 +• ; dominant hyperfine couplings were concentrated in PFPEB –• .…”

“…The method was chosen on the basis of available literature data concerning quantum-chemical calculations of A HF for fluorinated radical anions. According to refs and , the B3LYP functional provides a good agreement between the experimental and calculated A HF values for a wide array of different fluorinated radical anions, while UCAM-B3LYP was shown to correctly predict the self-consistent field electronic energies of the optimized radical anions . The integration grid was set to 99 radial shells and 590 angular points.…”

“…The first derivatives of dependencies of recombination fluorescence on external magnetic field (MARY spectra) were obtained on a MARY spectrometer described elsewhere. , The samples were irradiated with a BSV-27-Mo X-ray tube (Svetlana, Russia) operated at 40 kV and 20 mA. Fluorescence from the samples was registered using a Hamamatsu HD10493–012 photosensor module with a quartz light guide.…”

“…According to refs 37 and 67, the B3LYP functional provides a good agreement between the experimental and calculated A HF values for a wide array of different fluorinated radical anions, while UCAM-B3LYP was shown to correctly predict the self-consistent field electronic energies of the optimized radical anions. 57 The integration grid was set to 99 radial shells and 590 angular points. The convergence criteria for the self-consistent field were set to 10 −12 for the RMS change in the density matrix and 10 −10 for the maximum change in the density matrix.…”