Understanding and Tailoring Excited State Properties in Solution-Processable Oligo(p-phenyleneethynylene)s: Highly Fluorescent Hybridized Local and Charge Transfer Character via Experiment and Theory

Abstract

Rod-shaped oligo(p-phenyleneethynylene) (OPE) offers an attractive π-framework for the development of solution-processable highly fluorescent molecules having tunable hybridized local and charge transfer (HLCT) excited states and (reverse) intersystem crossing ((R)ISC) channels. Herein, an HLCT oligo(pphenyleneethynylene) library was studied for the first time in the literature in detail systematically via experiment and theory. The design, synthesis, and full characterization of a new highly fluorescent (Φ PL-solution ∼ 1) sky blue emissive 4′,4‴-((2,5bis((2-ethylhexyl)oxy)-1,4-phenylene)bis(ethyne-2,1-diyl))bis(N,N-diphenyl-[1,1′-biphenyl]-4-amine) (2EHO-TPA-PE) was also reported. The new molecule consists of a D′−Ar−π−D−π−Ar−D′ molecular architecture with an extended π-spacer and no acceptor unit, and detailed structural, physicochemical, single-crystal, and optoelectronic characterizations were performed. A high solid-state quantum efficiency (Φ PL-solid state ∼ 0.8) was achieved as a result of suppressed exciton−phonon/vibronic couplings (no π−π interactions and multiple (14 per dimeric form) strong C−H•••π interactions). Strong solution-phase/solid-state dipole-dependent tunable excited state behavior (local excited (LE) → HLCT → charge transfer (CT)) and decay dynamics covering a wide spectral region were demonstrated, and the CT state was observed to be highly fluorescent despite extremely large Stokes shift (∼130 nm)/fwhm (∼125 nm) and significant charge separation (0.75 charge• nm). Employing the Lippert−Mataga model, along with detailed photophysical studies and TDDFT calculations, key relationships between molecular design−electronic structure−exciton characteristics were elucidated with regards to HLCT and hot exciton channel formations. The interstate coupling between CT and LE states and the interplay of this coupling with respect to medium polarity were explored. A key relationship between excited-state symmetry breaking process and the formation of HLCT state was discussed for TPA-ended rod-shaped OPE π-systems. (R)ISC-related delayed fluorescence (τ ∼ 2−6 ns) processes were evident following the prompt decays (∼0.4−0.9 ns) both in the solution and in the solid-state. As a unique observation, the delayed fluorescence could be tuned and facilitated via small dielectric changes in the medium. Our results and the molecular engineering perspectives presented in this study may provide unique insights into the structural and electronic factors governing tunable excited state and hot-exciton channel formations in OPEs for (un)conventional solution-processed luminescence applications.

From symmetry breaking to symmetry swapping: is Kasha's rule violated in multibranched phenyleneethynylenes?

“…[26][27][28][29][30][31][32][33][34][35] Symmetry breaking also plays a crucial role in the generation of charge separation in multichromophoric assemblies. [36][37][38][39][40][41][42] In multibranched multipolar chromophores, whose low-lying excitations are dominated by charge transfer degrees of freedom, symmetry breaking is driven by polar solvation, as theoretically predicted 21,26 and experimentally veried. 29,43,44 In these multipolar systems, having formally equivalent branches, symmetry breaking implies chargelocalization in one branch, with a concomitant geometrical distortion, so that in the relaxed excited state the molecular branches are no more equivalent.…”

Watching Excited-State Symmetry Breaking in Multibranched Push–Pull Molecules

“…Over the past few years, symmetric molecules containing two or more electron donor–acceptor (D–A) branches have been attracting considerable attention. These molecules are being increasingly used as two-photon absorbers, electron acceptors, or new luminescent materials in applications as diverse as fluorescence microscopy, , photopolymerization, , organic photovoltaics, , or organic light-emitting diodes. , Although these compounds do not possess a permanent electric dipole moment, their fluorescence usually exhibits a strong solvatochromism that is, in some cases, as large as that measured with the single-branched D–A analogue. − This behavior was explained in terms of excited-state symmetry breaking (ES-SB), i.e., a transition from a symmetric and multipolar state to an asymmetric and dipolar state. , In principle, emission solvatochromism cannot be considered as an unambiguous evidence of ES-SB, unless it is as large as that of the single-branch analogue. Indeed, quadrupole–dipole and octupole–dipole interactions do also contribute to the solvation energy and, hence, give rise to a solvatochromism.…”

Progresses and Perspectives of Near-Infrared Emission Materials with “Heavy Metal-Free” Organic Compounds for Electroluminescence

“…The hybridized local and charge-transfer excited state (HLCT) possesses two combined and compatible characteristics with a large transition moment from a local excited (LE) state and a weakly bound exciton from a charge transfer (CT) state [ 137 , 138 , 139 ]. The former contributes to a high-efficiency radiation of fluorescence, while the latter is responsible for the generation of a high fraction of singlet excitons.…”