Solvation-Dependent Excited-State Dynamics of Donor–Acceptor Molecules with Hybridized Local and Charge Transfer Character

Abstract

Recently, an organic synthetic strategy based on hybridized local and charge transfer (HLCT) character has been attracting much attention because of its potential for designing high-efficiency organic light-emitting diode materials. In this work, two novel molecules, N,N-diphenyl-4-phenol-(1-phenyl-1H-phenanthro­[9,10-d]­imidazol-2-yl)­biphenyl-4-amine (TPA-PPI-OH) and N,N-diphenyl-4′-(1-phenyl-1H-phenanthro­[9,10-d]­imidazol-2-yl)-[1,1′-biphenyl]-4-amine (TPA-PPI), were investigated by quantum chemical calculations, steady-state spectroscopy, and femtosecond transient absorption spectroscopy (fs-TA) to explore the nature of HLCT. Computational results and steady-state spectra suggest that the lowest excited state is dominated by local excitation (LE) character in low-polar toluene (TOL), whereas the charge transfer (CT) character plays the main role in high-polar acetonitrile (ACN) for both TPA-PPI-OH and TPA-PPI. Relative to TPA-PPI, TPA-PPI-OH shows less sensitivity to solvent polarity with higher quantum yields because of the more planar geometric structure, fabricated by inserting an additional intramolecular hydrogen bond (H-bond) to enhance the inflexibility of the molecule. Ultrafast fs-TA clearly shows the conversion of excited states from LE to CT with the increase of solvent polarity. The stimulated emission is mainly from the LE-dominated lowest excited state in low-polar TOL, whereas CT dominates the final relaxation process in high-polar ACN because of strong solvation. Furthermore, the excited states being dominated by LE and CT simultaneously in medium-polar tetrahydrofuran is observed, while the quick equilibrium LE ↔ CT is established just after a femtosecond pulse excitation, indicating the typical HLCT character. The excited state deactivation process of TPA-PPI-OH is faster than that of TPA-PPI, which is attributed to the higher proportion of the LE component and the additional vibrational decay paths induced by the H-bond in TPA-PPI-OH. The results herein offer a guidance to understand the solvent-modulated excited state deactivation mechanism of HLCT molecules.

Tandem Nenitzescu Reaction/Nucleophilic Aromatic Substitution to Form Novel Pyrido Fused Indole Frameworks

“…Specifically, the PL spectra of 14 b and 14 c in toluene are vibronically structured, whereas only a shoulder peak is observed for 15 ( a – c ), indicating that the emission originates from the localized energy states. However in acetonitrile, only one broad band at a higher wavelength region could be discerned, implying the formation of a charge‐transfer state as the solvent polarity increases [56,57] . This solvatochromism is attributed to solvent relaxation, leading to charge transfer due to the high polarity of acetonitrile compared to toluene.…”

Hot Exciplexes in U-Shaped TADF Molecules with Emission from Locally Excited States

“…[19][20][21][22][23] Unfortunately, the emission characteristics of such HLCT geometries exhibit strong dependence on the polarity of the environment, thus limiting the choice of matrix and the OLED device architecture. [24][25][26] Moreover, hot exciton luminophores with HLCT states show detrimental roll-off behavior in current efficiency and low EQE. [27][28][29] New molecular designs that make use of precisely localized 1 LE states for emission, while utilizing hot excitonic states for fast transfer to these 1 LE states, would advance the field and understanding of TADF and enable the development of emissive materials with superior device performance in view of efficiency and color purity.…”

Scanning Ultrafast Spectral Dynamics of Triphenylamine-Modified Vinylbenzothiazole Derivative: Role of Solvent Polarity and Temperature

“…Apparently, the interaction becomes much stronger, since the corresponding interaction lifetime shortens and the PL red-shift becomes much more apparent. 44 Note that the lifetime of generating HLCT state decreases from 2.8 ps (in THF) to 1.6 ps (in DMF) on the basis of the global fitting results (Figure 2F). In addition, the transfer lifetime from the HLCT state to the HLCT′ state shortens a little from 15 to 13 ps, due to the increase of solvent polarity.…”

“…Apparently, the appearance of the HLCT state and the HLCT′ state increases the complexity of the relaxation pathway of excited BTTM in THF. 44 As seen in Figure 2C, the transient spectra of BTTM in DMF with high polarity are composed of a GSB band centered at around 425 and 480 nm, and an ESA band in the region from 520 to 800 nm only in the first 30 ps (Figure S3). Meanwhile, a new ESA band from 450 to 610 nm appears and replaces the SE signal in the initial picoseconds.…”