Solvation Controlled Excited-State Planarization in a Push–Pull Pyrene Dye

Abstract

The excited-state deactivation pathway of the push–pull pyrene derivative (EPP), a molecule emitting bright red fluorescence, has been studied in several solvents using quantum chemical calculations, femtosecond time-resolved fluorescence up-conversion spectroscopy, and femtosecond transient absorption spectra (fs-TA). From the steady-state spectra, EPP has shown a strong negatively solvatochromic behavior of the absorption spectra and non-solvatochromic emission spectra, revealing the decreased dipole moment upon excitation. Femtosecond up-conversion measurements have shown interesting rise–decay kinetics at the red emission side and fs-TA spectra also feature the SE (stimulated emission) formed and new ESA (excited-state absorption) developed in all investigated solvents. The careful analysis of the data gathered from fluorescence up-conversion and fs-TA spectra, supported by DFT quantum chemical calculations and temperature-dependent fluorescence measurements, unambiguously pointed out the excited-state relaxation pathway of EPP is controlled by solvent stabilized planar intramolecular charge transfer (PICT) dynamics. This finding confirms the surrounding solvents as a significant factor that affects the rate of excited-state conformation changes.

Ultrafast Photophysics of Multiple-Resonance Ultrapure Blue Emitters

“…As shown in Table , smaller dihedral angles (θ 1234 and θ 1564 , atom numbering as shown in Scheme ) in the S 1 state are observed than in the S 0 state for t-DABNA and TBN-TPA-1, which corresponds to the excited-state planarization of the B-N framework. − The photo-induced structural planarization was recently observed in ring-fused NBN-phenalenes, but is rarely reported for MR-TADF emitters. The observed S 1 state planarization of the B-N framework is opposite to that of the reported dihedral angle increase in MR-TADF molecules with the QAO (quinolino-[3,2,1- de ]­acridine-5,9-dione) framework .…”

Probing twisted intramolecular charge transfer of pyrene derivatives as organic emitters in OLEDs

“…Pyrene, a polycyclic aromatic hydrocarbon having four fused benzene rings, is an attractive material for electroluminescent devices, in particular, as a blue OLED emitter due to its high fluorescence quantum yield and stability. [20][21][22][23][24][25][26][27] Its synthetic controllability of the photophysical properties by addition of various functional groups have received lots of attention from the scientists as well as the engineers. 24,26 The optical properties of pyrene can be controlled by substituents that alter its conjugation length and CT character to tune emission wavelength and also to enhance emission quantum yield and the chemical stability for commercial applications such as OLEDs.…”

“…[28][29][30][31][32][33][34] Among a series of pyrene derivatives, pyrenes substituted with electron donors/acceptors frequently exhibit CT characteristics without structural changes, and show absorption and emission behavior that depend on the polarity of the solvent. [20][21][22][23][24][25][26][27][28][29][30][31] The pyrene derivatives (N- S1-S4, ESI †) in this study have donor (carbazole/N,N-di(biphenyl))-acceptor (pyrene) structures that exhibit CT characteristics and are also highly likely to show TICT because the N,N-di(biphenyl) group is well-known to undergo structural change in the excited state. 12,32,33 Moreover, these pyrene derivatives show an unique fluorescence behavior, the increased fluorescence intensity in the polar solvent, which is not typically observed in molecular systems having CT characters.…”

“…28–34 Among a series of pyrene derivatives, pyrenes substituted with electron donors/acceptors frequently exhibit CT characteristics without structural changes, and show absorption and emission behavior that depend on the polarity of the solvent. 20–31…”

Accurate Molecular Geometries in Complex Excited-State Potential Energy Surfaces from Time-Dependent Density Functional Theory

“…Theoretical and computational calculations aid photochemistry research in many different ways. In combination with experiments, they allow for a deeper understanding of the nature and structure of excited states − or of reaction pathways and deactivation mechanisms. − Beyond complementing experiments, excited-state calculations are employed in the design of new devices and materials for optoelectronic applications, − can lead to the discovery of new mechanisms, and allow studying potential energy surfaces which may reveal new reaction pathways and structures. , …”