Restricted rotation and tunable fluorescence in atropisomeric naphthyl pyridine chromophores.

Enhanced fluorescence quantum yields are enabled by simple reactions at the heterocyclic nitrogen in naphthyl-pyridine chromophores in which the electronic properties can be tuned through protonation, oxidation, and alkylation at the nitrogen center. Fluorescence quantum yield is increased by reacting the pyridine lone pair with either a proton or an alkyl group. Restricted intramolecular rotation (RIR) was observed upon alkylation, as evidenced by the presence of atropisomers. These simple structural changes allow application-driven tuning of electronic properties.

Effect of Oxidation on the Chiroptical Properties of Sulfur-Bridged Binaphthyl Dimers.

A series of axially chiral sulfur-bridged dimers were prepared from 1,1'-binaphthyl-2,2'-diol and subsequently oxidized to the respective sulfones. The chiroptical properties of the chiral chromophores were studied as a function of the oxidation state. Upon oxidation, an increase in quantum yields was observed for directly linked sulfur bridged binaphthyls (0.04 to 0.32), and a modest increase in dissymmetry factor was observed for diphenylsulfide-bridged binaphthyls (-8.9 × 10-4 to -1.4 × 10-3). Computational calculations were used to elucidate the changes in photophysical properties.

Fluorescence Switching of Intramolecular Lewis Acid-Base Pairs on a Flexible Backbone.

A class of intramolecular Lewis acid-base pairs containing Lewis acidic dimesitylboranes paired with phosphine oxide, sulfoxide, and sulfone Lewis basic groups are explored. The absorption and emission properties of the compounds are investigated in different solvent environments, and switching of the photophysical behavior between the Lewis acid-base adducts and free acid-base pairs is examined. We find that phosphine oxide Lewis base groups are effective partners in flexible Lewis pairs; however, the analogous sulfoxide and sulfone groups are not. Additionally, the absorption and emission wavelengths can be systematically tuned by varying the conjugation length and electron-donating and -accepting substituents on the backbone.

Tunable Emission of Iridium(III) Complexes Bearing Sulfur-Bridged Dipyridyl Ligands.

A series of six new cyclometalated iridium(III) complexes [Ir(ppy)2(N∧N)][PF6] (ppy = 2-phenylpyridine) is reported herein. Proligands bis(pyridin-2-yl)sulfane (1a), 2,2'-sulfinyldipyridine (1b), 2,2'-sulfonyldipyridine (1c), bis(4-methylpyridin-2-yl)sulfane (2a), 2,2'-sulfinylbis(4-methylpyridine) (2b), and 2,2'-sulfonylbis(4-methylpyridine) (2c) were synthesized, characterized, and employed as the N∧N ancillary ligand. Changing the oxidation state of the sulfur atom serves as a switch to alter the emissive state from that of mainly 3LC character (blue-green emission) to one of 3MLCT/3LLCT character (yellow emission). Sulfide and sulfoxide complexes Ir(1a), Ir(1b), Ir(2a), and Ir(2b) show identical, vibrationally structured emission profiles with maxima at 478, 510, 548 nm in CH2Cl2 solutions resulting from a 3LC state. In contrast, sulfone complexes Ir(1c) and Ir(2c) show broad, red-shifted 3CT emission (552 and 537 nm, respectively).

Control of photoluminescence quantum yield and long-lived triplet emission lifetime in organic alloys.

Two-component crystalline organic alloys with a wide range of compositional ratios (from 30% to 90% of one component) are employed to tune excited-state lifetimes and photoluminescence quantum yields (PLQYs). Alloy crystals exhibit homogeneous distribution of parent compounds by X-ray crystallography and differential scanning calorimetry. The alloys display a 1.5- to 5-fold enhancement in thermally activated delayed fluorescence (TADF) lifetime, compared to the parent compounds. PLQYs can also be tuned by changing alloy composition. The reverse intersystem crossing and long-lived lifetime of the parent compounds give rise to long-lived TADF in the alloys. Organic alloys enable tunability of both lifetime and efficiency, providing a new perspective on the development of organic long-lived emissive materials beyond the rules established for host-guest doped systems.

Switching between TADF and RTP: anion-regulated photoluminescence in organic salts and co-crystals.

Thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) are two photophysical phenomena which utilize triplet excitons. In this work, we demonstrate how variation of the anion in organic salts with carbazole and phenothiazine-5,5-dioxide donors and pyridinium and quinolinium acceptors may be used to switch between TADF and RTP. These compounds adopt similar molecular structures and packing modes with different anions and exhibit different types of photophysical behavior due to the electronic effects of the anions. With bromide anions, the salts exhibit TADF with some RTP. These compounds show fast reverse intersystem crossing and a short delayed lifetime, which is key to application in efficient and robust OLEDs. With BF4 - and PF6 - anions, RTP with long-lived lifetimes and afterglow are observed by eye. This behavior can be utilized for data encryption and anti-counterfeiting applications. Emission wavelengths and lifetimes are also anion-dependent. These results open up an avenue for developing novel luminescent materials through anion tuning and present a molecular model to understand the interplay of RTP and TADF.

Quinoline-containing diarylethenes: bridging between turn-on fluorescence, RGB switching and room temperature phosphorescence.

Simple structural modifications using oxidation and methylation of a quinoline-containing diarylethene result in dramatic variation of photophysical properties. Turn-on fluorescence, room temperature phosphorescence (RTP) and red-green-blue (RGB) switching were achieved in three different related compounds. Photoswitchable diarylethenes (DAEs) that exhibit turn-on fluorescence are in high demand for super-resolution microscopy, and the development of purely organic phosphorescent materials in the amorphous state is attractive but challenging. The findings reported here provide a novel toolkit for designing turn-on fluorescence DAEs for super-resolution microscopy and extending the scope of amorphous RTP materials. More importantly, we bridge between these two fundamentally significant photochemical and photophysical phenomena, and reveal structure-property relationships between DAE photochromism and RTP.

Controlling ultralong room temperature phosphorescence in organic compounds with sulfur oxidation state.

Sulfur oxidation state is used to tune organic room temperature phosphorescence (RTP) of symmetric sulfur-bridged carbazole dimers. The sulfide-bridged compound exhibits a factor of 3 enhancement of the phosphorescence efficiency, compared to the sulfoxide and sulfone-bridged analogs, despite sulfone bridges being commonly used in RTP materials. In order to investigate the origin of this enhancement, temperature dependent spectroscopy measurements and theoretical calculations are used. The RTP lifetimes are similar due to similar crystal packing modes. Computational studies reveal that the lone pairs on the sulfur atom have a profound impact on enhancing intersystem crossing rate through orbital mixing and screening, which we hypothesize is the dominant factor responsible for increasing the phosphorescence efficiency. The ability to tune the electronic state without altering crystal packing modes allows the isolation of these effects. This work provides a new perspective on the design principles of organic phosphorescent materials, going beyond the rules established for conjugated ketone/sulfone-based organic molecules.

Variable oxidation state sulfur-bridged bithiazole ligands tune the electronic properties of ruthenium(ii) and copper(i) complexes.

The synthesis of homoleptic and heteroleptic ruthenium(ii) and copper(i) complexes containing sulfur-bridged bithiazole ligands of varying oxidation states are reported. The complexes have been characterized using 1D and 2D NMR spectroscopy, X-ray single crystal diffraction, electrochemistry, UV-vis absorbance and fluorescence spectroscopy. The stability, photophysical and electrochemical properties were found to be dependent on the oxidation state of the non-coordinating sulfur. The ruthenium and copper species were found to be non-emissive in solution at room temperature, though all displayed weak emission when doped in a PMMA matrix, which increased in intensity on cooling to 77 K. The electrochemical HOMO-LUMO gap was found to be dependent on the oxidation state of the sulfur in the bridging ligand in all complexes, illustrating an additional handle for tuning electrochemical properties.