Involvement Of Triplet State Research Articles

Revealing the Origin of Fast Delayed Fluorescence in a Donor Functionalized Bisterpyridine.

A new carbazole-substituted bisterpyridine with pronounced delayed fluorescence is presented. While the molecular donor-acceptor-donor design suggests the origin of this to be thermally activated delayed fluorescence (TADF), results from various photophysical characterizations, OLED characteristics, temperature-dependent NMR spectroscopy, and DFT calculations all point against the involvement of triplet states. The molecule exhibits blue emission at about 440 nm with two or more fast decay channels in the lower nanosecond range in both solution and thin films. The delayed emission is proposed to be caused by rotational vibrational modes. We suggest that these results are generally applicable, especially for more complex molecules, and should be considered as alternative or competitive emissive relaxation pathways in the field of organic light emitting materials.

Green Phosphorescent Zn(II) Halide Complexes with N,N,N′,N′‐tetramethyl‐P‐indol‐1‐ylphosphonic Diamide as Ligand

AbstractTetrahedral Zn(II) complexes having general formula [ZnX2{O=P(NMe2)2Ind}2] (X=Cl, Br, I, NCS) were isolated from the reaction between the indol‐1‐yl substituted phosphoramide N,N,N′,N′‐tetramethyl‐P‐indol‐1‐ylphosphonic diamide O=P(NMe2)2Ind and anhydrous Zn(II) precursors under mild conditions. The structures of the three halide derivatives were ascertained by single‐crystal X‐ray diffraction. The bromo‐ and iodo‐derivatives revealed to be appreciably luminescent in the green region upon excitation with light below 300 nm, with emission bands centred between 520 and 530 nm. The large Stokes shifts and the excited state lifetimes in the milliseconds range indicated that triplet excited states are involved in the emission. TD‐DFT calculations indicated that the luminescence is related to a ligand‐centred transition with the involvement of triplet states.

The involvement of triplet states in the isomerization of retinaloids.

Rhodopsins form a family of photoreceptor proteins which utilize the retinal chromophore for light energy conversion. Upon light absorption the retinal chromophore undergoes a photoisomerization. This reaction involves a non-radiative relaxation through a conical intersection between the singlet excited state and the ground state. In this work we studied the possible involvement of triplet states in the photoisomerization of retinaloids using the extended multistate (XMS) version of CASPT2. To this end, truncated models of three retinaloids were considered: protonated Schiff base, deprotonated Schiff base and the aldehyde form. The optimized geometries of the reactant, the product and the conical intersection were connected by a linear interpolation of internal coordinates to describe the isomerization. The energetic position of the low-lying singlet and triplet states as well as their spin-orbit coupling matrix elements (SOCME) were calculated along the isomerization profile. The SOCME values peaked in vicinity of the conical intersection for all the retinaloids. Furthermore, the magnitude of SOCME is invariant to the number of double bonds in the model. The SOCME for the protonated Schiff base is negligible (1.5 cm-1) which renders the involvement of the triplet state as improbable. However, the largest SOCME value of 30 cm-1 was found for the aldehyde form, followed by 15 cm-1 for the deprotonated Schiff base.

Recent Advances in Organic Light-Emitting Diodes Based on Pure Organic Room Temperature Phosphorescence Materials.

Pure organic room temperature phosphorescence (RTP) materials have attracted extensive attention in recent years due to their unique characteristics, such as flexible design method, low toxicity, low cost, as well as the ease of production at scale. The involvement of triplet state and direct radiative transition from the triplet state show that RTP materials have great potential as a new generation emitter in organic light-emitting diodes (OLEDs). Based on the mechanism of phosphorescence, various methods have been developed to achieve RTP emissions in the crystal state. However, the observation of RTP in the thin film state is much more difficult to achieve because of the lower degree of rigidity and suppression of the non-radiative transition. In this mini-review, molecular design strategies developed to achieve RTP emissions and their application in OLEDs are summarized and discussed. The conclusion and outlook point to great potential as well as the challenges for the continued study of pure organic RTP materials-based OLEDs.

Proton Capture Dynamics in Quinoline Photobases: Substituent Effect and Involvement of Triplet States

Converting light into chemical energy often occurs through redox reactions that require transfer of several electrons and protons. Using light to control proton transfer has the potential for driving otherwise unfavorable protonation reactions or producing transient pH changes. Photoacids and photobases are fundamental functional elements that could serve this purpose. Previously, we have reported the thermodynamic drive for proton removal in a series of quinoline photobases using Forster cycle analysis of the singlet states. Because the existence of thermodynamic drive does not imply that the molecules can indeed capture protons in the excited state, in this work we report the kinetics of proton removal from water by 5-R-quinolines, R = {NH2, OCH3, H, Cl, Br, CN}, using ultrafast transient absorption spectroscopy. We found that the time constants and mechanisms of proton capture from water are highly sensitive to the substituent. In some cases, proton transfer occurs within the singlet manifold, whereas in some others intersystem crossing competes with this process. We have evidence that the triplet states are also capable of proton capture in two of the compounds. This renders the excited state proton transfer process more complicated than can be captured by the linear free energy relationships inferred from the energetics of the singlet states. We have measured proton capture times in this family to be in the range of several tens of picoseconds with no discernible trend with respect to the Hammett parameter of the substituents. This wide range of mechanisms is attributed to the high density of excited electronic states in the singlet and triplet manifolds. The ordering between these states is expected to change by substituent, solvent, and hydrogen bonding, thus making the rate of intersystem crossing and proton transfer very sensitive to these parameters. These results are necessary fundamental steps to assess the capabilities of photobases in prospective applications such as photomediated proton removal in redox reactions, steady state optical regulation of local pH, and pOH jump kinetics experiments.

Acid generation by 1,4-bis(phenylsulfonyloxy)benzene and their photochemical mechanism

Photochemical studies on 1,4-bis(phenylsulfonyloxy)benzene were performed in acetonitrile by steady-state photolysis at 254 nm. Analysis of photoproducts demonstrate that 1,4-bis(phenylsulfonyloxy)benzene undergoes S–O cleavage leading to the formation of phenylsulfonyl radical and phenylsulfonyloxyphenoxy radical which result in the formation of Fries rearrangement products or escape from the solvent cage to undergo further transformations. The involvement of triplet state in the photo-Fries rearrangement was detected. During the irradiation process, a large amount of acidic species were observed and the formation of benzenesulfonic acid was confirmed. Involvement of O2 in the acid generation was proved to be a primary pathway by comparative experiments. Another pathway of acid generation could be explained as a result of disproportionation of sulfonyl radicals due to the formation of thiobenzenesulfonate. The quantification of the acid was determined by the acid–base titration and investigation on steady-state photolysis was carried out by HPLC analysis and GC–MS.

Role of Photoionization on the Dynamics and Mechanism of Photoinduced Electron Transfer Reaction of Coumarin 307 in Micelles

The dynamics and mechanism of the photoinduced electron transfer (PET) reaction between coumarin 307 (C307) and aromatic amines in micelles have been studied by using steady-state (S-S) and time-resolved (T-R) absorption and fluorescence spectroscopy. Based on the fluorescence quenching time scale, PET in micelles is grouped into two types: (i) ultrafast electron transfer (ET) due to the close contact of the donor and acceptor in micelles and (ii) diffusion averaged dynamic electron transfer (DADET) which is controlled by the diffusion of the reactants in micellar Stern layer and diffusion of the micelles. The DADET does not affect the photoionization and solvation processes whereas ultrafast ET competes with the photoionization and faster than the solvation process. Both ultrafast and DADET shows Marcus inversion in the ET rates at the similar exergonicity and indicates that the role of diffusion and solvent reorganization is negligible toward the activation barrier for the ET reaction in micelles. The activation barrier for the ET reactions in micelles is mainly due to intramolecular reorganization energy. The intramolecular reorganization energy must be higher in CTAB due to the photoionization and subsequent recombination and also involvement of triplet state in the PET. The ET reaction between coumarin radical cation and amine is reported for the first time in the C307-amine systems in micelles which are confirmed by the effect on amine concentration of the decay of coumarin radical cation and the dynamics of the ground-state recovery of C307. A mechanism for the PET reaction between C307-amine systems is proposed in micelles including photoionization, ultrafast and dynamic ET, and solvation dynamics.

Involvement of triplet state in the photodissociation of hydrogen peroxide: experimental evidence from time-resolved EPR study

The dissociation of photoexcited hydrogen peroxide to generate a pair of hydroxyl radicals is generally believed to take place in a repulsive electronic singlet state. The results presented here, based on time-resolved EPR experiments on the spin polarisation pattern of the acetone ketyl radical (CH3)2C•OH, generated on photodissociation of H2O2 in 2-propanol with a 248 nm laser light, strongly indicate significant involvement of a repulsive triplet state of excited hydrogen peroxide.

ChemInform Abstract: THE 147‐NM PHOTOLYSES OF TETRAMETHYLENE SULFIDE, TETRAMETHYLENE SULFOXIDE, AND TETRAMETHYLENE SULFONE

Tetramethylene sulfide, sulfoxide, and sulfone have been photolyzed with 147-nm radiation in the gas and condensed phases. Mechanisms which assume diradical intermediates are presented for the photodecomposition of each compound. The major reaction in all three cases is the elimination of ethylene by a ..beta..-cleavage of the initially formed diradical: XCH/sub 2/CH/sub 2/CH/sub 2/CH/sub 2/. ..-->.. .XCH/sub 2/CH/sub 2/. + C/sub 2/H/sub 4/. The competing ..beta..-cleavage produces X==CH/sub 2/ and C/sub 3/H/sub 6/. This second ..beta..-cleavage is less important in the sulfoxide and the sulfone than it is in the sulfide. The direct elimination of the sulfur atom, and any attached oxygen atoms, is of no importance in the sulfide and of increasing importance as the oxidation state of the sulfur atom increases. The similarity between the mass spectral fragmentation and the vacuum ultraviolet photochemistry of these compounds is noted and the possible involvement of triplet states is mentioned.

The 147-nm photolyses of tetramethylene sulfide, tetramethylene sulfoxide, and tetramethylene sulfone

Tetramethylene sulfide, sulfoxide, and sulfone have been photolyzed with 147-nm radiation in the gas and condensed phases. Mechanisms which assume diradical intermediates are presented for the photodecomposition of each compound. The major reaction in all three cases is the elimination of ethylene by a ..beta..-cleavage of the initially formed diradical: XCH/sub 2/CH/sub 2/CH/sub 2/CH/sub 2/. ..-->.. .XCH/sub 2/CH/sub 2/. + C/sub 2/H/sub 4/. The competing ..beta..-cleavage produces X==CH/sub 2/ and C/sub 3/H/sub 6/. This second ..beta..-cleavage is less important in the sulfoxide and the sulfone than it is in the sulfide. The direct elimination of the sulfur atom, and any attached oxygen atoms, is of no importance in the sulfide and of increasing importance as the oxidation state of the sulfur atom increases. The similarity between the mass spectral fragmentation and the vacuum ultraviolet photochemistry of these compounds is noted and the possible involvement of triplet states is mentioned.