Suppressing molecular motions for enhanced room-temperature phosphorescence of metal-free organic materials

Metal-free organic phosphorescent materials are attractive alternatives to the predominantly used organometallic phosphors but are generally dimmer and are relatively rare, as, without heavy-metal atoms, spin-orbit coupling is less efficient and phosphorescence usually cannot compete with radiationless relaxation processes. Here we present a general design rule and a method to effectively reduce radiationless transitions and hence greatly enhance phosphorescence efficiency of metal-free organic materials in a variety of amorphous polymer matrices, based on the restriction of molecular motions in the proximity of embedded phosphors. Covalent cross-linking between phosphors and polymer matrices via Diels-Alder click chemistry is devised as a method. A sharp increase in phosphorescence quantum efficiency is observed in a variety of polymer matrices with this method, which is ca. two to five times higher than that of phosphor-doped polymer systems having no such covalent linkage.ope

Electronic excitation and singlet-triplet coupling in uracil tautomers and uracil-water complexes

Electronic spectra of uracil in its diketo (lactam) form and five enol (lactim) tautomeric forms have been investigated by means of combined density functional and configuration interaction methods. We have simulated the effects of hydrogen bonding with a protic solvent by recomputing the spectrum of uracil in the presence of two, four, or six water molecules. Geometries of the electronic ground state and several low-lying excited states have been optimized. Spin-orbit coupling has been determined for correlated wavefunctions employing a non-empirical spin-orbit mean-field approach. In accord with experiment, we find the diketo tautomer to be the most stable one. The calculations confirm that the first absorption band arises from the 1($\pi\to\pi^\ast$) ${\rm S}_0 \to {\rm S}_2$ excitation. The experimentally observed vibrational structure in this band originates from a breathing mode of the six ring. Complexation with water molecules is seen to cause a significant blue shift of $n\to\pi^\ast$ excitations while leaving $\pi\to\pi^\ast$ excitations nearly uninfluenced. Computed radiative lifetimes are presented for the experimentally known weak phosphorescence from the $\pi\to\pi^\ast$ excited T1 state. Among the uracil lactim tautomers, one is particularly interesting from a spectroscopic point of view. In this tautomer, the $\pi\to\pi^\ast$  excitation gives rise to the S1 state

Protonation-State-Driven Photophysics in Phenothiazinium Dyes: Intersystem Crossing and Singlet-Oxygen Production

The impact of altering the solvent pH value on the photodynamic activity of thionine has been studied computationally by means of density functional theory and multi‐reference interaction methods. To this end, we have investigated the electronic structure of the ground and excited states of diprotonated (TH22+) and neutral imine (T) forms of thionine (TH+). It is well known experimentally that the T1 state of TH+ undergoes acid–base equilibrium reactions resulting in a pronounced pH effect for the efficiency of singlet‐oxygen (1O2) production. Our results show that the energy‐transfer reactions from the T1 state of TH22+ and T to 3O2 correspond to reversible equilibrium processes, whereas in TH+ this process is very exothermic in a vacuum (−0.66 eV) and in aqueous solution (−0.49 eV). These facts explain the experimental observation of a much smaller efficiency of 1O2 production for TH22+ than for TH+. Moreover, we found that the pH value significantly effected the intersystem crossing (ISC) kinetics impacting the concentration of triplet‐state species available for energy transfer. In very acidic aqueous solution (pH22+ is the prevailing species, the ISC proceeds with a rate constant of ≈108 s−1. In a basic medium where T is the dominant species, ISC decay occurs by means of a thermally activated channel (≈108 s−1) which competes with fluorescence (5.32×107 s−1). According to these results, maximum ISC efficiency is expected for intermediate acidic pH values (TH+, ≈109 s−1)

Vibronic coupling in indole: II. Investigation of the l-1(a)-l-1(b) interaction using rotationally resolved electronic spectroscopy

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Rotationally resolved electronic spectroscopy of 2,3-bridged indole derivatives: Tetrahydrocarbazole

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Vibronic coupling in indole: I. Theoretical description of the l-1(a)-l-1(b) interaction and the electronic spectrum

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THE CONICAL INTERSECTIONS BETWEEN 1^{1}Lb_{b} and 1^{1}La_{a} STATES IN TRYPTAMINE AND INDOLE

Author Institution: Institut fur Physikalische Chemie I, Heinrich-Heine-Universitat; Universitatsstra{\ss}e 26.43.02.43 D-40225 Dusseldorf, Germany; Fritz-Haber-Institut der Max-Planck-Gesellschaft, 14195 Berlin, Germany; Molecular and Biophysics Group, Institute for Molecules and Materials,Radboud University; 6500 GL Nijmegen, The Netherlands; University of Pittsburgh, Department of Chemistry, Pittsburgh, PA 15260, USA; Chemical Physics Department, Weizmann Institute of Science, 76100 Rehovot, Israel; Institut fur Physikalische Chemie I, Heinrich-Heine-Universitat; Universitatsstra{\ss}e 26.43.02.43 D-40225 Dusseldorf, GermanyThe absorption spectrum of the lowest two excited singlet states of indole and tryptamine are calculated using Franck-Condon-Herzberg-Teller (FCHT) theory. The derivatives of the transition dipole moments with respect to the normal coordinates are computed numerically at the combined density functional theory multi-reference configuration interaction (DFT/MRCI) level of theory. All valence electrons were correlated in the MRCI runs and the eigenvalues and eigenvectors of the lowest singlet state were determined. The initial set of reference configuration state functions was generated automatically in a complete active space type procedure (including all single and double excitations from the five highest occupied molecular orbitals in the KS determinant to the five lowest virtual orbitals) and was then iteratively improved. The HT integrals are obtained from Doktorovs recursive relations used for the calculation of the FC integrals. A conical intersection (CI) between $^{1}$L$_{b}$ and $^{1}$L$_{a}$ states in indole is calculated using DFT/MRCI to be located 2000 cm$^{-1}$ above the $^{1}$L$_{b}$ origin, thus perturbing only slightly the vibronic spectrum of indole. For tryptamine the CI is calculated to be less than 1000 cm$^{-1}$ above the $^{1}$L$_{b}$ origin and strong perturbation of the vibronic spectrum is expected and observed

Sequential Hydrogen Tunneling in o Tolylmethylene

o Tolylmethylene 1 is a metastable triplet carbene that rearranges to o xylylene 2 even at temperatures as low as 2.7 K via [1,4] H atom tunneling. Electron paramagnetic resonance EPR and electron nuclear double resonance ENDOR spectroscopical techniques were used to identify two conformers of 1 anti and syn in noble gas matrices and in frozen organic solutions. Conformer specific kinetic measurements revealed that the rate constants for the rearrangements of the anti and syn conformers of 1 are very similar. However, the orbital alignment in the syn conformer is less favorable for the hydrogen transfer reaction than the orbital configuration in the anti conformer. Our spectroscopic and quantum chemical investigations indicate that anti 1 and syn 1 rapidly interconvert via efficient quantum tunneling forming a rotational pre equilibrium. The subsequent second tunneling reaction, the [1,4] H migration from anti 1 to 2, is rate limiting for the formation of 2. We here present an efficient strategy for the study of such tunneling equilibri