Color Polymorphism and Room Temperature Phosphorescence of 4-Bromo-2,7-Di-Tert-Butyl- 9-Methoxypyrene.

Color polymorphism combined with crystal packing-dependent luminescence properties of polymorphs reflects differences in intermolecular interactions in different molecular arrangements. The title compound has two polymorphic crystal structures having strikingly different absorption and luminescence spectra that result from different packing motifs in the crystal lattice. The polymorph with brick wall-like packing of molecules is white and shows very weak violet fluorescence whereas the second polymorph, where molecules are arranged in columnar stacks, is bright yellow and displays intense green fluorescence with maximum at 487 nm (20530 cm-1). In the white polymorph, where the distance between neighboring chromophores is increased, absorption and fluorescence spectra are similar to those of monomer in solution, and intersystem crossing to triplet manifold is the dominant pathway of relaxation. In the yellow polymorph, molecules within the columnar stacks are rotated which mitigates the steric hindrance and leads to closer π-stacking of the pyrene cores. That increases the ππ overlap and strengthens intermolecular interactions decreasing energy of the excited states. This affects emission spectra and photophysical processes-fluorescence yield grows whereas triplet formation yield decreases when S1 is lowered below higher triplet states and conditions for effective vibronic spin-orbit coupling are not favorable. The effect is not observed for other similar pyrene derivatives, testifying the uniqueness of the phenomenon.

Selective population of triplet excited states in heavy-atom-free BODIPY-C60 based molecular assemblies.

Photophysical studies on a BODIPY-fullerene-distyryl BODIPY triad (BDP-C60-DSBDP) and its reference dyads (BODIPY-fullerene; BDP-C60 and distyryl BODIPY-fullerene; DSBDP-C60) are presented herein. In the triad, the association of the two chromophore units linked by a fullerene moiety leads to strong near UV-Visible light absorption from 300 to 700 nm. The triplet-excited state was observed upon visible excitation in all these assemblies, and shown to be localized on the C60 or BODIPY moieties. Using quantitative nanosecond transient absorption, we provide a complete investigation on the lifetime and formation quantum yield of the triplet-excited state. In the BDP-C60 dyad, the triplet excited state of C60 (τ = 7 ± 1 µs) was obtained with a quantum yield of 40 ± 8%. For the DSBDP-C60 dyad and BDP-C60-DSBDP triad, a longer-lived triplet excited state with a lifetime of around 250 ± 20 µs centered on the DSBDP moiety was formed, with respective quantum yields of 37 ± 8 and 20 ± 4%. Triplet-triplet annihilation up-conversion is characterized in the BDP-C60 dyad and the bichromophoric triad in the presence of perylene and DSBDP-monomer as respective annihilators. The photo-induced formation of a long-lived 3DSBDP* in the triad coupled with panchromatic light absorption offers potential applications as a heavy-atom-free organic triplet photosensitizer.

Spin Orbit Coupling in Orthogonal Charge Transfer States: (TD-)DFT of Pyrene-Dimethylaniline.

The conformational dependence of the matrix element for spin-orbit coupling and of the electronic coupling for charge separation are determined for an electron donor-acceptor system containing a pyrene acceptor and a dimethylaniline donor. Different kinetic and energetic aspects that play a role in the spin-orbit charge transfer intersystem crossing (SOCT-ISC) mechanism are discussed. This includes parameters related to initial charge separation and the charge recombination pathways using the Classical Marcus Theory of electron transfer. The spin-orbit coupling, which plays a significant role in charge recombination to the triplet state, can be probed by (TD)-DFT, using the latter as a tool to understand and predict the SOCT-ISC mechanism. The matrix elements for spin-orbit coupling for acetone and 4-thio-thymine are used for benchmarking. (Time Dependent-) Density Functional Theory (DFT and TD-DFT) calculations are applied using the quantum chemical program Amsterdam Density Functional (ADF).

Ultrafast Dynamics of Sb-Corroles: A Combined Vis-Pump Supercontinuum Probe and Broadband Fluorescence Up-Conversion Study.

Corroles are a developing class of tetrapyrrole-based molecules with significant chemical potential and relatively unexplored photophysical properties. We combined femtosecond broadband fluorescence up-conversion and fs broadband Vis-pump Vis-probe spectroscopy to comprehensively characterize the photoreaction of 5,10,15-tris-pentafluorophenyl-corrolato-antimony(V)-trans-difluoride (Sb-tpfc-F2). Upon fs Soret band excitation at ~400 nm, the energy relaxed almost completely to Q band electronic excited states with a time constant of 500 ± 100 fs; this is evident from the decay of Soret band fluorescence at around 430 nm and the rise time of Q band fluorescence, as well as from Q band stimulated emission signals at 600 and 650 nm with the same time constant. Relaxation processes on a time scale of 10 and 20 ps were observed in the fluorescence and absorption signals. Triplet formation showed a time constant of 400 ps, with an intersystem crossing yield from the Q band to the triplet manifold of between 95% and 99%. This efficient triplet formation is due to the spin-orbit coupling of the antimony ion.

Understanding protein diffusion on force-induced stretched DNA conformation.

DNA morphology is subjected to environmental conditions and is closely coupled with its function. For example, DNA experiences stretching forces during several biological processes, including transcription and genome transactions, that significantly alter its conformation from that of B-DNA. Indeed, a well-defined 1.5 times extended conformation of dsDNA, known as Σ-DNA, has been reported in DNA complexes with proteins such as Rad51 and RecA. A striking feature in Σ-DNA is that the nucleobases are partitioned into triplets of three locally stacked bases separated by an empty rise gap of ∼ 5 Å. The functional role of such a DNA base triplet was hypothesized to be coupled with the ease of recognition of DNA bases by DNA-binding proteins (DBPs) and the physical origin of three letters (codon/anti-codon) in the genetic code. However, the underlying mechanism of base-triplet formation and the ease of DNA base-pair recognition by DBPs remain elusive. To investigate, here, we study the diffusion of a protein on a force-induced stretched DNA using coarse-grained molecular dynamics simulations. Upon pulling at the 3' end of DNA by constant forces, DNA exhibits a conformational transition from B-DNA to a ladder-like S-DNA conformation via Σ-DNA intermediate. The resulting stretched DNA conformations exhibit non-uniform base-pair clusters such as doublets, triplets, and quadruplets, of which triplets are energetically more stable than others. We find that protein favors the triplet formation compared to its unbound form while interacting non-specifically along DNA, and the relative population of it governs the ruggedness of the protein-DNA binding energy landscape and enhances the efficiency of DNA base recognition. Furthermore, we analyze the translocation mechanism of a DBP under different force regimes and underscore the significance of triplet formation in regulating the facilitated diffusion of protein on DNA. Our study, thus, provides a plausible framework for understanding the structure-function relationship between triplet formation and base recognition by a DBP and helps to understand gene regulation in complex regulatory processes.

Acceleration of Singlet Fission in an Aza-Derivative of TIPS-Pentacene.

The influence of the carbon to nitrogen substitution on the photoinduced dynamics of TIPS-pentacene was investigated by ultrafast transient absorption measurements on spin-coated thin films in the visible and in the near-infrared spectral region. A global target analysis was performed to provide a detailed picture of the excited-state dynamics. We found that the chemical modification has a high impact on the triplet formation and leads to shorter dynamics; hence it speeds up the singlet fission process. A faster relaxation from the singlet into the triplet manifold implies a higher efficiency because other relaxation channels are avoided. The air-stable aza-derivatives have the potential to exceed the energy conversion efficiency of TIPS-pentacene.

Mapping the Relation between Stacking Geometries and Singlet Fission Yield in a Class of Organic Crystals.

By generating two free charge carriers from a single high-energy photon, singlet fission (SF) promises to significantly improve the efficiency of a class of organic photovoltaics (OPVs). However, SF is generally a very inefficient process with only a small number of absorbed photons successfully converting into triplet states. In this Letter, we map the relation between stacking geometry and SF yield in crystals based on perylenediimide (PDI) derivatives. This structure-function analysis provides a potential explanation for the SF yield discrepancies observed among similar molecular crystals and may help to identify favorable geometries that lead to an optimal SF yield. Exploring the subtle relationship between stacking geometry and SF yield, this Letter suggests using crystal structure engineering to improve the design of SF-based OPVs.

The Calorimetric Detection of Excited States.

Calorimetric techniques offer the photophysicist and photochemist the opportunity to measure a number of parameters of excited states which may be difficult to obtain by other techniques. The calorimetric strategy seeks to measure the heating of a sample resulting from radiationless decays or chemical reactions of excited states. Heating is best measured through volume and pressure transducers, and four calorimeters based on these are described. With calorimetric instrumentation one can perform measurements on samples in the gas, liquid and solid phases over a wide temperature range. Moreover time dependent processes with time constants ranging from microseconds to seconds are amenable to study. Examples of the application of calorimetric techniques to the determination of quantum yields of fluorescence, triplet formation and photochemistry are given.