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.

Aggregation controlled photoluminescence of hexaazatri-naphthylene (HATN) - an experimental and theoretical study.

Photophysics of hexaazatrinaphthylene (HATN) in solution and in the solid state is determined by the nπ* character of its lowest excited singlet state and by the ππ* character of the first triplet state. The relaxation of an electronically excited state is dominated by nonradiative relaxation channels and only very weak fluorescence is observed for HATN monomers in solution. In liquid solution, the fluorescence quantum yield is 3-7 × 10-4 and the triplet formation yield ranges from 0.21 to 0.41. In methanol, the photodegradation of the molecule is observed. This molecule with a large delocalised π-electron system easily aggregates forming dimers in solution and in some crystalline forms. The band shifts observed in crystals as well as excitonic splitting in dimers provide evidence for a strong interaction in π-π stacked aggregates. The mixing of electronic states in aggregates changes their character and affects photophysical properties. The obtained experimental data are in good agreement with theoretical calculations performed using the ADC(2) method.

Electron transfer across a spiro link: extreme solvatofluorochromism of a compact spiro-bridged N,N-dimethylaniline-phthalide dyad.

Intramolecular electron transfer over the sp3-C-link in dimethyl-aniline-spiro-phthalide leads to extreme solvatofluorochromism (11 600 cm-1) and a fluorescence maximum shift from 357 (hexane) to 595 nm (acetonitrile). The spiro link enhances π-σ* negative hyperconjugation leading to C-O bond elongation and electron transfer over a longer distance than in a non-spirocyclic analogue.

Intramolecular Electron Transfer in Frozen Solvents: Charge Transfer and Local Triplet States Population Dynamics Revealed by Dual Phosphorescence.

In frozen solvents at 77 K, ultrafast (≤250 fs) photoinduced intramolecular electron transfer (ET) in bichromophoric donor-acceptor ([D-A]) diarylmethane lactones produces a covalently linked radical ion pair, 1[D•+-A•-]. Steady state and time-resolved luminescence measurements reveal that 1[D•+-A•-] decays to charge-separated (3[D•+-A•-]) and donor-centered ([3D*-A]) triplets, which display dual phosphorescence. 3[D•+-A•-] and [3D*-A] are formed in parallel via two intersystem crossing mechanisms: spin orbit charge transfer (SOCT) and hyperfine coupling (HFC), with solvent dependent branching ratio. The solvent drives the D-A alignment during the freezing process to adapt to increasing solvent polarity, producing inhomogeneous ground-state population distribution with solvent-dependent D-A exchange interaction, which plays a key role in partitioning into SOCT and HFC mechanisms. In polar glasses, a third phosphorescence band appears due to dissociative back ET in 3[D•+-A•-] resulting in excited open ring biradical.

Tuneable white fluorescence from intramolecular exciplexes.

Crystal violet lactone (CVL) in solution displays unusually broad (FWHM > 9100 cm(-1)) dual fluorescence with the characteristics of white light. The emission combines a blue CT band from a local chromophore with an orange CT band from an intramolecular exciplex formed adiabatically at appropriate medium polarity. The fluorescence spectrum can be controlled by solvent polarity to yield tuneable emission colours in a broad range of coordinates in the CIE chromaticity diagram including the white region. We show that such dual emission is a general property of CVL-like D-A structures built on sp(3) carbon atoms. The dependence of excited state energetics on molecular structure allows the prediction of width, shape and other parameters of the dual fluorescence spectrum, and so enables the engineering and customised design of white fluorophores. The photophysics-structure relationship found for CVL and its analogues can be generalized into a novel concept of white light generation by small molecules. These D-A systems are studied as a template basis for design and development of white fluorophores.

The key role of solvation dynamics in intramolecular electron transfer: time-resolved photophysics of crystal violet lactone.

The intramolecular electron-transfer reaction in crystal violet lactone in polar aprotic solvents is studied with femtosecond transient absorption spectroscopy. The initially excited charge transfer state (1)CT A is rapidly converted into a highly polar charge transfer state (1)CT B. This ultrafast electron transfer is seen as a solvent-dependent dual fluorescence in steady-state spectra. We find that the electron-transfer process can be followed by a change from a double-peaked transient absorption spectrum to a single-peak one in the low picosecond range. The transient absorption kinetic curves are multiexponential, and the fitted time constants are solvent dependent but do not reproduce the known solvation times. For 6-dimethylaminophthalide, the optically active constituent of crystal violet lactone, only a small temporal evolution of the spectra is found. To explain these findings, we present a model that invokes a time-dependent electron-transfer rate. The rate is determined by the instantaneous separation of the two charge-transfer states. Because of their differing dipole moments, they are dynamically lowered to a different extent by the solvation. When they temporarily become isoenergetic, equal forward and backward transfer rates are reached. The intrinsic electron-transfer ( (1)CT A --> (1)CT B) reaction is probably as fast as that in the structurally analogous malachite green lactone (on the 100 fs time scale). The key element for the dynamics is therefore its control by the solvent, which changes the relative energetics of the two states during the solvation process. With further stabilization of the more polar state, the final equilibrium in state population is reached.

Photoinduced intramolecular charge transfer to meta position of benzene ring in 6-aminophthalides.

Substitution of non-fluorescent phthalide (Pd) with amino group at meta (6) position in relation to the electron-accepting part of the lactone ring completely changes Pd photophysics: a new long-wavelength absorption band arises and the molecule becomes highly fluorescent. The experimental data and the analysis of vertical electronic transitions with TDDFT method indicate that the first absorption band in 6-aminophthalides (6-APds) comprises a single CT transition to the S1 state. Almost equal absorption and emission transition dipole moments indicate that S0 <--> S1 transition in all 6-APds is not affected by any mixing with other electronic states, the excited-state vibrational relaxation is not accompanied by significant conformational changes and the Stokes shifts reflect mainly solvation energetics of these molecules. Excited state dipole moments obtained from solvatochromic plots and from CASSCF calculations confirm large charge displacement from amino group towards the meta position of the benzene ring upon excitation of 6-APds to S1 state. Long fluorescence lifetimes and high fluorescence quantum yields demonstrate efficient and stable excited state charge separation in 6-APds. Taken together with sensitivity of 6-APds to polarity and proticity of the environment these properties make them good candidates for fluorescent probes of long-time scale molecular dynamics.

A kinetic description of dioxygen motion within alpha- and beta-subunits of human hemoglobin in the R-state: geminate and bimolecular stages of the oxygenation reaction.

Laser flash photolysis technique is used to study human hemoglobin (HbA) oxygenation. Monomolecular geminate oxygenation of triliganded R-state HbA molecules is described by a function of three exponentials. Geminate oxygenation of the alpha-subunit within R-state HbA is characterized by two components with time constants of 0.14 and 1 ns, while geminate oxygenation of the beta-subunit within HbA is characterized by two components with time constants of 1 and approximately 30 ns. Bimolecular oxygenation of triliganded R-state HbA molecules is described by a biexponential law. Two observed rate constants are assigned to oxygenation of the alpha- and beta-subunit within HbA. The bimolecular association rate constants for O(2) rebinding with the alpha- and beta-subunit within triliganded R-state HbA are k(alpha) = 18.8 +/- 1.3 (microM x s)(-1) and k(beta) = 52 +/- 4 (microM x s)(-1), respectively. The apparent quantum yields of photodissociation of the beta- and alpha-subunit within completely oxygenated R-state HbA differ from each other by a factor of 3.6 and are equal to 0.041 +/- 0.004 and 0.0114 +/- 0.0012, respectively. The apparent quantum yield of photodissociation of completely oxygenated R-state HbA is equal to 0.026 +/- 0.003.

Determination of triplet formation efficiency from kinetic profiles of the ground state recovery.

A procedure is presented which allows determination of the triplet formation efficiency from the analysis of transient ground state bleaching curves obtained after short laser pulse excitation. Crucial factors that control the applicability and the accuracy of the method are discussed, such as a proper choice of the detection wavelength, which allows avoidance of interference from transient absorption. The technique is tested on porphycene and octaethylporphyrin and applied for several porphyrin derivatives used as photodynamic therapy agents.