@article{StrehmelSarkerLahtietal.2005, author = {Strehmel, Veronika and Sarker, A. M. and Lahti, P. M. and Karasz, F. E. and Heydenreich, Matthias and Wetzel, Hendrik and Haebel, Sophie and Strehmel, Bernd}, title = {One- and two-photon photochemistry and photophysics of poly(arylenevinylene)s containing a biphenyl moiety}, issn = {1439-4235}, year = {2005}, abstract = {Photochemical and photophysical properties were investigated for poly(arylenevinylene)s containing a flexible biphenyl "hinge" unit by applying one-photon (OP) and two-photon (TP) excitation to explore excited-state properties. The poly(arylenevinylene)s were poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(4,4'-dihexyloxy-3,3'-biph enylenevinylene)] (1), poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2'-dihexyloxy-3,3'-biph enylenevinylene)] (2), and poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2'-biphenylene-vinylene )] (3). Effective emission quantum yields and related photonic properties were evaluated on a realistic per-chromophore basis using effective conjugation lengths based on the Strickler-Berg relationship. intramolecular photocyclization was deduced to occur in the one case where the biphenyl molecular connectivity permitted the reaction, based on matrix- assisted loser desorption/ionization time-of-flight (MALDI-TOF), heteronuclear multiple-quantum coherence (HMQC)-NMR, and gel-permeation chromatography (GPC) results. The various photoprocesses could be induced by either OP or TP excitation, though the first excited singlet state is the photoactive state. The higher excitation energy 1 of the TP excited state favors indirect population of the S, state by electronic coupling between the TP and OP excited states [lambda(max)(TPE) (nm): 726; delta (GM)([9]): 1 = 229, 2 = 215, 3 = 109). Photochemical processes occurring from the lowest OP excited state (S-1) could therefore also be indirectly induced by TP excitation}, language = {en} } @article{StrehmelSarkerLahtietal.2004, author = {Strehmel, Veronika and Sarker, A. M. and Lahti, P. M. and Karasz, F. E. and Strehmel, Bernd}, title = {One and two-photon optical properties of ionic block copolymers with phenylenediethylenebispyridinium- chromophores}, issn = {0065-7727}, year = {2004}, language = {en} } @misc{KaafaraniWexStrehmeletal.2002, author = {Kaafarani, Bilal R. and Wex, Brigitte and Strehmel, Bernd and Neckers, Douglas C.}, title = {Structural concept for fluorinated Y-enynes with solvatochromic properties}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-13168}, year = {2002}, abstract = {An approach to the development of fluorescent probes to follow polymerizations in situ using fluorinated cross-conjugated enediynes (Y-enynes) is reported. Different substitution patterns in the Y-enynes result in distinct solvatochromic behavior. β,β-Bis(phenylethynyl)pentafluorostyrene 7, which bears no donor substituents and only fluorine at the styrene moiety, shows no solvatochromism. Donor substituted β,β-bis(3,4,5-trimethoxyphenylethynyl) pentafluorostyrene 8 and β,β-bis(4-butyl-2,3,5,6-tetrafluorophenylethynyl)-3,4,5-trimethoxystyrene 9 exhibit solvatochromism upon change of solvent polarity. Y-enyne 8 showed the largest solvatochromic shift (94 nm bathochromic shift) upon changing solvent from cyclohexane to acetonitrile. A smaller solvatochromic response (44 nm bathochromic shift) was observed for 9. Lippert-Mataga treatment of 8 and 9 yields slopes of -10,800 and -6,400 cm -1, respectively. This corresponds to a change in dipole moment of 9.6 and 6.9 D, respectively. The solvatochromic behavior in 8 and 9 supports the formation of an intramolecular charge transfer (ICT) state. The low fluorescence quantum yields are caused by competitive double bond rotation. The fluorescence decay time of 9 decreases in methyltetrahydrofuran from 2.1 ns at 77 K to 0.11 ns at 200 K. Efficient single bond rotation in 9 was frozen at -50 °C in a configuration in which the trimethoxyphenyl ring is perpendicular to the fluorinated rings. 7-9 are photostable compounds. The X-ray structure of 7 shows it is not planar and that its conjugation is distorted. Y-enyne 7 stacks in the solid state showing coulombic, actetylene-arene, and fluorine-π interactions.}, language = {en} } @article{StrehmelWishartPolyanskyetal.2009, author = {Strehmel, Veronika and Wishart, James F. and Polyansky, Dmitry E. and Strehmel, Bernd}, title = {Recombination of photogenerated lophyl radicals in imidazolium-based ionic liquids}, issn = {1439-4235}, doi = {10.1002/cphc.200900594}, year = {2009}, abstract = {Laser flash photolysis is applied to study the recombination reaction of lophyl radicals in ionic liquids in comparison with dimethylsulfoxide as an example of a traditional organic solvent. The latter exhibits a similar micropolarity as the ionic liquids. The ionic liquids investigated are 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (1), 1-hexyl-3-methylimidazolium hexafluorophosphate (2), and 1-butyl-3- methylimidazolium tetafluoroborate (3). The recombination of the photolytic generated lophyl radicals occur significantly faster in the ionic liquids than expected from their macroscopic viscosities and is a specific effect of these ionic liquids. On the other hand, this reaction can be compared with the macroscopic viscosity in the case of dimethylsulfoxide. Activation parameters obtained for lophyl radical recombination suggest different, anion-dependent mechanistic effects. Quantum chemical calculations based on density functional theory provide a deeper insight of the molecular properties of the lophyl radical and its precursor. Thus, excitation energies, spin densities, molar volumes, and partial charges are calculated. Calculations show a spread of spin density over the three carbon atoms of the imidazolyl moiety, while only low spin density is calculated for the nitrogens.}, language = {en} } @article{StrehmelBerdzinskiStrauchetal.2014, author = {Strehmel, Veronika and Berdzinski, Stefan and Strauch, Peter and Hoffmann-Jacobsen, Kerstin and Strehmel, Bernd}, title = {Investigation of molecular solvents and ionic liquids with a dual probe}, series = {Zeitschrift f{\"u}r physikalische Chemie : international journal of research in physical chemistry and chemical physics}, volume = {228}, journal = {Zeitschrift f{\"u}r physikalische Chemie : international journal of research in physical chemistry and chemical physics}, number = {2-3}, publisher = {De Gruyter}, address = {Berlin}, issn = {0942-9352}, doi = {10.1515/zpch-2014-0453}, pages = {155 -- 169}, year = {2014}, abstract = {A dual probe was investigated by UV-Vis, fluorescence, and ESR spectroscopy. It comprises the pyrene chromophore and the paramagnetic 2,2,6,6-tetramethylpiperidinyl-N-oxyl radical that are covalently linked together via an ester bridge. The dual probe was used to investigate molecular solvents of different polarity as well as ionic liquids bearing either imidazolium or pyrrolidinium cations and various anions, such as bis(trifluoromethylsulfonyl)imide, tetrafluoroborate, tris(pentafluoroethyl)trifluorophosphate, or dicyanamide. The dual probe does not show solvatochromism that is typical for some pyrenes. Furthermore, the dual probe is considerable less mobile compared to 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) without additional substituent as detected by ESR spectroscopy. This is caused by the bulky pyrenyl substituent bound at the dual probe resulting in a reduced mobility of the dual probe.}, language = {en} } @article{Strehmel2001, author = {Strehmel, Bernd}, title = {Fluorescence Probes for Material Science}, year = {2001}, language = {en} } @article{StrehmelHenbestSarkeretal.2001, author = {Strehmel, Bernd and Henbest, K. B. and Sarker, A. M. and Malpert, J. H. and Chen, D. Y. and Rodgers, M. A. J. and Neckers, D. C.}, title = {Ion induced man ipulation of photochemical pathways in crown ether compounends based on fluorinated oligophenylenevinylenes}, publisher = {Journal of nanoscience and nanotechnology. - 1 (2001), 1, S. 107 - 124}, year = {2001}, language = {en} } @book{StrehmelSchraderSeifert1999, author = {Strehmel, Bernd and Schrader, Sigurd and Seifert, H.}, title = {Applied fluorescence in chemistry, biology and medicine}, editor = {Rettig, W.}, publisher = {Springer}, address = {Berlin}, isbn = {3-540-64451-2}, pages = {XIX, 562 S. : Ill., graph. Darst}, year = {1999}, language = {en} }