We performed a detailed theoretical analysis of femtosecond transient infrared spectra to determine
excited-state structures involved in photoinduced intramolecular charge transfer (ICT) in 4-(dimethylamino)benzonitrile (DMABN). For comparison, 4-aminobenzonitrile (ABN) is studied. We present the first ab initio
CASSCF study with all states under consideration being fully optimized. We derive two different models for
the locally excited (LE) states: a planar and a novel pyramidal conformation. Three different mechanisms are
treated for the ICT state formation: the twisted ICT (TICT), the pseudo-Jahn−Teller ICT (PICT), and the
rehybridized ICT (RICT) models. By use of this combined theoretical and experimental approach, we can
evaluate the respective models and thus provide new insight into the ICT process. We assign a pyramidal LE
state to ABN, and, in contrast, a planar LE state to DMABN. We can conclusively rule out RICT as the ICT
mechanism in DMABN. Although our results favor TICT as the ICT mechanism in DMABN, a final statement
cannot be made. We predict, however, that determination of the position of the, as yet, unobserved phenyl−amino stretching vibration will substantiate a definitive explanation of the ICT mechanism in DMABN.
We report the first time-resolved site-specific mid-infrared study of the photo-induced excited state hydrogen transfer reaction in 2-(2'-hydroxyphenyl)benzothiazole (HBT) with 130 fs time resolution. The transient absorption of the C=O stretching band marking the keto*-S1-state appears delayed on a time scale of 30-50 fs after electronic excitation to the enol*-S1-state. Its line center subsequently shifts up by about 3-5 cm(-1) after excitation, depending on the excitation wavelength tuned between 315 and 349 nm. This effect is attributed to intramolecular vibrational energy redistribution (IVR) and vibrational energy relaxation (VER) processes. We observe for the first time the coherent effects of anharmonic coupling of low frequency modes (approximately 60 cm(-1), approximately 120 cm(-1)), on the C=O mode marking the product state. We ascribe the 120 cm(-1) mode to a Raman-active in-plane deformation mode that is coherently excited by the UV-pump pulse. We tentatively explain the coherent excitation of the infrared active 60 cm(-1) out-of-plane deformation mode by nonradiative processes within the excited enol state after electronic excitation.
We present three-dimensional microfluidic structures with integrated optical fibers, mirrors and electrodes for flow cytometric analysis of blood cells. Ultraprecision milling technique was used to fabricate different flow cells featuring single-stage and two-stage cascaded hydrodynamic focusing of particles by a sheath flow. Two dimensional focussing of the sample fluid was proven by fluorescence imaging in horizontal and vertical directions and found to agree satisfactorily with finite element calculations. Focussing of the sample stream down to 5 microm at a particle velocity of 3 m s(-1) is accessible while maintaining stable operation for sample flow rates of up to 20 microL min(-1). In addition to fluorescence imaging, the micro-flow cells were characterised by measurements of pulse shapes and pulse height distributions of monodisperse microspheres. We demonstrated practical use of the microstructures for cell differentiation employing light scatter to distinguish platelets and red blood cells. Furthermore, T-helper lymphocytes labelled by monoclonal antibodies were identified by measuring side scatter and fluorescence.
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