Christophe Lefumeux

Vibrational and structural analysis of the radical cation of N,N,N′,N′-tetramethylbenzidine based on ab initio calculations and time-resolved resonance Raman spectroscopyElectronic supplementary information (ESI) available: Potential energy distributions (PED) expressed in terms of the local symmetry coordinates for each isotopomer (Table 1S and 3S); description of the symmetry coordinates (Table 2S) and related internal coordinates (Fig. 1S). See http://www.rsc.org/suppdata/cp/b2/b210837m/

New time-resolved resonance Raman spectra of the radical cation of N,N,N′,N′-tetramethylbenzidine (TMB) in solution have been probed at 752 nm, in resonance with a strong electronic transition, for several isotopic derivatives. These spectra and those obtained previously at 488 nm, in resonance with a second electronic transition, have been interpreted by means of the density functional theory (B3LYP) calculations. An unambiguous assignment of the observed vibrations has been deduced. For comparison, similar calculations have been done for the parent ground state molecule. In addition, configuration interaction singles (CIS) calculations have been used to assign the two resonant electronic excited states of the radical cation and identify the molecular orbitals involved in the corresponding transitions. On this basis, the Raman intensities of the radical cation in resonance with these two transitions have been evaluated in the Franck–Condon approximation by projecting the excited state potential energy surface gradients, determined from CASSCF calculations, on the ground state normal modes. The analysis of both the vibrational frequencies and resonance Raman intensities suggests that the TMB radical cation adopts a coplanar structure (D2h point group).

The photophysics of phenyl disubstituted polyacetylenes: picosecond photoluminescence and femtosecond photoinduced absorption studies of PDPA-n-Bu

We have obtained picosecond (ps) photoluminescence (PL) decays of phenyl disubstituted polyacetylenes (PDPA) with an attached n-butyl group (PDPA-n-Bu) in solution and as thin films. In solution, we observe only a single exponential decay at all emission wavelengths over the range from 2.15 to 2.50 eV. The PL lifetime in solution is 190 ps. The PL decays in thin films cannot be fit well using a single exponential, and are significantly longer in thin films than in solution. In addition, the PL decay times vary with emission wavelength in thin films with longer PL decay times observed at lower emission energies. We have also obtained the femtosecond (fs) photoinduced absorption (PA) spectra of PDPA-n-Bu in solution over the range from 1.70 to 2.95 eV. These data exhibit fast ground state bleaching at high energy, along with the growth of a broad PA feature at low energies that decays with a time constant consistent with the PL decay time in solution. At long times (>200 ps) we observe the growth of a long-lived PA feature peaked at 2.3 eV that persists for hundreds of nanoseconds. We also observe a broad feature in the infrared PA at 0.23 eV whose dynamics are the same as the band at 2.3 eV. We assign these two long-lived bands to polaron transitions.

Towards STED microscopy with nanometric optical sectioning

Circumventing the limit imposed by diffraction is a major issue in the instrumental development to realize finer resolutions in biological samples. With STED microscopy, we exploit the molecular transitions of the fluorescent marker to image well below the Rayleigh criterion. Also in combination with STED, we propose to use an alternative technique for optically sectioning fluorescent emitters close to the water-glass interface by selectively filtering the supercritical emission at the pupil plane. We discuss the instrumental development of such a system and its combination with other imaging techniques.