Ž. The reaction dynamics of the photochromic ring-opening reaction of 1,2-bis 5-formyl-2-methyl-thien-3-yl perfluoro-Ž . cyclopentene CHO-BMTFP in dichloromethane solution was investigated using femtosecond transient absorption spectroscopy. The data were analyzed in terms of a model potential and single-electron density matrices, which were calculated Ž . using the collective electronic oscillator CEO approach and the INDOrS semiempirical Hamiltonian. The S -S and 0 1 S -S transitions of the closed isomer were resonantly excited using 120 fs pump pulses at 610 and 410 nm, respectively. A 0 2 temporally delayed white light continuum probe pulse monitors the decay of the S or S state as well as the recovery of the 1 2 S state. Within the first picosecond after excitation, CHO-BMTFP was observed to undergo a fast structural relaxation 0 along the S potential energy surface into a minimum constituting a precursor of the ring-opening process. The rather long 1 lifetime of the precursor, t s 13 ps, was consistent with the calculated potential barrier in front of the conical intersection 2 with the S potential energy surface, which may arise from stabilization of the nearly planar closed isomer by an efficiently 0 delocalized p-electron system. q

A series of 2‐chloro‐1,3,2‐diazaphospholenes with different substitution patterns has been prepared from 1,4‐diazabutadienes according to a general synthetic methodology. Subsequent chloride abstraction with Lewis acids affords the corresponding 1,3,2‐diazaphospholenium salts. All products have been characterised spectroscopically and by single‐crystal X‐ray diffraction studies. A detailed analysis of trends in the structural parameters supports the interpretation that the unusual P–Cl bond lengthening in the diazaphospholenes is attributable to n(N)/σ*(P‐Cl) hyperconjugation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

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