Photochemical synthesis of 2,3,9,10-tetrabromopentacene: its unusual photodimerization.

The photogeneration of 2,3,9,10-tetrabromopentacene from its precursor dione by photodecarbonylation was investigated. An unusual photodimer, 3,3',9,9',10,10'-hexabromo-2,2'-bipentacene, is produced.

Analysis of the time-resolved FTIR spectra produced by the photolysis of alkyl phenylglyoxylates.

The photochemistry of alkyl phenylglyoxylates (APG) was further investigated using time-resolved infrared spectroscopy. The primary focus was on the analysis of weak transient bands around 1828 and 1730 cm(-1) in the time-resolved FTIR spectra of glyoxylates. The observed transients were assigned to benzoyl and alkyl mandelate ester radicals, respectively.

Observation of the two triplet state conformations of alkyl phenylglyoxylates.

Step-scan time-resolved FT-IR spectra of alkyl phenylglyoxylates in hexane with a 4 cm(-1) spectral resolution reveal splitting of the transient absorption signal near 1650 cm(-1) into two closely located peaks with different lifetimes on the nanosecond time scale. This signal had been previously assigned to the triplet state of the starting material that gives rise to the alpha-hydroxyphenylketene. In the current article, evidence is presented to assign these two peaks to different triplet state conformers only one of which undergoes fast Norrish Type II photoelimination.

Dynamics of the excited states of chromenes studied by fast and ultrafast spectroscopies.

In this paper, the photodynamics of three chromenes (2,2-spiro-adamantyl-7,8-benzo(2H)chromene, 2,2-diphenyl(2H)chromene and 2,2-diphenyl-5,6-benzo(2H)chromene) has been investigated by nano- and femtosecond time resolved techniques in hydrocarbon solutions at room temperature. Using pump-probe techniques, ultrafast steps of molecular dynamics characterizing the photoresponse of chromenes have been investigated: the breakage of the C-O bond was found to occur within a few hundred femtoseconds producing a short-lived transient that converts to the cisoid-cis open form in less than one picosecond and then to the metastable transoid-cis structure within a few picoseconds. The effect of different excitation wavelengths on the formation rate of the first shortest-lived transient of 5,6DPBC is in agreement with the model previously proposed to explain the wavelength dependence of the reaction and emission quantum yields in photostationary experiments.

Observations of different triplet conformations in time-resolved infrared spectra of alkyl phenylglyoxylates.

Two different conformations of the triplet state of alkyl phenylglyoxylates were observed by means of time-resolved step-scan FT-IR spectroscopy. The amplitude of the peak corresponding to the sterically hindered conformation decreases as the size of the alkyl group increases. Both conformations exhibit similar reactivity in intermolecular hydrogen abstraction, but only one of them undergoes Norrish Type II photoelimination.