1.3- Butadienes excited singlet states

The fact that dimers (10)—(12), produced in the direct photolysis of butadiene, were not observed in the photosensitized experiments may indicate that these are derived from the excited singlet state of butadiene/ ... 

Diphenyl-1,3-butadiene. The excited-state behavior of this diene differs significantly from stilbene and is the subject of a review. Unlike tS in which the lowest vertical excited singlet state is the 1 B state and S2 is the 2 Ag state in solution, these two excited states lie very close to each other in all-trans-1,4-diphenyl-1,3-butadiene (DPB). The additional carbon-carbon double bond introduces a new conformational equilibrium involving the s-trans and s-cis rota-mers. Most spectroscopic studies in solution have concluded that the l B state is S. The DPB compound has a low quantum yield for photoisomerization, so the use of DPB in time-resolved spectroscopic studies on photoisomerization, especially those that monitor only fluorescence decay, needs to be considered cautiously and critically. 

A test of the basis-set effects on the excitation energy and transition moments of vertically excited singlet states in pyrazine has been published [43]. Table IV shows some recent data on the basis-set dependency of the excitation energy for the lowest vertically excited singlet states of neutral butadiene and its positive and negative ions. In accord with the previous results, we conclude that basis sets of the contracted size (4s3pld/2s)... 

From experiment one merely knows that the photochemical cyclization of butadiene starts from an excited singlet state and terminates at the ground state of the cyclobutene appropriate labelling indicates the cyclization to be stereospecific. What intervenes between start and finish is not clear, but it is certainly not excited singlet cyclobutene. What seems more likely is that at... 

This photorearrangement was not sensitized by triplet sensitizers such as benzophenone and MK and was not quenched by molecular dioxygen and 2-methyl-1,3-butadiene, and thus a singlet mechanism is proposed. The homolysis of the C1-C2 bond of 7 from the excited singlet state of 7 generates the 1,3-singlet... 

The red line follows the progress of the reaction path. First, a butadiene compound b excited into its first excited state (either the cis or trans form may be used—we will be considering the cis conformation). What we have illustrated as the lower excited state is a singlet state, resulting from a single excitation from the HOMO to the LUMO of the n system. The second excited state is a Ag state, corresponding to a double excitation from HOMO to LUMO. The ordering of these two excited states is not completely known, but internal conversion from the By state to the Ag state i.s known to occur almost immediately (within femtoseconds). 

The first electronic transition in butadiene has been the subject of many experimental and theoretical studies.The absorption, which has a maximum at 2100 A., is strong and represents a tt tt transition from a ground singlet to an upper singlet state. Analysis of the spectrum, which shows very little structure, has not been carried out. Since no fluorescent radiation has ever been detected on excitation of any of the simple dienes even at low temperature, a definite assignment of the 0 — 0 band has not been made. The 0 — 0 band had been placed at 2300 A. (124 kcal./mole), at which point the absorption is only /so as intense as at its maximum. The oscillator strength is 0.53, which leads to a radiative lifetime of 10 sec. Since emission of radiation has not... 

The efficiency in the transfer of energy from the triplet states of various compounds to ground singlet states of isoprene, 1,3-pentadiene, and 2,3-dimethylbutadiene have been studied. The diene molecules become excited to their lowest triplet states and undergo chemical reactions which may be dimerization or stereoisomerization. As in the case of butadiene, the energy level of the sensitizer in its lowest triplet state determines the magnitude of the energy that is transferred to the diene and the course of its subsequent reactions. Typically, in the case of... 

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