Abstract:Steady-state photolysis of haloanthracenes (XA; 9,10-dichloro, 9,10-dibromo, 9-chloro and 9-bromo compounds) in acetonitrile-amine (triethylamine or N,N-dimethylaniline) causes the consecutive reactions; 9,10-dihaio compounds-*9-halo compounds-, anthracene. Although both the lowest excited singlet [~XA(S1)] and triplet [3XA(T1)] states of XA are quenched by amine, the appearance of absorption spectra due to the haloanthracene radical anions (XA"-) within the duration of nanosecond pulse excitation indicates th… Show more
“…It was proposed that the intermediates for the photoinduced dehalogenation of aromatic halo compounds in the presence of amine were the radical anions of halo compounds generated by decomposition of exciplexes formed between the lowest excited singlet states of halo compounds and ground-state amine. − By means of nanosecond laser photolysis and steady-state photolysis, we thus studied photoinduced debromination of haloanthracenes (XA, i.e., the 9-bromo, 9,10-dibromo, 9-chloro, and 9,10-dichloro compounds) in acetonitrile containing N , N -dimethylaniline (DMA) or triethylamine (TEA). − Although no absorption bands responsible for the lowest excited singlet state [XA*(S 1 )] of XA and an exciplex [ 1 (XA-amine)*] of XA*(S 1 ) with amine could be detectable, following scheme (Scheme ) was proposed: i.e., (1) the intermediate for photoinduced dehalogenation of XA was its radical anion (XA •- ) generated by decomposition of 1 (XA−amine)* which is formed by a diffusion-controlled reaction of XA*(S 1 ) with amine; (2) since the lowest excited triplet state [XA*(T 1 )] of XA in the absence of amine was populated via the indirect XA*(S 1 ) → XA*(T n ) → XA*(T 1 ) intersystem crossing through an adjacent higher excited triplet state [XA*(T n )], − amine-assisted population of XA*(T 1 ) was ascribed to the intersystem crossing from 1 (XA−amine)* to its triplet exciplex [ 3 (XA−amine)*] followed by rapid decomposition into XA*(T 1 ) and amine. 1 …”
Picosecond laser photolysis reveals the formation of an exciplex between the lowest excited singlet state [DBA*(S 1 )] of 9,10-dibromoanthracene (DBA) and ground-state amine in acetonitrile (CH 3 CN) and ethanol (EtOH) containing N,N-dimethylaniline (DMA) [and n-heptane (HP) containing DMA or triethylamine (TEA)]. Only in CH 3 CN-DMA, however, can decomposition of the DBA-DMA exciplex into the DBA radical anion (DBA •-) and the DMA radical cation be seen, indicating very small generation of DBA •-from the DBA-amine exciplex formed in EtOH-DMA and HP-amine (DMA or TEA). Interestingly, no DBA-TEA exciplex is formed in CH 3 CN and EtOH containing TEA but nanosecond laser photolysis reveals the existence of DBA •-in the former solvent. Furthermore, the rate of DBA f 9-bromoanthracene debromination upon steady-state photolysis in CH 3 CN-TEA is 1 order of magnitude smaller than that in CH 3 CN-DMA but 2 or 3 orders of magnitude greater than those in EtOH and HP containing amine. This suggests that diffusion-controlled quenching of DBA*(S 1 ) by TEA in CH 3 CN and EtOH gives rise to the formation of a nonemissive short-lived encounter complex or ion pair. It thus be concluded that generation of DBA •-from the DBA-amine exciplex or the encounter complex (or the ion pair) is affected by the dielectric constant of a pure solvent.
“…It was proposed that the intermediates for the photoinduced dehalogenation of aromatic halo compounds in the presence of amine were the radical anions of halo compounds generated by decomposition of exciplexes formed between the lowest excited singlet states of halo compounds and ground-state amine. − By means of nanosecond laser photolysis and steady-state photolysis, we thus studied photoinduced debromination of haloanthracenes (XA, i.e., the 9-bromo, 9,10-dibromo, 9-chloro, and 9,10-dichloro compounds) in acetonitrile containing N , N -dimethylaniline (DMA) or triethylamine (TEA). − Although no absorption bands responsible for the lowest excited singlet state [XA*(S 1 )] of XA and an exciplex [ 1 (XA-amine)*] of XA*(S 1 ) with amine could be detectable, following scheme (Scheme ) was proposed: i.e., (1) the intermediate for photoinduced dehalogenation of XA was its radical anion (XA •- ) generated by decomposition of 1 (XA−amine)* which is formed by a diffusion-controlled reaction of XA*(S 1 ) with amine; (2) since the lowest excited triplet state [XA*(T 1 )] of XA in the absence of amine was populated via the indirect XA*(S 1 ) → XA*(T n ) → XA*(T 1 ) intersystem crossing through an adjacent higher excited triplet state [XA*(T n )], − amine-assisted population of XA*(T 1 ) was ascribed to the intersystem crossing from 1 (XA−amine)* to its triplet exciplex [ 3 (XA−amine)*] followed by rapid decomposition into XA*(T 1 ) and amine. 1 …”
Picosecond laser photolysis reveals the formation of an exciplex between the lowest excited singlet state [DBA*(S 1 )] of 9,10-dibromoanthracene (DBA) and ground-state amine in acetonitrile (CH 3 CN) and ethanol (EtOH) containing N,N-dimethylaniline (DMA) [and n-heptane (HP) containing DMA or triethylamine (TEA)]. Only in CH 3 CN-DMA, however, can decomposition of the DBA-DMA exciplex into the DBA radical anion (DBA •-) and the DMA radical cation be seen, indicating very small generation of DBA •-from the DBA-amine exciplex formed in EtOH-DMA and HP-amine (DMA or TEA). Interestingly, no DBA-TEA exciplex is formed in CH 3 CN and EtOH containing TEA but nanosecond laser photolysis reveals the existence of DBA •-in the former solvent. Furthermore, the rate of DBA f 9-bromoanthracene debromination upon steady-state photolysis in CH 3 CN-TEA is 1 order of magnitude smaller than that in CH 3 CN-DMA but 2 or 3 orders of magnitude greater than those in EtOH and HP containing amine. This suggests that diffusion-controlled quenching of DBA*(S 1 ) by TEA in CH 3 CN and EtOH gives rise to the formation of a nonemissive short-lived encounter complex or ion pair. It thus be concluded that generation of DBA •-from the DBA-amine exciplex or the encounter complex (or the ion pair) is affected by the dielectric constant of a pure solvent.
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