Abstract: Ultrafast photolysis (lambda(ex) = 308 nm) of p-biphenylyltrifluoromethyl diazomethane (BpCN2CF3) releases singlet p-biphenylyltrifluoromethylcarbene (BpCCF3) which absorbs strongly at 385 nm in cyclohexane, immediately after the 300 fs laser pulse. The initial absorption maximum shifts to longer wavelengths in coordinating solvents (nitrile, ether, and alcohol). In low viscosity coordinating solvents, the initial absorption maximum further red shifts between 2 and 10 ps after the laser pulse. Similar effects are observed upon ultrafast photolysis of 2-fluorenyltrifluoromethyl diazomethane (FlCN2CF3) and therefore cannot be associated with torsional motion around the two phenyl rings of the biphenyl compound. Instead, the effect is attributed to the dynamics of solvation of the singlet carbene. The time constant of solvation in normal alcohols lengthens with solvent viscosity in a linear manner. Furthermore, the time constants of the red shift in methanol-O-d (16 ps), ethanol-O-d (26 ps), 2-propanol-OD (40 ps), and 2,2,2-trifluoroethanol-O-d (14 ps) are longer than those recorded in methanol (9.6 ps, KIE = 1.7), ethanol (14.3 ps, KIE = 1.8), 2-propanol (28 ps, KIE = 1.4), and 2,2,2-trifluoroethanol (4.4 ps, KIE = 3.2), which indicates that the solvent reorganization involves formation of hydrogen bonds. The kinetic data are consistent with motion of the solvent to achieve a specific interaction with the carbene, with the creation of a new hydrogen bond. The solvated carbene reacts with the solvent over tens, hundreds, and thousands of ps, depending upon the solvent.
Abstract: The photochemistry of 5-diazo-10,11-dihydrodibenzo[a,d]diazocycloheptene (DDBC) and 9-diazoanthrone (DAN) were studied by ultrafast time resolved techniques. The excited states of these diazo compounds were observed by UV-Vis spectroscopy and were found to decay in 300 fs. The diazo excited state decays led to the appearance and first direct observation of singlet 5-diazo-10,11-dihydrodibenzo[a,d]cycloheptenylidene (DBC) and 9-anthronylidene (AN). The dynamics of DBC and AN were studied in acetonitrile, cyclohexane and methanol. The lifetimes of (1)DBC are 83 ps and 72 ps in acetonitrile and cyclohexane, respectively. The lifetime of (1)DBC shortens to 9 ps in methanol due to rapid reaction with the solvent. The lifetime of (1)AN is 87 ps and 66 ps in acetonitrile and cyclohexane, respectively. In methanol, the lifetime of (1)AN cannot be determined due to spectral overlap of (1)AN and cation ANH+. The decays of (1)DBC and (1)AN are controlled by intersystem crossing (ISC) in acetonitrile and cyclohexane and the rates of ISC of (1)DBC or (1)AN are similar in these two solvents. This differs from the solvent dependence of other diarylcarbene intersystem crossing rates. This is attributed to the relatively large singlet-triplet (S-T) gaps of these carbenes and this factor dominates the influence of solvent.
Abstract: Photoinduced Wolff rearrangements were studied by femtosecond time-resolved UV-vis and IR transient absorption spectroscopy. For BpCN2COCH3 in acetonitrile the IR data indicate the presence of at least two mechanisms of ketene formation. The first process is fast proceeding in either 1BpCN2COCH3*, or in a hot carbene, or in both species, while the second is slow proceeding through the intermediacy of a relaxed carbene. The slow time constant of the ketene formation dynamics obtained by ultrafast IR (700 ps) spectroscopy agrees with the relaxed carbene decay of 800 +/- 100 ps obtained by UV-vis absorption spectroscopy.
Abstract: Time-resolved resonance Raman (TR3) studies of the photochemistry of phenyl azide, 3-hyroxyphenyl azide, 3-methoxyphenyl azide and 3-nitrophenyl azide in acetonitrile:water solutions is reported. After photolysis of these four aryl azides in room temperature solutions, only one species was observed in the TR3 spectra for each azide, respectively at the probe wavelengths employed in the TR3 experiments. The species observed after photolysis of 3-nitrophenyl azide was assigned to 3,3'-dinitroazobenzene, an azo compound formed from the dimerization reaction of triplet 3-nitrophenylnitrene. In contrast, the species observed after photolysis of phenyl azide, 3-hydroxyphenyl azide and 3-methoxyphenyl azide were tentatively assigned to intermediates formed from the dimerization of didehydroazepines that are produced from the ring expansion reaction of the respective singlet arylnitrene. To our knowledge, this is the first time-resolved vibrational spectroscopic observation of the dimerization reaction of didehydroazepines in solution. In addition, these are the first resonance Raman spectra reported for dimers formed from didehydroazepines. We briefly discuss the structures, properties and chemical reactivity of the dimer species observed in the TR3 spectra and possible implications for the photochemistry of aryl azides.
Abstract: The influence of coordinating solvents on singlet-to-triplet carbene intersystem crossing (ISC) rates has been studied with diphenylcarbene (DPC) and para-biphenyltrifluoromethylcarbene (BpCCF 3) by using ultrafast time-resolved spectroscopy. DPC has a triplet ground state in all of the solvents considered, and the concentration of singlet carbene at equilibrium is too small to be measured. It is found that the lifetime of (1)DPC is extended in acetonitrile, benzene, tetrahydrofuran, dichloromethane, and halobenzene solvents relative to cyclohexane. The solvent effect does not well correlate with bulk measures of solvent polarity. The singlet-triplet energy separation of BpCCF 3 is close to zero. The data demonstrates that BpCCF 3 has a triplet ground state in benzene, fluorobenzene, and hexafluorobenzene. Halogenated solvents are found to dramatically retard the rate of ISC in (1)BpCCF 3. We postulate that the empty p orbital of a singlet carbene coordinates with a nonbonding pair of electrons of a halogen atom of the solvent to form a pseudoylide solvent complex, stabilize the singlet carbene, and decrease the singlet-triplet (S-T) energy gap. The "golden rule" of radiationless transitions posits that the smaller the energy gap between the two states, the faster their rate of interconversion. To explain the apparent violation of the golden rule of radiationless transitions for the carbene ISC processes monitored in this study, we propose that the significantly different specific solvation of the singlet and triplet carbenes imposes a Franck-Condon-like factor on the ISC process. Those solvents that most solvate the singlet carbene will also have the greatest structural difference between singlet carbene-solvent complex and their triplet spin isomer-solvent complex, the smallest S-T gap, and the slowest ISC rate. Alternatively, one can propose that a highly solvated singlet carbene must desolvate prior to ISC, and that this requirement decelerates the radiationless transition.
Abstract: p-biphenylyl, o-biphenylyl, and 1-naphthyl azides were deposited in argon at low temperature in the presence and absence of HCl. In the absence of HCl, the known electronic and vibrational spectra of the corresponding triplet nitrenes, azirines, and didehydroazepines were observed, whereas in the presence of HCl, photolysis of these azides produces new electronic spectra assigned to the corresponding nitrenium cations. For p-biphenylyl azide the resulting spectrum of the nitrenium ion is very similar to the previously observed solution-phase spectrum of this species. The vibrational spectrum of this cation was recorded for the first time. Spectroscopic evidence for the previously unknown o-biphenylylnitrenium cation and 1-naphthylnitrenium cation are provided. The spectra of p- and o-biphenylylnitrenium cations and 1-naphthylnitrenium cation are well reproduced by CASSCF and CASPT2 calculations. The same nitrenium cations were detected in solution by femtosecond time-resolved laser flash photolysis of the appropriate azides in 88% formic acid. The transient spectra of the nitrenium cations recorded in solution are in good agreement with the spectra obtained in HCl matrices. The rates of formation of these cations equal the rates of decay of the singlet nitrenes in 88% formic acid and are as follows: p-biphenylyl (taugrowth = 11.5 ps), o-biphenylyl (taugrowth = 7.7 ps), and 1-naphthylnitrenium cations (taugrowth = 8.4 ps). The decay lifetimes of p- and o-biphenylylnitrenium cations are 50 and 27 ns, respectively. The decay lifetimes of 1-naphthylnitrenium cation is 860 ps in 88% formic acid.
Abstract: Ultrafast photolysis of p-biphenylyldiazoethane (BDE) produces an excited state of the diazo compound in acetonitrile, cyclohexane, and methanol with lambdamax = 490 nm and lifetimes of less than 300 fs. The decay of the diazo excited state correlates with the growth of singlet carbene absorption at 360 nm. The optical yields of diazo excited states produced by photolysis of p-biphenylyldiazomethane (BDM) and BDE are the same; however, the optical yield of singlet p-biphenylylmethylcarbene (1BpCMe) is 30-40% less than that of p-biphenylylcarbene (1BpCH) in all three solvents. The results are explained by rearrangement in the excited state (RIES) of BDE to form p-vinylbiphenyl (VB) in parallel with extrusion of nitrogen to form 1BpCMe in reduced yield. This interpretation is consistent with product studies (ethanol-OD in cyclohexane) which indicate that there is an approximately 25% yield of VB that is formed by a mechanism that bypasses the relaxed singlet carbene. The decay of 1BpCMe is biexponential, and that of 1BpCH is monoexponential. This is attributed either to efficient relaxation of vibrationally excited 1BpCMe by 1,2 migration of hydrogen to form VB (minor) or to the increased number of low-frequency vibrational modes provided by the methyl group (major). A methyl group retards the rate of intersystem crossing (ISC), relative to a hydrogen atom, and ISC is more rapid in nonpolar solvents. Reaction of 1BpCMe with methanol is much faster than spin equilibration. Both the lifetime of 1BpCMe and 1BpCH are the same in cyclohexane and in cyclohexane-d12. This demonstrates that spin equilibration is faster than reaction of either carbene with the solvent. The lifetimes of 1BpCMe and 1BpCMe-d3 are the same in cyclohexane. This indicates that 1,2 hydrogen migration of 1BpCMe to form VB is slower than spin equilibration in cyclohexane. In acetonitrile, however, the lifetime of 1BpCMe-d3 is 1.5 times longer than that of 1BpCMe in the same solvent. Thus, in acetonitrile, where ISC is slow, the rate of 1,2 hydrogen shift of 1BpCMe is competitive with ISC. In cyclohexene, the lifetime of 1BpCH is shortened relative to that in cyclohexane. The lifetime of 1BpCMe is the same in cyclohexene and cyclohexane. The data indicate that spin relaxation is slow relative to reaction of 1BpCH with neat alkene but that spin relaxation is fast for 1BpCMe relative to reaction with neat cyclohexene.
Abstract: Ultrafast photolysis (lambda(ex) = 308 nm) of phenyl azide in 100% formic acid produces a broadly absorbing transient within the instrument time resolution (300 fs), which is assigned to an excited state of the azide. The azide excited state fragments within 300 fs to form singlet phenylnitrene. The decay of the nitrene (tau = 12.0 ps) produces a new species with absorption centered at 500 nm, which is assigned to phenylnitrenium ion. The lifetime of phenylnitrenium ion is 110 ps in 100% formic acid. This is the first spectroscopic observation of phenylnitrenium ion.
Abstract: Ultrafast photolysis of 9-diazofluorene (DAF) produces a broadly absorbing transient within the instrument time resolution (300 fs), which is assigned to an excited state of the diazo compound. The diazo excited state fragments to form fluorenylidene (Fl) in both its lowest energy singlet state (1Fl, 405-430 nm, depending on the solvent) and a higher energy singlet state (370 nm, 1Fl*). The excited singlet carbene has a lifetime of 20.9 ps in acetonitrile and decays to the lower energy singlet state (1Fl), which relaxes to the triplet ground state (3Fl) in acetonitrile, cyclohexane, benzene, and hexafluorobenzene. The equilibrium mixture of singlet and triplet fluorenylidene reacts with these solvents. Singlet fluorenylidene reacts with methanol and cyclohexene in competition with relaxation to 3Fl. One of the reaction products in methanol is the 9-fluorenyl cation. The rate of intersystem crossing (ISC) in hexafluorobenzene and other halogenated solvents is remarkably slow given that carbene ISC rates are generally fastest in nonpolar solvents. An explanation of this effect is advanced.
Abstract: Exposure of 2-naphthyl azide in acetonitrile at ambient temperature to femtosecond pulses of 266 nm light produces a transient absorption with maxima at 350 and 420 nm. The carrier of the 350 nm band decays more rapidly than that of the 420 nm band which has a lifetime of 1.8 ps. Analogous experiments with 1-chloro-2-naphthyl azide in methanol allow the assignment of the 350 nm band to a singlet excited state of 2-naphthyl azide and the carrier of the 420 nm band to singlet 2-naphthylnitrene. This reactive intermediate has the shortest lifetime of any singlet nitrene observed to date and is a true reactive intermediate. Computational studies at the RI-CC2 level of theory support these conclusions and suggest that initial excitation populates the S2 state of 2-naphthyl azide. The S2 state, best characterized as a pi --> (pi*, aryl) transition, has a geometry similar to S0. S2 of 2-naphthyl azide can then populate the S1 state, a pi --> (in-plane, pi*, azide) excitation, and in the S1 state, electron density is depleted along the proximal N-N bond. S1 is dissociative along that N-N coordinate to form the singlet nitrene, and with a barrier of only approximately 5 kcal/mol for N2 extrusion.
Abstract: The rate of protonation of singlet 2-fluorenylnitrene was studied in eight protic solvents. The protonation rates vary between 10(11) and 10(9) s(-1) and are well-correlated with Swain's acity parameters. This demonstrates that Swain acity parameters and nitrene protonation rates can be used to evaluate bulk solvent acidity.
Abstract: Laser flash photolysis of the quinol ester 2b in O2-saturated aqueous phosphate buffer at pH 7.1 with excitation at 266 nm generates a transient intermediate with lambda(max) 460 nm that decays in a first-order manner with an aqueous solution lifetime of (170 +/- 10) ns at 22 degrees C. This intermediate is not affected by O2, but reacts rapidly with N3- with an apparently diffusion-limited rate constant of (6.6 +/- 0.2) x 10(9) M-1 s-1. Steady state photolysis of 2b yields the quinol 3b as a major reaction product with a yield of ca. 30-35% after correction for photolytic decomposition of 3b. This is the same product that is quantitatively produced by hydrolysis of 2b in the dark. Photolysis of 2b in the presence of 40 mM N3- completely suppresses the yield of 3b The photolytic intermediate is identified as the aryloxenium ion 1b, that was previously indirectly detected by N3--trapping during the hydrolysis of 2b, based on the chemical behavior of the transient and the quantitative agreement of the N3-/solvent selectivity ratio, kaz/ks, measured directly during the flash photolysis experiment, and indirectly by the azide clock procedure during the hydrolysis reaction. Other, as of yet unidentified, transients are produced during the photolysis reaction. A strong transient absorbance band observed at 360 nm decays in a biphasic manner with two first-order rate constants, neither of which are affected by O2 or N3-. The lifetimes of the two intermediates of ca. 12 and 75 mus are considerably longer than that of 1b. Another very short-lived species can be detected at early reaction times (<or=20 ns) in Ar-saturated buffer. This species is suppressed, but not completely eliminated, in O2-saturated buffer.
Abstract: p-Biphenylyldiazomethane was excited by femtosecond pulses of UV light in acetonitrile, in cyclohexane, and in methanol. Ultrafast photolysis produces a singlet excited state of p-biphenylyldiazomethane with lambdamax = 490 nm, and lifetimes of less than 300 fs in acetonitrile, in cyclohexane, and in methanol. The decay of the excited state is accompanied by the growth of transient absorption with lambdamax = 360 nm. The carrier of this transient absorption is attributed to singlet p-biphenylylcarbene, a result that is consistent with the predictions of TD-DFT calculations. The singlet carbene lifetimes are 200 and 77 ps in acetonitrile and cyclohexane, respectively, and are controlled by intersystem crossing to the lower energy triplet state. The transient absorption does not decay to baseline in acetonitrile, because of the formation of nitrile ylide. The equilibrium mixture of singlet and triplet p-biphenylylcarbene reacts with acetonitrile to form a nitrile ylide (lambdamax = 370 nm), and with cyclohexane by C-H insertion 1-20 ns after the laser pulse. The singlet carbene lifetime is only 7.9 ps in methanol, owing to a rapid reaction with the solvent. Reaction with the solvent gives rise, in part, to a p-biphenylylbenzyl cation (lambdamax = 450 nm, tau = 6.3 ps) in methanol.
Abstract: Ultrafast photolysis (lambdaex = 270, 350, or 360 nm) of bromophenyl, chlorophenyl, fluorophenyl, and fluoro-para-trifluoromethylphenyl diazirines produces transient species which absorb broadly in the UV and visible regions. Transient decay can be fit to either mono- or biexponential functions (tau1 approximately 0.3-10 ps, tau2 approximately 10-350 ps; dependent on solvent and halogen). Fluoro- and chlorophenylcarbene are formed within the time resolution of the spectrometer (300 fs, 270 nm excitation). Bromophenyl diazirine decay (270 nm excitation) correlates with the growth of bromophenylcarbene. Solvent and substituent effects on the slower decays of the transient absorptions are consistent with assigning the carriers of transient absorption in the visible region to ring-opened zwitterionic species.
Abstract: The photochemistry of para- and ortho-biphenylyl azides and 1-naphthyl azide was studied by ultrafast spectroscopy. In every case, the singlet azide second excited states were observed by transient absorption spectroscopy and were found to have lifetimes of hundreds of femtoseconds. The decay of the S(2) states of the azides was accompanied by the growth of transient absorption of the corresponding singlet nitrenes. The intermediate S(1) state of the azides could not be observed due to its low instantaneous concentration resulting from fast fragmentation and nitrene formation. Quantum chemical calculations predict that the S(2) state of the azide is bound and that there is a much lower barrier toward arylnitrene formation from the S(1) state of the azide. Vibrational cooling of para-biphenylnitrene (11 ps) was experimentally observed. The lifetime of singlet ortho-biphenylnitrene was 16 ps in acetonitrile and was not affected by perdeuteration of the aryl ring. The lifetime of singlet 1-naphthylnitrene is 12 ps in acetonitrile at ambient temperature.
Abstract: A hairpin polyamide-chlorobenzenesulfonyl conjugate was synthesized in solution by a haloform reaction and the dicyclohexylcarbodiimide/1-hydroxybenzotriazole (DCC/HOBT) coupling reaction. Its ability to cleave DNA was investigated with supercoiled DNA (pBluescript SK), and the DNA cleaving efficiency of chlorobenzenesulfonyl group was enhanced by the hairpin polyamide under UV irradiation (365 nm).
Abstract: The electrospray ionization tandem mass spectrometric (ESI-MS/MS) characteristics and fragmentation mechanisms of eight distamycin analogues containing N-methylpyrrole and N-methylimidazole were investigated. The members of two isomeric groups of distamycin analogues with the same elemental composition can be distinguished by MS/MS spectra of protonated molecules and of significant fragment ions.
