Decay Of Benzene Research Articles

Multi-walled carbon nanotubes as electrode materials for electrochemical studies of organometallic compounds in organic solvent media

Films of vertically aligned multi-walled carbon nanotubes (MWCNT) were selectively synthesized on silicon dioxide substrate by catalytic chemical vapor deposition using either benzene or acetonitrile as carbon source and ferrocene (1% w/w) as catalyst. The MWCNT were extensively characterized by using scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and Raman spectroscopy. In order to examine the prospective application of the fabricated MWCNT films for the detection of electro-active compounds in organic solvent media, electrochemical studies of the oxidation of cobaltocene (CoCp2) to cobaltocenium cation (CoCp 2 + ) (Cp = cyclopentadienyl anion) in acetonitrile were performed on these films. For this purpose, cyclic voltammetry and electrochemical impedance spectroscopy were employed. The electrochemical parameters for the CoCp 2 +/0 couple in acetonitrile were derived and compared with those obtained using a conventional glassy carbon electrode. The results demonstrate that the synthesized MWCNT films are promising electrode materials for the electrochemical detection of electro-active species in organic solvents. The MWCNT film formed upon decay of benzene has higher capacitance, less Warburg impedance, and less charge transfer resistance, and consequently it provides faster electron transfer kinetics.

Synthesis and electrochemistry of multiwalled carbon nanotube films directly attached on silica substrate

Vertically aligned multiwalled carbon nanotubes (MWCNT) on silica substrate were selectively produced by the procedure of chemical vapor deposition (CVD). For the synthesis of the MWCNT films, either acetonitrile (ACN) or benzene (BZ) was used as carbon sources while ferrocene (FeCp2) was adopted as catalyst. The packing organization of the aligned carbon nanotubes on the silica substrate, and thus the degree of disorder of the produced MWCNT films, was found to be different. Namely, the MWCNT2 film, produced upon decay of BZ, seems to be more disordered compared to MWCNT1, produced upon decomposition of ACN. In order to examine their prospective application as electrodes for the detection of electroactive compounds in organic solvent media, the techniques of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed. FeCp2 was selected as the suitable standard electroactive substance for probing the fabricated MWCNT electrodes, in view of the fact that FeCp2 undergoes a fast one-electron oxidation process forming the ferrocenium cation (FeCp2+), which is rather stable during the time scale of the experiments. All electrochemical experiments were performed in ACN as solvent medium including n-tetrabutylammonium hexafluorophosphate as supporting electrolyte at room temperature. The extracted CV and EIS results were compared with those obtained using glassy carbon electrode. The findings demonstrate the successful detection of FeCp2 in ACN at both MWCNT films. However, among the films investigated, the electrode produced upon decay of BZ seems to be better capacitor, most probably due to its higher surface area as well as to its small film thickness. Evidently, the high degree of disorder, which has been observed for MWCNT2, plays an important role for the increase of its effective surface area and thus, its capacitance. It is, however, very interesting that the more disorder MWCNT2 film provides, the greater charge transfer resistance.

Rotational Effects on the Channel Three Decay in Benzene: in Support of the Preisomerization Mechanism

Abstract The “Channel 3” phenomena in benzene were investigated concerning a model scheme as an extension of that applied for a typical intermediate case molecule, pyrazine. Using a multiple doorway-states formalism, the difficulties in interpreting such as insufficient density of states for the intermediate dark levels were eliminated based on a preisomerization diagram involving a benzene isomer, most probably benzvalene. By a detailed examination of information about the title subject in the literature concerning both experiments and theory, several questions were raised and answered while making proposals for further study.

Model study of the abrupt excess‐energy dependence of radiationless decay in benzene and azabenzenes

AbstractModel studies are reported aimed at accounting for the abrupt dependence of radiationless decay‐rate constants on excess energy (known as channel‐three decay in the case of S1 benzene) in singlet and triplet manifolds of benzene and azabenzenes. The favored model involves La(ππ*) state, strongly distorted along an out‐of‐plane coordinate, crossing the ground‐state potential at an energy close to the minimum of the lowest excited state. The results are compared with experimental observations on benzene and three azabenzenes. Implications for photochemical reactions are also discussed.

Doppler-free time-resolved polarization spectroscopy of large molecules: Measurement of excited state rotational constants

Measurement of the rotational spectra and constants of molecules can be a powerful probe of excited state geometries and intramolecular dynamics. The conventional approach for obtaining rationally resolved spectra is to use high-resolution (frequency domain, time-integrated) laser excitation. For medium-sized molecules, recent advances in these high-resolution techniques have made it possible to obtain Doppler-free spectra of benzene(1) (using two-photon excitation), and jet-cooled spectra of tetrazines,(2) pyrazine,(3) and others.(4) These results on medium-sized molecules have provided valuable information on geometries, (1,2) and on the dynamics of intramolecular singlet-triplet coupling(3,5) and the “channel 3” decay in benzene.(1) For large molecules, to obtain rationally resolved spectra one needs stable, ultra-narrow bandwidth lasers together with a scheme to reduce Doppler broadening to less than several megahertz.

Channel-three decay in benzene a picosecond fluorescence investigation

Fluorescence quantum yields, decay times and spectra have been measured following excitation of benzene vapour above the channel-three threshold. Experiments have been carried out on the static gas and on molecules jet-cooled to a vibrational temperature of 160 K. The excitation source was a recently developed high-power, picosecond, narrow-bandwidth laser continuously tunable from 224 to 252 nm. Channel-three decay was observed for all vibrational levels, even those not excited via line-broadened transitions. In addition it was observed that fluorescence decay curves resulting from excitation just above the channel-three threshold were non-exponential. It is proposed that the cause of absorption line broadening — intramolecular vibrational redistribution — is not directly related to channel-three, which is postulated to be coupling to a state X, which could be an isomeric form of benzene or a hidden singlet electronic state.

ChemInform Abstract: INTERSYSTEM CROSSING IN COLD ISOLATED MOLECULES OF NAPHTHALENE

We report the results of an extensive study of the rate of radiationless decay in very cold, isolated, naphthalene molecules. The experimental method employed, namely, pulsed narrow band laser excitation of a pulsed seeded supersonic free jet, avoids the complications imposed by the sequence congestion generated by the thermal excitation characteristic of ordinary vapor samples. Our analysis of the fluorescence decay includes a method for transforming room temperature quantum yield data to quantum yields characteristic of the molecules in the supersonic free jet. It is concluded that (a) the out of plane vibration 2(b2g) is very likely a promoting mode for intersystem crossing; (b) there is no need to invoke near resonant coupling of levels in the prepared singlet and final triplet manifolds to explain features of the radiationless decay; and (c) in general, the broad features of radiationless decay in naphthalene are very much like those of the corresponding decay in benzene, and can be similarly explained.

Intersystem crossing in cold isolated molecules of naphthalene

We report the results of an extensive study of the rate of radiationless decay in very cold, isolated, naphthalene molecules. The experimental method employed, namely, pulsed narrow band laser excitation of a pulsed seeded supersonic free jet, avoids the complications imposed by the sequence congestion generated by the thermal excitation characteristic of ordinary vapor samples. Our analysis of the fluorescence decay includes a method for transforming room temperature quantum yield data to quantum yields characteristic of the molecules in the supersonic free jet. It is concluded that (a) the out of plane vibration 2(b2g) is very likely a promoting mode for intersystem crossing; (b) there is no need to invoke near resonant coupling of levels in the prepared singlet and final triplet manifolds to explain features of the radiationless decay; and (c) in general, the broad features of radiationless decay in naphthalene are very much like those of the corresponding decay in benzene, and can be similarly explained.

Radiationless decay of vibronically coupled electronic states. III. Strong coupling and its effect on triplet decay in aromatic hydrocarbons

Radiationless transitions from an optically prepared state to the ground state are studied on a model consisting of three electronic states and four modes of vibration. Two ’’promoting’’ modes induce transitions from the two excited states; they are taken harmonic and treated in the weak-coupling approximation. One mode couples the two excited states; its coupling is allowed to be strong enough to lead to anharmonic adiabatic potentials. The fourth mode is a displaced harmonic oscillator acting as an accepting mode, a role it shares with the coupling mode if the coupling is strong enough. The model differs from that studied in part II [J. Chem. Phys. 72, 1641 (1980)] in allowing stronger coupling as well as indirect (second-order) transition via the upper electronic state. Solutions are obtained by numerical methods that avoid the use of the Condon approximation. Radiationless decay rate constants are calculated for cold and hot bands and related to the energy distribution between the accepting modes. They are plotted as a function of energy separations, displacements, and coupling strengths. The results which confirm and extend the conclusions of part II are applied to radiationless triplet decay in benzene, naphthalene, and their perdeuterated analogs. It is concluded that in the two benzenes, in naphthalene-d8, but not in naphthalene-h8, the mode coupling T1(3La) with T(3Ba) is the dominant accepting mode. This result is used to account for anomalies in the deuterium effect and energy-gap dependence of triplet decay rate constants in aromatic hydrocarbons.

Non-condon effects on radiationless triplet decay in benzene and naphthalene

Model calculations that avoid the Condon approximation are reported for T 1 \\ ̃ ar S 0 intersystem crossing in normal and perdeuterated benzene and naphthalene. In both benzenes and in naphthalene- d 8, skeletal bending vibrations coupling T 1 to T 2 are found to be more efficient as accepting modes than CH-stretching vibrations.

THE NATURE OF THE “CHANNEL 3” DECAY MECHANISM IN BENZENE

Abstract A simple but realistic mechanism for the “channel 3” decay in benzene is proposed where energy levels of isomers are incooporated as intermediate states of S1 → S0 internal conversion.

Radiationless transitions and the “third channel” problem in benzene

Calculations are presented in support of the suggestions that the onset of “channel three” decay in benzene is due to enhanced internal conversion involving multiple excitation of CC stretching vibrations.

Fluorescence lifetimes and relative quantum yields of 9,10-diphenylanthracene in dilute solutions of cyclohexane and benzene

The lifetimes of 9,10-diphenylanthracene in dilute solutions of cyclohexane and benzene at 25°C have been found to be 7.58 ± 0.04 and 6.95 ± 0.04 ns respectively. Measurements of the relative quantum yields show that the dependence on the solvent is caused by an increased probability for non-radiative decay in benzene compared with cyclohexane. This behaviour is shown to partly reconcile previous conflicting data on the radiative properties of this molecule.