The deuterium effect on the triplet radiative decay rate of aromatic molecules

Measurements of relative quantum yields of phosphorescene have been made for naphthalene versus perdeuteronaphthalene and for toluene versus perdeuterotoluene, all contained in argon crystals at 20°K. In combination with triplet lifetime measurements these quantum yield ratios give the ratios of radiative triplet decay rates. No radiative deuterium effect is found for toluene and only a small effect is seen in naphthalene in contrast to the rather large radiative deuterium effect seen for naphthalene in EPA by Li and Lim.     

Radiative and radiationless triplet exciton decay in benzophenone

The EPR of triplet excitons and of shallow triplet traps in crystalline benzophenone is observed by an optical detection technique. The predominantly radiationless mode of de-excitation of the traps is demonstrated and shown to account for most of the quenching of electronic excitation energy in this system.     

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

Model calculations that avoid the Condon approximation are reported for T₁$\text{}\text{?}$ar S₀ intersystem crossing in normal and perdeuterated benzene and naphthalene. In both benzenes and in naphthalene-d₈, skeletal bending vibrations coupling T₁ to T₂ are found to be more efficient as accepting modes than CH-stretching vibrations.     

What is the leading mechanism for the nonradiative decay of the lowest triplet state of aromatic hydrocarbons?

It is shown that in aromatic hydrocarbons the ISC rate constants (T^α₁→S₀) are governed by HT vibronic coupling. Some established rules for deciding between different coupling mechanisms are proved to be invalid. Application to naphthalene yields excellent agreement with experimental results.     

Laser flash photolysis studies on the triplet state of aromatic hydrocarbons in the vapour phase

Triplet—triplet (T₁ → T_n) absorption spectra under low resolution and decay times have been measured for a variety of simple fluorine-substituted benzenes in the vapour phase. Assignments for the observed transitions were made on the basis of CNDO calculations and comparison with the corresponding ³B_1u → ³E_2g⁻ transition in benzene. Decay times measured were in the region 200 – 250 ns, orders of magnitude shorter than the corresponding decay time for benzene itself. Reasons for this shortening are briefly discussed.     

Extremely fast radiationless transition from the triplet state of 9,10-diazaphenanthrene

The decay of the lowest excited triplet state of 9,10-diazaphenanthrene (DAP) was investigated. A surprisingly high sublevel decay rate (k_y = 3.3 × 10⁷ s⁻¹) in biphenyl and fluorene hosts was found at 3.0 K from the decay of a time-resolved EPR (TREPR) signal. This high decay rate was confirmed by a lifetime measurement of the delayed fluorescence using a picosecond laser and photon counting system. The nature of the fast radiationless transition is discussed.     

Energy transfer in the course of triplet state quenching of aromatic hydrocarbons by nitroxyl radicals

Quenching of triplet states of aromatic hydrocarbons by nitroxyl radicals has been investigated by the flash photolysis method. There are two different mechanisms of triplet quenching: quenching occurs via enhanced intersystem crossing on exchange interaction with the radical for the triplet states of aromatic hydrocarbons which have low triplet energy (E_T < 14700 cm^?1); for very high triplet energies, energy transfer from the triplet molecule to the nitroxyl radical occurs. The energy of the excited nitroxyl radical was estimated to be 18000 cm^?1.     

Intermediate case radiationless decay: the excited state dynamics of pyrazine

This paper reports new measurements of the non-exponential fluorescence decay of pyrazine, covering the time range 0.1-200 ns. The restits of our measurements remove the inconsistencies between experiment and the Frad-Lahmani-Tramer-Tric model of the radiationless decay in this molecule. In particular, our data show that the triplet component of the mixed singlet-triplet levels does increase with increasing triplet density of states. The effective density of triplet levels determined from our experimental data exceeds the theoretical density of vibrational levels. We propose that at the excitation levels achieved in the triplet manifold there is extensive scrambling of rotational states, but that conservation of nuclear spin states permits a level with total angular momentum quantum number f to couple to only (2J + 1/4) of the 2J+1 rotational levels built on one vibration. The appropriate theoretical density of states to be compared with experiment is then obtained by multiplying the vibrational density of states by J2. Good agreement is found between experimentally determined and calculated densities of states.     

Formation and decay of the triplet excited state of pyridine

A pulse radiolysis study of the formation and decay of the triplet excited state of liquid pyridine has been performed using quenching techniques. The pyridine triplet excited state is observed with an absorption band at lambda = 310 nm and has a first-order decay with a lifetime of 72 ns. Stern-Volmer plots of the quenching of the pyridine triplet excited state with anthracene, naphthalene, and biphenyl give its yield to be 1.3 molecules/100 eV. This value is very similar to the previously determined yield of 1.25 molecules/100 eV for dipyridyl, the predominant condensed-phase product in the gamma-radiolysis of liquid pyridine. The rate coefficient for pyridine triplet excited-state scavenging by oxygen is estimated to be 6.6 x 10(9) M(-1) s(-1). Oxygen may also scavenge the electron precursor to the pyridine triplet excited state, whereas nitrous oxide is observed to have little effect. A pyridyl radical-pyridine (dimer) complex produced in the pulse radiolysis of neat liquid pyridine is detected at lambda = 390 nm and is consistent with iodine scavenging effects. Formation of the pyridiniumyl radical cation-pyridine charge-transfer complex is proposed to be insignificant in liquid pyridine.     

Acridine dyes in the triplet state as reagents for the selective luminescence determination of polycyclic aromatic hydrocarbons

The possibility of the selective determination of several polycyclic aromatic hydrocarbons (PAH) by sensitized room-temperature phosphorescence (SRTP) in sodium dodecylsulfate (SDS) micelles was studied. Acridine dyes (trypaflavine, acridine yellow, and acridine orange) were used as triplet-energy donors. It was found that the presence of external heavy atoms of thallium(I) is a prerequisite to SRTP in the system of an acridine dye (donor) and a PAH (acceptor). The linear concentration ranges, detection limits, and selectivity factors for the determination of pyrene, anthracene, and 1,2-benzanthracene by fluorimetry, room-temperature phosphorescence (RTP), and proposed SRTP were compared.     

Direct measurement of optical spin orientation in the excited triplet state of aromatic hydrocarbons at room temperature

Optically induced magnetization has been observed in polycrystals and solutions of various aromatic carbonyls, quinones, and aza-aromatics at room temperature using a simple pickup-coil detector. The magnetic-field dependence of the magnitude of the induced magnetization provided clear evidence of the creation of spin polarization in the photo-excited triplet state. The dependence of the relaxation rate of the magnetization on the magnetic field and the temperature has been measured.     

Excited state radiationless decay process with Duschinsky rotation effect: formalism and implementation

Duschinsky rotation effect is a simple and effective way to characterize the difference between the ground state and excited state potential energy surfaces. For complex molecules, harmonic oscillator model is still the practical way to describe the dynamics of excited states. Based on the first-order perturbation theory a la Fermi golden rule, the authors have applied the path integral of Gaussian type for the correlation function to derive an analytic formalism to calculate the internal conversion rate process with Duschinsky rotation effect being taken into account. The validity of their formalism is verified through comparison with previous work, both analytically for the case of neglecting Duschinsky rotation and numerically for the ethylene molecules with two-mode mixing. Their expression is derived for multimode mixing.     

Temperature effects on the intramolecular decay of the lowest triplet state of benzophenone

By monitoring the emission specturm, yield, and lifetime from 77 to 400°K for benzophenone in poly(chlorotrifluoroethylene), we have been able to establish the thermally activated delayed fluorescence process and have also observed a large temperature effect on the rate constant for nonradiative decay from the lowest triplet state to the ground singlet state.     

Electron-capture detector and multiple negative ions of aromatic hydrocarbons

Multiple electron affinities are identified in the temperature dependence of the electron-capture detector: naphthalene, 0.16, 0.13+/-0.01, anthracene, 0.69, 0.60, 0 53+/-0.01; tetracene 1.1, 0.88+/-0.03, 0.53+/-0.05; pyrene, 0.61, 0.50+/-0.02; azulene 0.90, 0.80, 0.70+/-0.02, 0.65, 0.55+/-0.05; acenaphthylene, 0.80, 0.69, 0.60, 0.50+/-0.05; and c-C8H8, 0.80, 0.70, 0.55+/-0.02; (all in eV). These are obtained from a rigorous least squares procedure incorporating literature values and uncertainties. The adiabatic electron affinities for about 40 hydrocarbons listed in the US National Institute of Standards and Technology (NIST) tables are evaluated. The adiabatic electron affinity values not listed in NIST are biphenylene, 0.45+/-0.05 eV and coronene. 0.8+/-0.05 eV. Morse potential energy curves in the C-H dimensions illustrate multiple states for benzene and naphthalene.     

Novel sign-alternation rule for the triplet state in aromatic systems

Within the framework of the LCAO -MO theory, a novel sign-alternation rule has been found empirically for the lowest excited ππ* triplet state in the alternant aromatic system. This rule can be applied to identify the phosphorescent state.     

Dynamics of the tyrosine triplet state from magnetic resonance-saturated phosphorescence decay measurements

Analysis of the phosphorescence decays measured during magnetic resonance saturation of sublevel populations has been carried out on tyrosine and tyrosinate triplet states at 1.17°K in zero field. The individual sublevel decay constants and spin-lattice relaxation rate constants are derived. Relative intersystem crossing rates to the sublevels are obtained from flash excitation microwave-induced delayed phosphorescence measurements. Intersystem crossing is not spin-selective in tyrosine, but becomes so upon ionization near pH = 12. The spin-selective intersystem crossing mechanism in tyrosinate is discussed in terms of a model proposed by Bersohn.