Triplet Benzene Research Articles

Deciphering the role of (anti)aromaticity in cofacial excimers of linear acenes

AbstractExcited state aromaticity is a stimulating area of research, widely used as a probe to describe and rationalize many photochemical phenomena. Herein, we review some of the recent findings of unprecedented aromatic stabilization in spin singlet excimer and through‐space aromatic character in triplet excimers of a series of linear [n]acenes, as paramount examples of polycyclic aromatic hydrocarbons (PAHs). This review also provides insights on the aromatic stabilization profile of singlet benzene excimer formation, which can be related to antiaromaticity alleviation of the molecular (localized) S1 through exciton delocalization. The theoretical investigation of excimer stabilization using magnetic, electronic, and geometric aromatic indices manifested the presence of through‐space ring current in triplet cofacial excimers. The antiaromaticity of the sandwich (D6h) spin singlet and triplet benzene excimers was also investigated by decomposing the excimer wave function as a linear combination of local exciton (LE) and charge transfer (CT) diabats and by identifying the contribution of these terms to the nucleus independent chemical shift (NICS) of the two 6‐membered rings. These results provide a detailed description of the unique (anti)aromatic properties in PAH excimers, establishing strong connection between this important chemical concept and the electronic structure intricacies of excimers.

Exploring potential crossing seams in periodic systems: Intersystem crossing pathways in the benzene crystal.

The intersystem crossing (ISC) pathways of triplet benzene molecules in a benzene crystal were investigated theoretically. A combination of the gradient projection (GP) method, which is a standard method for optimizing the crossing seam of two potential energy surfaces, and the single-component artificial-force-induced reaction (SC-AFIR) method (GP/SC-AFIR) was used. This is the first reported use of a GP/SC-AFIR calculation using a density functional theory calculation with periodic boundary conditions. A systematic search for the minimum-energy structures in the seams of crossing of the singlet (S0) and triplet (T1) potential energy surfaces (S0/T1-MESX structures) found 39 independent S0/T1-MESX structures. Energy barriers between the S0/T1-MESX and the stationary structure of the triplet state (T1-MIN) were computed, and then two competing ISC pathways were extracted; the calculated overall energy barrier to the intermolecular C-C-bonded type (SX3) and the out-of-plane bent C-H type (SX15) S0/T1-MESX structures from T1-MIN were 0.26 and 0.27 eV, respectively. The rate constants for SX3 and SX15 formation were estimated to be 5.07 × 108 and 2.17 × 108 s-1 (at 273 K), respectively, or 9.73 × 10-5 and 4.78 × 10-6 s-1 (at 77 K), respectively. At 273 K, which is close to the melting point of the benzene crystal (278.5 K), SX3 and SX15 are easily accessible from T1-MIN, and ISC could occur through the S0/T1-MESX points. By contrast, at 77 K, T1-MIN survives long enough for phosphorescence to compete with ISC.

Triplet states of zigzag edged hexagonal graphene molecules C6m**2H6m (m = 1, 2, 3, …, 10) and carbon based magnetism

The geometry and magnetization (spin distribution) of the series of flat hexagonal zigzag edged molecules C(6m∗∗2)H(6m) (m = 1,2, ..., 10) in their lowest triplet state (S(z) = 1) has been calculated using density functional theory and a connection established from the known benzene (m = 1) triplets to the triplets and singlet ground state of the largest molecules (m = 9, 10). The triplet state potential energy surface has two minima corresponding to distortions from the ground state geometry, such that CC bonds bisected by a C(2)" rotation axis are either longer or shorter. For both geometries, the spin on the carbon atoms forms a pattern that peaks at the middle of an edge and for large index (m) values is the same (apart from sign) as the edge pattern of the hexagonally sectored singlet radical ground state of the largest member C(600)H(60). This similarity suggests that the singlet ground state of the larger (m = 9, 10) zigzag edged hexangulenes is possibly a hex-radical, in some ways analogous to the di- and higher multiradical ground state of the linear acenes C(4m + 2)H(2m + 4) starting around m ≥ 8 and 9. The spin patterns provide guidance in interpreting the multiradical nature of ground and low lying excited states of large hexangulenes and how magnetism evolves with size in molecules with graphene cores.

Molecular applications of analytical gradient approach for the improved virtual orbital-complete active space configuration interaction method

The improved virtual orbital-complete active space configuration interaction (IVO-CASCI) method is extended to determine the geometry and vibrational frequencies for ground and excited electronic states using an analytical total energy gradient scheme involving both first and second order analytical derivatives. Illustrative applications consider the ground state geometries of the benzene (C(6)H(6)), biphenyl (C(12)H(10)), and alanine dipeptide (CH(3)CONHCHCH(3)CONHCH(3)) molecules. In addition, the IVO-CASCI geometry optimization has been performed for the first excited singlet ((1)B(2u)) and triplet states ((3)B(1u)) of benzene to assess its applicability for excited and open-shell systems. The D(6h) symmetry benzene triplet optimization produces a saddle point, and a descent along the unstable mode produces the stable minimum. Comparisons with Hartree-Fock, second order Moller-Plesset perturbation theory, complete active space self-consistent field (CASSCF), and density functional theory demonstrate that the IVO-CASCI approach generally fares comparable to or better for all systems studied. The vibrational frequencies of the benzene and biphenyl molecules computed with the analytical gradient based IVO-CASCI method agree with the experiment and with other accurate theoretical estimates. Satisfactory agreement between our results, other benchmark calculations, and available experiment demonstrates the efficacy and potential of the method. The close similarity between CASSCF and IVO-CASCI optimized geometries and the greater computational efficiency of the IVO-CASCI method suggests the replacement of CASSCF treatments by the IVO-CASCI approach, which is free from the convergence problems that often plague CASSCF treatments.

Products of the Triplet Excited State Produced in the Radiolysis of Liquid Benzene

The radiation chemical yields of the products derived from the triplet excited state produced in the radiolysis of liquid benzene with gamma-rays, 10 MeV 4He ions, and 10 MeV 12C ions have been determined. Iodine scavenging techniques have been used to examine the formation and role of radicals, especially the H atom and phenyl radical. For all irradiation types examined here, the increase in hydrogen iodide yields with increasing iodine concentration matches the increase in iodobenzene yields. This agreement suggests that the benzene triplet excited state is the common precursor for the H atom and the phenyl radical. Pulse radiolysis studies in liquid benzene have determined the rate coefficients for the reactions of phenyl radicals with iodine and with the solvent benzene to be 9.3 x 10(9) M(-1) s(-1) and 3.1 x 10(5) M(-1) s(-1), respectively. Direct measurements of polymer formation, which refers to trimers (C18) and higher order compounds (>C18), in liquid benzene radiolysis using gamma-rays, 4He ions, and 12C ions at relatively high doses have been performed using gel permeation chromatography. The yields of trimers increase from gamma-rays to 12C ions due to the increased importance of intratrack radical-radical reactions that can be scavenged by the radical scavenging reactions of iodine. On the other hand, the >C18 product yields decrease from gamma-rays to 12C ions. The structure of the polymer consists of a partly saturated ring as determined by infrared and gas chromatography/mass spectrometry studies. A schematic representation for the radiolytic decomposition of the benzene triplet excited state is presented.

Radiolytically generated benzene triplets as sensitizers for energy and combined electron/proton transfer processes

Pulse radiolysis technique has been employed to investigate energy and electron transfer reactions involving triplets of naphthols and hydroxybiphenyls. The transient absorption spectra obtained on pulse radiolysis of N2-saturated solution of naphthols and hydroxybiphenyls in benzene are assigned to triplet–triplet absorption. It was found that biphenyl triplets undergo energy transfer to naphthols and hydroxybiphenyls forming the acceptor triplets. On the other hand, benzophenone triplets, favor electron transfer followed by H+ transfer reaction forming benzophenone ketyl radical and phenoxyl radical of the acceptor. An analogous sequence mimics with 2-naphthol triplets and using benzophenone, acetophenone or chloranil as electron acceptor.

Photosensitized Ring Opening in Phenyl Oxiranes#

Under energy transfer sensitization by pulse-radiolytically or laser-flash-photolytically generated benzene, acetone, or chromone triplet, the ring opening in cis or trans-stilbene oxide leads to a mixture of exo,endo and exo,exo forms of carbonyl ylides that are spectrally and kinetically distinguishable from each other. The fact that the same molar ratio of the two ylide forms is produced from the two isomeric oxiranes suggests the involvement of a common ylide triplet. On the other hand, under reversible electron transfer sensitization by 1,4-dicyanonaphthalene singlet excited state, the two ylide forms are produced in significantly different ratios when cis and trans-stilbene oxides are used as donors; this indicates the intermediacy of distinct, non-equilibrating, ring-opened radical cations in the two cases. Spectrally observable carbonyl ylide transients with short lifetimes (85-550 ns) are also produced from tri- and tetraphenyl oxiranes under both energy and electron transfer sensitization. Unlike the ylides from di- and trisubstituted phenyl oxiranes, that from tetraphenyl oxirane is practically non-quenchable by electron-poor dipolarophiles (e.g., methyl acrylate), suggesting a steric hindrance in attaining the "two-plane orientation complex" in the course of 1,3-dipolar cycloaddition.

On the formation of triplet state of crystal violet in solutions—a pulse radiolysis study

Sensitization of Crystal Violet (CV), in benzene solutions, by triplet states of benzophenone, biphenyl and benzene itself has been observed. The resulting transient spectrum and its decay kinetics have been studied in the 360–450 nm region. The observed spectrum has been attributed to triplet state of CV. The rate constant for energy transfer from triplet benzene to CV has been estimated to be 5 × 10 10dm 3mol −1s −1. Triplet state of CV is found to decay by reacting with the parent dye molecule with a rate constant 4 × 10 6dm 3mol −1s −1.

Vibrational energy dependence of nonradiative decay rate: Triplet benzene studied by time-resolved and steady-state photosensitized phosphorescence

Abstract The nonradiative decay rate of triplet benzene was determined as a function of excitation energy utilizing the phosphorescence of biacetyl induced by the energy transfer from triplet benzene. The triplet benzene was formed by intersystem crossing from the photoexcited singlet molecule. The nonradiative decay rate determined by combination of time-resolved and steady-state measurements increased from 8.5 × 10 3 s −1 to 4 × 10 6 s −1 as the excitation energy was varied from 38000 to 41000 cm −1 , i.e. 8500 to 11500 cm −1 excess vibrational energy in the triplet state. The increasing trends were in reasonable agreement with previously reported experimental results and also with theoretical calculations.

Triplet states of symmetrically disubstituted biphenyl derivatives. Triplet exciplex formation between benzophenone triplet and benzidine

Abstract The absorption spectra of the triplet states of 4,4′-diaminobipheyl (benzidine, BZD) and 4,4′-dibromobiphenyl (DBRBP) in benzene and of 4,4′-dihydroxybiphenyl (DHXBP) in a mixture of 90% benzene and 10% ethanol were recorded in the 300–600 nm region using the pulse radiolysis technique. The extinction coefficients at λ max were estimated to be 30 600,21 630 and 45 060 dm 3 mol −1 cm −1 for BZD, DBRBP and DHXBP respectively and the corresponding triplet energies were 61±1.5, 63±0.5 and 63±0.5 kcal mol − . The rate constants for energy transfer from benzene and biphenyl triplets to these compounds and from the compound triplets to acceptor molecules such as naphthalene and coumarin-1 were determined. The triplets of the compounds BZD, DBRBP and DHXBP were found to react with the corresponding ground state molecules with rate constants of 5.4×10 6 , 4.1×10 6 and 2.1×10 7 dm 3 mol −1 s −1 respectively. In the case of the benzidine triplet reaction with the ground state gave transient species with a λ max at 390 and 620 nm, which has been suggested to be a mixture of semi-reduced and semi-oxidized species. The benzophenone triplet reacts with benzidine to give an exciplex species with an absorption maximum at 380 nm.

The phosphorescence of benzene obtained byab initio and semi-empirical calculations

Radiative decay and phosphorescence of triplet stare benzene is doubly -orbital and spin- forbidden and is only activated through vibronic coupling among the manifold of triplet states. For this reason the determination of lifetime and transition moments for the decay of triplet benzene has posed a considerable challenge to both theory and experiment. In the present work we have addressed the triplet benzene problem at several levels of theory; by truncated perturbation theory and semiempirical, CNDO/S-CI, calculations; by complete sum-over-state calculations as implemented in recentab initio multiconfiguration quadratic response (MCQR) theory; and by direct MCQR calculations of vibronic phosphorescence. The vibronic coupling is in the two former cases treated by the Herzberg-Teller (H-T) perturbation theory, involving four main mechanisms for the phosphorescent decay of triplet benzene. The results and interpretations given by these approaches as well as their merits and limitations are presented and discussed in some detail. Our calculations indicate that the phosphorescent decay of the3B1u state takes place predominantly through vibronic coupling along thee2g mode. We obtain a phosphorescence that is almost completely out-of-plane polarized, which is in line with more recent measurements by the microwave-induced delayed phosphorescence technique, and could reproduce quite well the intensity ratios for different vibronic bands obtained in that experiment. The final triplet state lifetime is the result of a delicate sum of contributions from several vibronic degenerate and non-degenerate modes. The direct vibronic phosphorescence calculations predict a long lifetime, about one minute — 68 seconds for the best wavefunction — and seem to focus on a doubling of the assumed, albeit not established, “best experimental” value for the radiative lifetime of triplet benzene; ⋍ 30 seconds.

The vibronically induced phosphorescence in benzene

The phosphorescence spectrum of benzene is particularly rich in vibronic structure and provides the best-known example of a spectrum of electronic transitions that by symmetry is both spin and orbital forbidden. Such transitions, only allowed through the coupling of nuclear and electronic motions, are notoriously difficult to analyze both on theoretical and experimental grounds. We investigate the vibronically induced phosphorescence by means of multi-configuration quadratic response theory calculations and explore vibronic intensities, polarization directions, transition moments for benzene phosphorescence and the radiative lifetime of triplet benzene. We find that the radiative decay of the 3B 1u state takes place predominantly through vibronic coupling among the e 2g CC stretching modes ν 8=1601 cm −1 and ν 9=1178 cm −1, with a close to complete out-of-plane polarization. The calculations predict relative intensities of different vibronic bands in good agreement with experiment. The oscillator strength for the 3B 1u← 1A 1g absorption is predicted to 0.74×10 −10, to be compared with the experimental value of ≈10 −10. The computed averaged radiative lifetime falls in the interval of 22 to 96 seconds, depending on the quality of the basis set, with the best prediction being 64 seconds.

Non-radiative decay rate of vibrationally excited triplet molecules

The non-radiative decay rates of triplet benzene and 2-chloronaphthalene were determined as a function of excitation energy. As the excitation energies were increased, the non-radiative decay rates increased gradually. In the case of 2-chloronaphthalene it increased rapidly for the excitation energies above about 38000 cm-1.

Studies of the triplet state of biphenyl derivatives by nanosecond pulse radiolysis

The triplet characteristics of two biphenyl derivatives (4,4′-dimethylbiphenyl and 4,4′-dimethoxybiphenyl) were studied in benzene solution using the pulse radiolysis technique. Their triplet-triplet absorption spectra were recorded in the 300–550 nm region. The triplet lifetimes were determined to be 16.1 /gms for 4,4′-dimethylbiphenyl and 7.7 /gms for 4,4′-dimethoxybiphenyl. The extinction coefficients at γ max were determined. The triplet energy levels of both these molecules were determined to be 65 ± 1.5 kcal mol −1. Rate constants for energy transfer from benzene and benzophenone triplets to these compounds and from the triplets of these compounds to acceptor molecules such as naphthalene, coumarin-1, benzil and anthracene were determined.

Resonance theoretical description of magnetic zero field splitting energies of benzenoid hydrocarbons. Structural dependance of the parameterD

The zero field splitting (ZFS) parameters D of the triplet ground states of benzene, naphthalene, anthracene, and phenanthrene are calculated within the framework of a semiempirical valence bond (VB) theory. The VB theoretical results are utilized to develop a much simpler resonance theoretical model which stresses the relationship between molecular structure, aromaticity, and magnitude of the ZFS parameter. The resonance theoretical approximation of the electronic wavefunction is anlysed in terms of triplet benzene subunits which describe a benzene-type resonance of the “triplet electrons” on a particular hexagon of the molecular structure. The ZFS parameter can be approximated by contributions from all triplet benzene subunits of the molecule. The resonance theoretical model can be applied successfully to larger hydrocarbons where a VB theoretical description becomes too complicated. Moreover, the model allows for a simple qualitative understanding of the dependence of the ZFS parameter D on the molecul...

A study of the triplet state of 7-amino coumarin laser dyes by the nanosecond pulse radiolysis technique

The triplet characteristics of two 7-amino coumarin laser dyes (7-diethylamino 4-methyl coumarin (C 1) and 7-diethylamino 4-trifluoromethyl coumarin (C 1F)) were studied in benzene solution using the nanosecond pulse radiolysis technique. Their T—T absorption spectra, rate constants for triplet to ground state transition and triplet—triplet annihilation and G values were determined. The energy transfers from coumarin triplets to various acceptor molecules such as pyrene, t-stilbene and anthracene and from benzene, biphenyl and naphthalene triplets to these dyes as acceptors were studied. Estimates of their energy levels were made.

Triplet state of coumarin 153 studied by nanosecond pulse radiolysis

The triplet—triplet absorption spectrum of the rigidized 7-amino coumarin dye C 153 has been investigatd in benzene solution in the region 450 – 750 nm by the pulse radiolysis technique. The triplet decay was first order with a half-life of 5 μs. Its formation was confirmed by energy transfer from the pulse radiolytically generated biphenyl triplet. The extinction coefficient of C 153 triplet in benzene was calculated to be 10 000 ± 500 dm 3 mol −1 cm −1. Using various triplet donors and acceptors its energy level was estimated to be 50 ± 2 kcal mol −1. The energy transfer rate constants from benzene, biphenyl and naphthalene triplets to C 153 and from C 153 triplet to anthracene were found to be diffusion controlled.

The benzene-sensitized cycloreversion of 9,10-peroxy,9,10-dimethylanthracene in solution

Rates of 9,10-dimethylanthracene (DMA) formation have been determined for N 2-purged solutions of 9,10-peroxy-9, 10-dimethylanthracene (DMAPO) and benzene ( B) in dichloromethane at 254 nm as a function of concentration of DMAPO and of B. The rate equation is integrated to accommodate time-dependent absorption of the actinic radiation by DMA, necessary even for cycloreversions of 1%. The data are quantitatively consistent with triplet benzene sensitized adiabatic cycloreversion from the endoperoxide triplet (T π) state with an estimated efficiency of 0.16 ± 0.06.

Photolysis and benzene sensitized dissociation of 1,1,3-trimethylsilacyclobutane

Benzene photosensitization may serve to select reaction products in 1,1,3-trimethylsilacyclobutane. Increasing the population of the benzene triplet state using a heavy atom, xenon, has the effect of enhancing the elimination of dimethylsilylene from the ring. Singlet and triplet benzene quenching efficiencies are 8 × 10 10 and 2 × 10 9 M −1 s −1, respectively for 1,1,3-trimethylsilacyclobutane.

ChemInform Abstract: PHOTOIONIZATION MEASUREMENT OF THE TRIPLET LIFETIME OF BENZENE

The lifetime of triplet benzene produced by laser excitation of the 6/sup 1/ level of the S/sub 1/ state in a supersonic beam has been measured by photoionization with an ArF excimer laser. This collision-free lifetime pertaining to the vibrationally unrelaxed triplet was found to be 470 +- 50 ns. Measurement of a component of the photoion signal due to decay of S/sub 1/ permitted, in addition, an estimate of the quantum yield for intersystem crossing.