Triplet State T1 Research Articles

Theoretical study on electronic structure of (CNC)Fe_2N2 and its N2 elimination mechanism

AbstractThe density function theory (DFT) is to elucidate the electronic structure of bis(dinitrogen) Fe(0) complex, (CNC)Fe_2N2, and its N2 elimination mechanism. (CNC)Fe_2N2 has a low‐spin singlet (S = 0) ground state with a distorted square pyramidal structure. Fragment orbital interaction analysis yields total occupancy of π* orbitals (LUF(4)O and LUF(4)O−1) of apical N3N4 is 0.188 while that of basal N1N2 is 0.187 in S0(CNC)Fe_2N2, suggesting nearly the same activation extent for both basal and apical N2 ligands. The lowest‐lying triplet state T1 (3‐A′) has a repulsive potential energy surface along the FeN3 bong length by PBE functional, while a minimum on T2 state (3‐A″) with higher energy is found by B3LYP functional. The nonadiabatic N2 elimination mechanism of (CNC)Fe_2N2 involves an S0‐T1 states crossing, which lowers the activation energy to 9.7 kcal/mol and produces high‐spin intermediate (CNC)FeN2. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Insights into Photoinduced Electron Transfer between [Ru(bpy)3]2+ and [S2O8]2− in Water: Computational and Experimental Studies

We examine the mechanism of electron transfer between [Ru(bpy)(3)](2+) and [S(2)O(8)](2-) in aqueous solutions using time-dependent density functional and transition charge density model levels of theory. The calculations support the existence of a short-lived, optically bright S1 state that decays to lower-lying triplet states T1 and T2. The T1 state leads to the charge transfer products, that is, [Ru(bpy)(3)](3+) + SO(4)(-*) + SO(4)(2-). It was shown that photoinduced (in the 420-520 nm wavelength range) electron transfer between [Ru(bpy)(3)](2+) and [S(2)O(8)](2-) may proceed via both the "unimolecular" and "bimolecular" pathways, in agreement with the previous experimental findings. This distinction arises on the basis of a weak interaction between the two reactants. Analysis of excited electronic states and their spin-orbit mixing suggests that a photon excites a MLCT S1 state of [Ru(bpy)(3)](2+)*/{[Ru(bpy)(3)](2+)...[S(2)O(8)](2-)}* that converts to the lower-lying T2 state via spin-orbit interaction, which is an intermediate in the S1 --> T2 --> T1 chain. Once the system has converted to T1, it can evolve toward charge transfer from [Ru(bpy)(3)](2+)* to [S(2)O(8)](2-) via elongation of the peroxo O-O bond that brings the "zero order" states of the [Ru(bpy)(3)](2+)* and [S(2)O(8)](2-) fragments into (near) resonance. An exciton interaction model in combination with the transition charge density model provides excellent agreement with TD-DFT and allows us to obtain insight into the electron transfer process. The steady-state luminescence studies of the quenching of [Ru(bpy)(3)](2+)* in aqueous solutions by [S(2)O(8)](2-) at pH 7.2 and 20 mM sodium phosphate buffer, the exact conditions used in our previous study of water oxidation [Geletii, Y. V.; Huang, Z. Q.; Hou, Y.; Musaev, D. G.; Lian, T. Q.; Hill, C. L. J. Am. Chem. Soc. 2009, 131, 7522-7523], show that 61% of the [Ru(bpy)(3)](2+) forms a ground state ion-pair complex [Ru(bpy)(3)](2+)...[S(2)O(8)](2-), and the unimolecular ET rate is 2.5 times faster than for the bimolecular process. The concentration of the ion-pair complexes decreases at higher sodium phosphate buffer.

Transient absorption spectra of tetraazaporphine and its reduced derivatives upon picosecond excitation

Transient absorption spectra of tetraazaporphine and substituted derivatives of tetraazachlorin and tetraazabacteriochlorin were obtained upon picosecond excitation with various probe pulse delay times in order to resolve the open question about the reasons for fluorescence quenching in tetraazaporphine hydrogenated derivatives. The quantum yield of triplet state T 1 formation has been estimated. It has been shown that in all investigated cases radiationless de-excitation of the fluorescent level S 1 occurs by two channels: S 1 ⤳ T 1 and S 1 ⤳ S 0, the latter being predominant. As the S 1 level becomes lower, the transition rate for this channel increases. For tetraazaporphine and its derivatives, a quasi-photochemical mechanism is proposed that accounts for the anomalous efficiency of the S 1 ⤳ S 0 channel in the dissipation of the electronic excitation energy.

Synthesis and Comparison of the Optical Properties of Platinum(II) Poly-ynes with Fused and Non-Fused Oligothiophenes

The synthesis and characterization of the thieno[3,2-b]thiophene and dithieno[3,2-b:2′,3′-d]thiophene containing platinum(II) poly-ynes and their molecular precursors is described and the electronic structure is established by absorption, luminescence and photoinduced absorption measurements. A comparison of the electronic structure of the fused and the nonfused oligothiophenes, thieno[3,2-b]thiophene, dithieno[3,2-b:2′,3′-d]thiophene, 2,2′-bithiophene, and 2,2′:5′,2′′-terthiophene incorporated in platinum(II) poly-ynes is reported. We find the singlet S1 and triplet T1 and Tn excited states to be at higher energy in thin films made from the fused systems than from the nonfused systems. For ligands with the same number of rings, we attribute this to the decreased number of double bonds in the fused system and to the presence of an additional sulfur atom in spacers with the same number of double bonds.

The Photo-Favorskii Reaction of p-Hydroxyphenacyl Compounds Is Initiated by Water-Assisted, Adiabatic Extrusion of a Triplet Biradical

The p-hydroxyphenacyl group 1 is an effective photoremovable protecting group, because it undergoes an unusual photo-Favorskii rearrangement concomitant with the fast release (<1 ns) of its substrates in aqueous solution. The reaction mechanism of the diethyl phosphate derivative 1a was studied by picosecond pump−probe spectroscopy, nanosecond laser flash photolysis, and step−scan FTIR techniques. The primary photoproduct is a triplet biradical, 33, with a lifetime of about 0.6 ns. The release of diethyl phosphate determines the lifetime of the triplet state T1(1a), τ(T1) = 60 ps in wholly aqueous solution. Formation of a new photoproduct, p-hydroxybenzyl alcohol (6), was observed at moderate water concentrations in acetonitrile. It is formed by CO elimination from the elusive spirodione intermediate (4), followed by hydration of the resulting p-quinone methide (5). Computational studies show that CO elimination from the spirodione is a very facile process.

Electronic Relaxation Processes in the T1 State of Phosphorescent fac-Tris(2-phenylpyridine) Iridium Molecule in 4,4'-N,N'-Dicarbazole-biphenyl Host

We have analyzed the time-resolved photoluminescence spectra of fac-tris(2-phenylpyridine) iridium [Ir(ppy)3] doped in 4,4'-N,N'-dicarbazole-biphenyl (CBP) using the spin–phonon relaxation processes among three zero-field splitting substates of the T1 triplet state under the Franck–Condon principle. Good agreement was obtained between the calculated and measured results regarding temperature dependence of three decay components, red peak shift during time evolution at 5 K, and luminescence line shape with vibronic structure.

Photodissociation Dynamics of Bromofluorobenzenes Using Velocity Imaging Technique

Velocity imaging technique combined with (2 + 1) resonance-enhanced multiphoton ionization (REMPI) has been used to detect the Br fragment in photodissociation of o-, m-, and p-bromofluorobenzene at 266 nm. The branching ratio of ground state Br(2P3/2) is found to be larger than 96%. Its translational energy distributions suggest that the Br fragments are generated via two dissociation channels for all the molecules. The fast route, which is missing in p-bromofluorobenzene detected previously by femtosecond laser spectroscopy, giving rise to an anisotropy parameter of 0.50-0.65, is attributed to a direct dissociation from a repulsive triplet T1(A' ') or T1(B1) state. The slow one with anisotropy parameter close to zero is proposed to stem from excitation of the lowest excited singlet (pi,pi*)state followed by predissociation along a repulsive triplet (pi,sigma*) state localized on the C-Br bond. For the minor product of spin-orbit excited state Br(2P1/2), the dissociating features are similar to those found in Br(2P3/2). Our kinetic and anisotropic features of decomposition obtained in m- and p-bromofluorobenzene are opposed to those by photofragment translational spectroscopy. Discrepancy between different methods is discussed in detail.

Time-resolved step scan FTIR spectroscopy and DFT investigation on triplet formation in Peridinin-Chlorophyll-a-Protein from Amphidinium carterae at low temperature

We have used time-resolved step-scan FTIR spectroscopy to investigate the process of triplet formation in the peridinin–chlorophyll-a–protein (PCP) at 100 K. Results have led to the identification of possible marker bands for peridinin (Per) and 3Per. Band assignment has been carried out performing a DFT investigation on Per, 3Per and on model molecules. The main spectral feature is a couple of bands at 1746 (negative) and 1719 (positive) cm–1. The assignment of these two bands to the lactonic C═O of Per in (respectively) its fundamental S0 and triplet T1 state is based on: (1) comparison with FTIR spectra of isolated Per in literature; (2) good agreement with DFT calculations for Per and 3Per; (3) the downshift of the band upon triplet formation as predicted by DFT calculations in Per and similar lactonic systems; (4) the kinetic of band disappearance, in agreement with literature data. Another possible band for Per in its fundamental state has been identified at 1523 cm–1.

Role of charge-transfer complexes in oxygen quenching of excited states of anthracene derivatives in the gas phase

Oxygen quenching of excited triplet and singlet states of gas-phase anthracene and its derivatives that have similar energies of the lower triplet levels but widely different oxidation potentials (0.44 < Eox < 1.89 V) was studied. Quenching rate constants for singlet (kSO2) and triplet (kTO2) states in addition to the fraction of oxygen-quenched singlet and triplet states qS1(T1O2 were determined from the decay rates, fluorescence intensities, and delayed fluorescence as functions of oxygen pressure. It was found that kSO2 values vary from 2·104 (9,10-dicyanoanthracene) to 1.2·107 sec−1·torr−1 (anthracene, 9-methylanthracene, 2-aminoanthracene) and kSO2 values from 5·102 to 1·105 sec−1·torr−1. The kSO2 values for anthracene, 9-methylanthracene, and 2-aminoanthracene, which have fast rates of interconversion from S1 to T1, are close to the rate constants for gas-kinetic collisions and are independent of the oxidation potentials (Eox). The quenching rate constants kSO2 for the other anthracene derivatives and kTO2 for all studied compounds decrease with increasing free energy of electron transfer ΔGET, which indicates the important role of charge-transfer interactions in the oxygen quenching of singlet S1- and triplet T1 states.

The Benzophenone S1(n,π*) → T1(n,π*) States Intersystem Crossing Reinvestigated by Ultrafast Absorption Spectroscopy and Multivariate Curve Resolution

The well-known benzophenone intersystem crossing from S(1)(n,pi*) to T(1)(n,pi*) states, for which direct transition is forbidden by El-Sayed rules, is reinvestigated by subpicosecond time-resolved absorption spectroscopy and effective data analysis for various excitation wavelengths and solvents. Multivariate curve resolution alternating least-squares analysis is used to perform bilinear decomposition of the time-resolved spectra into pure spectra of overlapping transient species and their associated time-dependent concentrations. The results suggest the implication of an intermediate (IS) in the relaxation process of the S(1) state. Therefore, a two step kinetic model, S(1) --> IS --> T(1), is successfully implemented as an additional constraint in the soft-modeling algorithm. Although this intermediate, which has a spectrum similar to the one of T(1)(n,pi*) state, could be artificially induced by vibrational relaxation, it is tentatively assigned to a hot T(1)(n,pi*) triplet state. Two characteristic times are reported for the transition S(1) --> IS and IS --> T(1), approximately 6.5 ps and approximately 10 ps respectively, without any influence of the solvent. Moreover, an excitation wavelength effect is discovered suggesting the participation of unrelaxed singlet states in the overall process. To go further discussing the spectroscopic relevancy of IS and to rationalize the expected involvement of the T(2)(pi,pi*) state, we also investigate 4-methoxybenzophenone. For this neighboring molecule, triplet energy level is tunable through solvent polarity and a clear correlation is established between the intermediate resolved by multivariate data analysis and the presence of a T(2)(pi,pi*) above the T(1)(n,pi*) triplet. It is therefore proposed that the benzophenone intermediate species is a T(1)(n,pi*) high vibrational level in interaction with T(2)(pi,pi*) state.

Triplet State Properties of the OLED Emitter Ir(btp)2(acac): Characterization by Site-Selective Spectroscopy and Application of High Magnetic Fields

The well-known red emitting complex Ir(btp)2(acac) (bis(2-(2'-benzothienyl)-pyridinato-N,C3')iridium(acetylacetonate)), frequently used as emitter material in OLEDs, has been investigated in a polycrystalline CH2Cl2 matrix. The studies were carried out under variation of temperature down to 1.2 K and at magnetic fields up to B=10 T. Highly resolved emission and excitation spectra of several specific sites are obtained by site-selective spectroscopy. For the preferentially investigated site (I-->0 at 16268 cm-1), the three substates I, II, and III of the T1 triplet state are separated by DeltaEII-I=2.9 cm-1 and DeltaEIII-I=25.0 cm-1, respectively. DeltaEIII-I represents the total zero-field splitting (ZFS). The individual decay times of these substates are tauI=150 micros, tauII=58 micros, and tauIII=2 micros, respectively. The long decay time of the lowest substate I indicates its almost pure triplet character. The time for relaxation from state II to state I (spin-lattice relaxation, SLR) is as long as 22 micros at T=1.5 K, while the thermalization between the two lower lying substates and substate III is fast. Application of a magnetic field induces Zeeman mixing of the substates of T1, resulting in an increased splitting between the two lower lying substates from 2.9 cm-1 at zero field to, for example, 6.8 cm-1 at B=10 T. Further, the decay time of the B-field perturbed lowest substate IB decreases by a factor of about 7 up to 10 T. The magnetic field properties clearly show that the three investigated states belong to the same triplet parent term of one single site. Other sites show a similar behavior, though the values of ZFS vary between 15 and 27 cm-1. Since the amount of ZFS reflects the extent of MLCT (metal-to-ligand charge transfer) parentage, it can be concluded that the emitting state T1 is a 3LC (ligand centered) state with significant admixtures of 1,3MLCT (metal-to-ligand charge transfer) character. Interestingly, the results show that the MLCT perturbation is different for the various sites. An empirical correlation between the amount of ZFS and the compound's potential for its use as emitter material in an OLED is presented. As a rule of thumb, a triplet emitter is considered promising for application in OLEDs, if it has a ZFS larger than about 10 cm-1.

Effect of pyrene and acetophenone on photostability of poly(methylphenylsilane) films

We have used investigations of low-temperature photoluminescence and thermally stimulated luminescence to study the effect of acetophenone and pyrene dopants on the photochemical stability of poly(methylphenylsilane) (PMPS) films. We have established that pure PMPS films and acetophenone-doped PMPS films degrade on exposure to UV radiation due to breaking of Si-Si bonds followed by formation of crosslinks between the macromolecules. At the same time, pyrene inhibits degradation of PMPS to a significant extent. This proves that photoinduced breaking of the Si-Si bond can occur upon excitation of the macromolecule to the first triplet T1(σσ*) state, since acetophenone sensitizes while pyrene quenches the intrinsic phosphorescence of PMPS. So the photochemical stability of PMPS can be improved by introducing a dopant which quenches triplet excitations of the macromolecule.

Quenching of ππ* triplet states of vaporous polycyclic aromatic compounds by oxygen

The oxygen quenching rate constants k T O2 of the triplet state T 1 of vapors of polycyclic aromatic hydrocarbons (PAHs) with strongly different oxidation potentials 0.44 eV < E OX < 1.61 eV and energies of the triplet levels 14800 cm−1 < E T < 24500 cm−1 (anthracene, 2-aminoanthracene, 9-nitroanthracene, chrysene, phenanthrene, fluoranthene, and carbazole) are estimated from the measured dependences of the decay rates and intensities of delayed fluorescence on the oxygen pressure P O2. It is found that the rate constants k T O2 vary from 4 × 103 (9-nitroanthracene) to 4 × 105 s−1 Torr−1 (2-aminoanthracene) and increase with decreasing oxidation potentials E OX of PAHs. The rate constants k T O2 for vapors and solutions are compared. The dependences of k T O2 on the free energy of two intermolecular processes, namely, triplet energy transfer to oxygen and electron transfer, are analyzed. It is shown that the rate constants k T O2 increase with decreasing electron transfer free energy, which proves that, along with energy transfer, charge-transfer interactions contribute to the quenching of the triplet states of PAH vapors.

Efficient Electrogenerated Chemiluminescence from Bis-Cyclometalated Iridium(III) Complexes with Substituted 2-Phenylquinoline Ligands

The ECL behavior of bis-cyclometalated (pq)2Ir(LX) complexes, in which pq is a 2-phenylquinoline anion and LX is a monoanionic bidentate ligand (e.g., acetylacetonate, picolinate, etc.), and the specific influences of the electrochemical stability and photoluminescence quantum yield (PL QY) of luminophores on ECL generation have been investigated. In this study, efficient ECLs, some of which even approached ΦECL = 0.88 (18× higher than that of Ru(bpy)32+), have been observed in the annihilation process. The simultaneous accumulation to excited singlet S1 and triplet T1 states and the spin−orbit coupling characteristics of transition metal complexes are expected to produce efficient annihilation ECL, which permits the high efficiency to exceed PL QY. A cyclic voltammetric study has revealed that the ECL intensity depends primarily on the electrochemical stability of the redox precursors of (pq)2Ir(LX)s. For example, (pq)2Ir(acac) (acac = acetylacetonate anion), which shows irreversible reduction, has produ...

Theoretical study of the phosphorescence of 8-azasteroids

We have used quantum chemical methods for a theoretical study of the luminescent characteristics (including estimating the rate constants for intramolecular T1 → S0 electronic transitions) of the molecules of four immunoactive compounds of the 8-azasteroid class. We have established that the T1 state is localized on the enaminodicarbonyl moiety that is common to all the molecules. We have shown that for the studied compounds, the phosphorescence spectra and the physical mechanisms for deactivation of the lower triplet state T1 are similar. For all the compounds, the results of the theoretical study are consistent with the corresponding conclusions of experimental studies of 8-azasteroids.

Methyl 3-Methyl-2H-azirine-2-carboxylate Photochemistry Studied by Matrix-isolation FTIR and DFT Calculations

The aliphatic 2H-azirine, methyl 3-methyl-2H-azirine-2-carboxylate (MMAC), has been synthesized and its monomeric form investigated by IR spectroscopy in an argon matrix, at 10 K, as well as theoretically (DFT/B3LYP/6-311++G(d,p)). Two low-energy conformers of MMAC (Ct and Cc) were found in the matrix, both exhibiting the cis conformation around the C-O bond but differing in the arrangement around the C-C(alpha) bond. The two conformers were photoreactive upon in situ broadband UV excitation (lambda > 235 nm), yielding nitrile ylide (P1) and ketene imine (P2) type products, which resulted from cleavage of the C-C or C-N bond, respectively. The kinetics of the reactions leading to the formation of P1 and P2 are of first order, with the processes being favored when the reactant is in the Cc conformation. Very interestingly, the C-N bond photocleavage, which is unusual for aliphatic 2H-azirines, was found to be preferred over the generally favored in 2H-azirines C-C bond breakage. This behavior is attributed to the presence in the molecule of the electron-withdrawing methoxycarbonyl substituent, which accelerates the intersystem crossing toward the T(1) triplet state and, in this way, favors the C-N bond cleavage. In addition to the primary photoprocesses leading to formation of P1 and P2, secondary photoprocesses leading to the decarboxylation and decarbonylation of P2 have been also observed.

Formation of intersystem crossing transitions in Pd(II) and Pt(II) porphyrins: Nonplanar distortions of the macrocycle and charge transfer states

We found new bands in the absorption spectra of Pt(II) and Pd(II) complexes of octaethylporphyrin and tetraphenylporphyrin that differ in the nature, number, and position of their side substituents. The bands are observed at 295 K in the range 570–690 nm and are attributed to spin-forbidden transitions from the ground S 0 state to the excited T 1 and T 2 triplet states (the internal heavy atom effect). We determined the frequency distribution, number, and nature of these transitions, as well as their extinction coefficients (ɛ = 6.0–210.0 M−1 cm−1), using computer decomposition of complex contours into Gaussian components and additional data obtained from the phosphorescence and phosphorescence excitation spectra of these complexes (295–77 K). In comparison to Pd complexes of porphyrins with planar macrocycles, nonplanar distortions of the tetrapyrrole macrocycle in the ground S 0 state of the sterically hindered PdOETPP molecule cause a bathochromic shift of the bands of the electronic spin-forbidden S 0 → T 1 and S 0 → T 2 transitions, as well as an increase in their extinction coefficients. For the PdOEP-Ph(o-NO2) molecule, which contains the electron acceptor nitro group, an absorption band attributed to an electronic transition from the ground state S 0 to a charge transfer state (λmax = 905 nm, ɛ = 10.0 M−1 cm−1) is observed at 295 K.

Absorption spectra of organic semiconductors in IR-range measured by constant photocurrent method

Constant photocurrent method has been used to study spectral dependence of the absorption coefficient in copper phthalocyanine thin films formed by the molecules with different peripheral groups. Spectral dependence of the absorption coefficient reveals exponential growth in the energy range 1.4–1.6eV and a series of three bands in the energy below 1.4eV. The maximum observed at 1.12eV is attributed to optical transition from ground state S0 to excited triplet state T1 of a molecule. This transition results in creation of triplet excitons. The correspondence between photoluminescence and absorption spectra measured by constant photocurrent method both with respect to peak positions and relative intensities has been found.

Characterizing the Fluorescence Intermittency and Photobleaching Kinetics of Dye Molecules Immobilized on a Glass Surface

The blinking behavior of single Atto565 molecules on a glass surface is studied under air or nitrogen atmospheres using confocal microscopy. The broad distributions for both on- and off-time durations obey power law kinetics that are rationalized using a charge tunneling model. In this case, a charge is transferred from the Atto565 molecule to localized states found on the glass surface. Subsequent charge recombination by back charge tunneling from trap to Atto565 cation (i.e., dark state) restores the fluorescence. The off-time distribution is independent of excitation intensity (I), whereas the on-time distribution exhibits a power law exponent that varies with I. Two pathways have been identified to lead to the formation of the radical dark state. The first involves direct charge tunneling from the excited singlet S1 state to charge traps in the surrounding matrix, and the second requires charge ejection from the triplet T1 state after intersystem crossing from S1. Monte Carlo simulation studies complement the two-pathway model. Photobleaching curves of both single and ensemble molecules do not exhibit monoexponential decays suggesting complex bleaching dynamics arising from triplet and radical states.

The effect of the heteroatom on the radiative deactivation of the lowest triplet state in heterocyclic analogues of fluorene

The vibronic transition strengths F VSO s for the transitions from the spin sublevels s of the triplet state T 1 to the energy levels of the nontotally symmetric vibrations of dibenzothiophene (DB(S)) are calculated. For a series of heterocyclic analogues of fluorene (DB(S) and previously studied carbazole, dibenzofuran, and phenyldibenzophosphole), the regular features of the effect of the valence state of the heteroatom and of the spin-orbit interactions in individual atomic groups of the molecule are revealed. The factors affecting changes in the radiative deactivation rate constant of the T 1 state of the molecules studied are established. The intensity distribution of the vibronic lines in the Herzberg-Teller component of the fine structure phosphorescence spectrum of phenyldibenzophosphole is calculated taking into account different populations of the triplet sublevels.