Multiple-State Emissions from Neat,Single-Component Molecular Solids: Suppression of Kasha's Rule

Three rigid and structurally simple heterocyclic stilbene derivatives, (E)-3H,3′H-[1,1′-biisobenzofuranylidene]-3,3′-dione, (E)-3-(3-oxobenzo[c] thiophen-1(3H)-ylidene)isobenzofuran-1(3H)-one, and (E)-3H,3′H-[1,1′-bibenzo[c] thiophenylidene]-3,3′-dione, are found to fluoresce in their neat solid phases, from upper (S₂) and lowest (S₁) singlet excited states, even at room temperature in air. Photophysical studies, single-crystal structures, and theoretical calculations indicate that large energy gaps between S₂ and S₁ states (T₂ and T₁ states) as well as an abundance of intra and intermolecular hydrogen bonds suppress internal conversions of the upper excited states in the solids and make possible the fluorescence from S₂ excited states (phosphorescence from T₂ excited states). These results, including unprecedented fluorescence quantum yields (2.3–9.6 %) from the S₂ states in the neat solids, establish a unique molecular skeleton for achieving multi-colored emissions from upper excited states by “suppressing” Kasha's rule.     

Multiple‐State Emissions from Neat,Single‐Component Molecular Solids: Suppression of Kasha's Rule

Three rigid and structurally simple heterocyclic stilbene derivatives, (E)‐3H,3′H‐[1,1′‐biisobenzofuranylidene]‐3,3′‐dione, (E)‐3‐(3‐oxobenzo[c] thiophen‐1(3H)‐ylidene)isobenzofuran‐1(3H)‐one, and (E)‐3H,3′H‐[1,1′‐bibenzo[c] thiophenylidene]‐3,3′‐dione, are found to fluoresce in their neat solid phases, from upper (S₂) and lowest (S₁) singlet excited states, even at room temperature in air. Photophysical studies, single‐crystal structures, and theoretical calculations indicate that large energy gaps between S₂ and S₁ states (T₂ and T₁ states) as well as an abundance of intra and intermolecular hydrogen bonds suppress internal conversions of the upper excited states in the solids and make possible the fluorescence from S₂ excited states (phosphorescence from T₂ excited states). These results, including unprecedented fluorescence quantum yields (2.3–9.6 %) from the S₂ states in the neat solids, establish a unique molecular skeleton for achieving multi‐colored emissions from upper excited states by “suppressing” Kasha's rule.     

Unoccupied titanium 3d states due to subcluster formation in stoichiometric TiO2 nanoparticles

The source of unoccupied Ti 3d states in the case of stoichiometric anatase structured (TiO₂)_n clusters has been investigated using ab initio methods. These unoccupied gap states appear for example in the case of a stoichiometric (TiO₂)₃₈ cluster. We show that the origin of these gap states is related to effective subcluster formation which gives rise to empty defect‐like gap states, when these states are split off from conduction band. © 2015 Wiley Periodicals, Inc.     

Small nickel clusters: Two low-lying Ni<Subscript>3</Subscript> states

Matrix isolation IR spectroscopy and quantum-chemical calculations were jointly used to identify the system of bands related to Ni₃ clusters. The positions of two low-lying electronic states were determined, and vibrational frequencies and geometry in the ground and excited states were estimated. In all the calculated states, Ni₃ had the structure of an isosceles triangle. In the X ³ B ₂ ground and a ³ B ₁ lower excited states, this was an acute-angled triangle. In the b ³ B ₂ and c ³ B ₁ excited states, the triangle was obtuse-angled.     

Triplet excited states of nitronaphthalenes. IV. 1,2- and 1,8-dinitronaphthalenes

Nanosecond flash photolysis of 1,2- and 1,8-dinitronaphthalenes (1,2-DNO₂N; 1,8-DNO₂N) in nonpolar and polar solvents shows transient species with absorption maxima and lifetimes dependent on solvent polarity. In deaerated n-hexane the absorption maxima and lifetimes (1/K) are 490 nm and 1.0 μsec for 1,2-DNO₂N and 550 nm and 2.5 μsec for 1,8-DNO₂N. In deaerated ethanol the corresponding values are 550 nm and 4.3 μsec for 1,2-DNO₂N and 590 nm and 5.3 μsec for 1,8-DNO₂N. The transient absorptions are attributed to the lowest triplet excited states T₁ of the 1,2- and 1,8-DNO₂N. The observed red shifts in the absorption maxima of the T₁ states are indicative of the extent to which electronic charge is transferred intramolecularly during the T₁ → T_n transitions. Furthermore, the increased lifetime of the T₁ states with increasing solvent polarity indicates the intramolecular charge transfer character of the T₁ states. Changes of dipole moments accompanying the T₁ → T_n transitions as well as rate constants for electron or proton transfer and hydrogen abstraction reactions involving the T₁ states of 1,2- and 1,8-DNO₂N and tributyl tin hydride (Bu₃SnH) as the hydrogen donor were determined together with the activation energy of the hydrogen abstraction reaction for the case of 1,2-DNO₂N. The spectroscopic and kinetic data obtained in this work demonstrate that the triplet states of 1,2- and 1,8-DNO₂N behave like n → π* states in nonpolar media while in polar solvents the n → π* character of these states is reduced with a simultaneous increase in their intramolecular charge transfer character.     

STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS — V. AB INITIO CONFIGURATION INTERACTION CALCULATIONS ON THE GROUND AND LOWER EXCITED SINGLET AND TRIPLET STATES OF ETHYL CHLOROPHYLLIDE a AND ETHYL PHEOPHORBIDE a

Abstract— Ab initio configuration interaction wavefunctions and energies are reported for the ground state and many low-lying excited singlet and triplet states of ethyl pheophorbide a (Et-Pheo a) and ethyl chlorophyllide a (Et-Chl a), and are employed in an analysis of the electronic absorption spectra of these systems. In both molecules the visible spectrum is found to consist of transitions to the two lowest-lying ¹(π, π*) states, S₁ and S₂. The configurational compositions of S₁ and S₂ in both molecules are similar, and are described qualitatively in terms of a four-orbital model. The S₁← S₀ transition in each case is predicted to be intense, and is largely in-plane y-polarized, while the S₂← S₀ transition is predicted to be extremely weak and in-plane polarized. The orientation of the S₂← S₀ transition dipole is not conclusively established in the present calculations. The Soret band in both molecules is composed of transitions to no less than ten states (S_3-S₁₂ in Et-Chl a and S₃-S₇S₉-S₁₂. and S₁₄ in Et-Pheo a), which exhibit primarily (π, π*) character. The configurational compositions of these states are generally a complex mixture of excitations from both occupied macrocyclic π molecular orbitals and occupied orbitals with electron density in the cyclopen-tanone ring and the carbomethoxy chain. No clear correspondences are evident between respective Soret states of the two systems. Transitions to these states are generally intense and display a variety of in-plane polarizations. Two additional Soret states of Et-Pheo a, S₈ and S₁₃, exhibit primarily (n. π*) character. S₈ is characterized by excitations from u and non-bonding regions of the carbomethoxy chain, while S₁₃ is described by n →π* excitations involving the nitrogen atom of ring II. No corresponding (n, π*) states were found for Et-Chl a. In both molecules the lowest two triplet states, T₁ and T₂, are found to lie lower in energy than S₁. while T, and S₁ are approximately degenerate. The configurational compositions of T₁-T₄ of both molecules are nearly identical, and may be described by a four-orbital model. However, the compositions of T₁-T₄ differ sharply from those of S₁ and S₂. A number of higher-lying ³(π, π*) states of both molecules (T₅-T₁₃ in Et-Chi a and T₈-T₉, T₁₁-T₁₃ in Et-Pheo a) are found to have energies similar to the singlet Soret states, relative to S₀. They are characterized by a complex mixture of configurations which do not include significant contributions involving the four-orbital model. In addition, two ³(n, π*) states of Et-Pheo a, T₁₀ and T₁₄, are found, which are somewhat analogous to S₈ and S₁₃. Additional data presented include the charge distributions and molecular dipole moments of the S₀. S₁, and T₁ states of both molecules, as well as energies and oscillator strengths of computed S_n←S₁ and T_n←₁ transitions.     

Theoretical electronic structure of the molecule ScI

Theoretical investigation of the 18 lowest electronic states of the molecule ScI in the representation ^2S+1Λ^(±) has been performed via CASSCF and MRCI (single and double excitation with Davidson correction) calculations. To the best of our knowledge these calculated electronic states are the first ones from ab initio methods. Thirteen electronic states between 4,500 cm^?1 and 21,000 cm^?1 have been studied for the first time and have not yet been observed experimentally. The harmonic frequency ω_e, the internuclear distance R_e, the electronic transition energy with respect to the ground state T_e, and the rotational constant B_e have been calculated for the considered electronic states. By using the canonical functions approach the eigenvalues E_υ and the rotational constants B_υ have also been calculated for the six lowest‐lying electronic states. The comparison of these results with the theoretical and the experimental data available in the literature shows a good agreement. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

A CAS study on S‐loss and O‐loss dissociation mechanisms of the SO2+ ion in the C,D, and E states

In the present work, we mainly study dissociation of the C ²B₁, D²A₁, and E²B₂ states of the SO₂⁺ ion using the complete active‐space self‐consistent field (CASSCF) and multiconfiguration second‐order perturbation theory (CASPT2) methods. We first performed CASPT2 potential energy curve (PEC) calculations for S‐ and O‐loss dissociation from the X, A, B, C, D, and E primarily ionization states and many quartet states. For studying S‐loss predissociation of the C, D, and E states by the quartet states to the first, second, and third S‐loss dissociation limits, the CASSCF minimum energy crossing point (MECP) calculations for the doublet/quartet state pairs were performed, and then the CASPT2 energies and CASSCF spin‐orbit couplings were calculated at the MECPs. Our calculations predict eight S‐loss predissociation processes (via MECPs and transition states) for the C, D, and E states and the energetics for these processes are reported. This study indicates that the C and D states can adiabatically dissociate to the first O‐loss dissociation limit. Our calculations (PEC and MECP) predict a predissociation process for the E state to the first O‐loss limit. Our calculations also predict that the E²B₂ state could dissociate to the first S‐ and O‐loss limits via the A²B₂ ← E²B₂ transition. On the basis of the 13 predicted processes, we discussed the S‐ and O‐loss dissociation mechanisms of the C, D, and E states proposed in the previous experimental studies. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Quantum-Chemical Analysis of Electronic Excitation and Reactivity of Olefins and Dienes

Results obtained in studying the structure of olefin and diene molecules, and complexes of these, in the ground and lower excited states by RHF, ROHF, GVB/DN, and 6-31G^* quantum-chemical methods are presented. Attention is paid to the identity of the main structural and electronic parameters of triplet T ₁ and singlet S ₁ states forming a reactive fourfold spin-degenerate diradical equilibrium excited state (S·T)₁ having the lowest energy. A new mechanism of cyclodimerization of ethylene and tetrafluoroethylene and anionic polymerization of dienes, involving the (S·T)₁ states, is suggested.     

Mechanism of vibrational relaxation and intersystem crossing within excited ion-pair states of I2

Visible and ultraviolet fluorescence of I₂, following excitation by ArF/193nm excimer laser pulses, was recorded for different pressures of argon buffer gas in a flow system. Dispersed fluorescence spectra due to the transitionsD’(2_g) → A’(2_y andD(0 _n ⁺ )→X0 _g ⁺ ) were analysed by inversion and spectral simulations. Thus vibrational distributions in the emitting states were obtained as a function of pressure to determine the mechanism of relaxation to populate the lowest quantum levels of theD’ state, which are the emitting states in the iodine laser. Fast intersystem crossing is found to occur from initially populated vibrational levels of theD state to other ion-pair states correlating with the ground state ions, followed by rapid relaxation, involving both direct vibrational relaxation within individual states and intersystem crossing between states.     

Theoretical studies on quinones I. The structure of p-benzoquinone and two of its excited triplet states

Ab initio calculations on the ground and two excited triplet states (³B_1g and ³B_1u) of p-benzoquinone are described. The geometries of the three states were fully optimised at the SCF level using the 3-21G basis set. For the excited states, both D_2h and C_2v geometries were investigated. Comparison was made between UHF and ROHF levels of theory.     

Ab initio study of the structure and conformations of 2-fluoroethanal in the ground and lowest excited electronic states

The structure of the conformationally flexible 2-fluoroethanal molecule (CH₂FCHO, FE) in the ground (S₀) and lowest excited triplet (T₁) and singlet (S₁) electronic states was investigated by ab initio quantum-chemical methods. The FE molecule in the S₀ state was found to exist as two conformers, viz., as cis (the F—C—C—O angle is 0°) and trans (the F—C—C—O angle is 180°) conformers. On going both to the T₁ and S₁ states, the FE molecule undergoes substantial structural changes, in particular, the CH₂F top is rotated with respect to the core and the carbonyl CCHO fragment becomes nonplanar. The potential energy surfaces for the T₁ and S₁ states are qualitatively similar, viz., six minima in each of the excited states of FE correspond to three pairs of mirror-symmetrical conformers. Based on the potential energy surfaces calculated for the FE molecule in the T₁ and S₁ states, the one-dimensional problems on the torsion and inversion nuclear motions as well as the two-dimensional torsion-inversion problems were solved.     

Inverting the external-potential-to-electron density map in systems of electrons by minimization of a least-squares fit functional for analytic potentials

In a system of electrons, there is a map connecting any external potential v with its electron density ρ _v. In this work, we describe a procedure for inverting this potential-to-density map, so that potentials (if any) corresponding to a target density ρ_t can be obtained. We give the trial external potential v _α , an analytic expression depending on a number of parameters α = (α₁, …) and then minimize the least-squares integral ∫(ρ _α − ρ_t)² d r by the conjugate gradient method, where ρ _α is the density corresponding to v _α . The implementation takes advantage of the analytic nature of v _α . The procedure can be applied to any system and quantum chemistry model, and works both for ground and excited states, as well as for ensembles of states. The method is tested with some excited states of the particle-in-a-box model, confirming the lack of a Hohenberg–Kohn theorem for excited states. It is also applied to the first singlet excited state of the helium atom, where, apart from the nucleus–electron attraction potential, some generalized external potentials are found.     

High-Resolution Experimental Study on Photodissocaition of N2O

We study the photodissociation dynamics of nitrous oxide using the time-sliced ion velocity imaging technique at three photolysis wavelengths of 134.20, 135.30, and 136.43 nm. The O(¹S_J=0)+N₂(X¹∑_g⁺) product channels were investigated by measuring images of the O(¹S_J=0) products. Vibrational states of N₂(X¹∑_g⁺) products were fully resolved in the images. Product total kinetic energy releases (TKER) and the branching ratios of vibrational states of N₂ products were determined. It is found that the most populated vibrational states of N₂ products are v=2 and v=3. The angular anisotropy parameters (β values) were also derived. The β values are very close to 2 at low vibrational states of the correlated N₂(X¹∑_g⁺) products at all three photolysis wavelengths, and gradually decrease to about 1.4 at v=7. This indicates the dissociation is mainly through a parallel transition state to form products at lower vibrational states, and the highly vibrational exited products are from a more bent configuration. This is consistent with the observed shift of the most intense rotational structure in the TKER as the vibrational quantum number increases.     

Charge and spin correlation structures of conjugated π systems: Analysis of full CI wave functions of the PPP hamiltonian

An analysis of the electronic correlation structures by means of the charge and spin correlation functions is carried out for full CI wave functions of four, five, and six membered conjugated π systems described by the Pariser–Parr–Pople Hamiltonian. The low-lying states of these systems are classified as covalent (CV ) and ionic (IN ) states depending on whether the probability of finding two electrons simultaneously at the same position is small or large. It is found that many of excited CV states, the typical ones of which are the 2¹A_g state of linear π systems, have stronger CV character than the ground CV state, and their spin coupling structures are different from each other as well as from that of the ground CV state. The spin coupling structure in the ground CV state has an “antiferromagnetic” spin arrangement in favor of antiparallel coupling between nearest neighbor spins while in excited CV states the extent of the antiparallel spin coupling between nearest neighbor sites is decreased. IN states, which are less common for low-lying states than CV ones, are also found to have characteristic modulations in the charge correlation. In particular, the charge correlations in the lowest singlet IN states, 1¹B_u of linear π systems, 1¹B_2g of cyclobutadiene and 1¹B_1U of benzene, are alternating.     

Ab initio study of low-lying electronic states of the FNO2 molecule

Ab initio electronic structure calculations are reported for low-lying electronic states, ¹A₁, ¹A₂, ³A₂, ¹B₁, ³B₁, ¹B₂, and ³B₂ of the FNO₂ molecule. Geometric parameters for the ground state ¹A₁ are predicted by MRSDCI calculations with a double-zeta plus polarization basis set. The vertical excitation energies for these electronic states are determined using MRSDCI/DZ+P calculations at the ground-state equilibrium conformation. The oscillator strengths and radiative lifetimes for some electronic states are calculated based on the MRSDCI wave functions. © 1993 John Wiley & Sons, Inc.     

Acceleration of Reverse Intersystem Crossing using Different Types of Charge Transfer States

There is a need to boost the rate constant of reverse intersystem crossing (k_RISC) in thermally activated delayed fluorescence (TADF) materials for applications to organic light-emitting diodes. Recently, energy level matching of the locally excited state (LE) and charge transfer state (CT) has been reported to enhance k_RISC. In this study, we conceptually demonstrate that k_RISC can be improved even between CT states without LE states, through the use of different types of CT states. On the basis of this concept, we design a new compound, named DMAC-bPmT, where two phenyl groups of a well-known TADF material DMAC-TRZ are substituted by pyrimidine groups. Theoretical calculations indicated that the energy levels of the different CT states of DMAC-bPmT are very close and enhanced spin orbit coupling may be expected between them. As predicted, DMAC-bPmT experimentally exhibited a k_RISC three times as high as that of DMAC-TRZ.     

Potential energy surfaces of pseudoaromatic molecules: An MMVB and CASSCF study of pentalene

The S₀ and S₁ potential energy surfaces of pentalene were studied using MMVB—a hybrid force-field/parametrized valence bond (VB) method designed to simulate CASSCF calculations for ground and covalent excited states. The results were calibrated against full CASSCF calculations. Four distinct critical points were optimized: on S₀, a C_2h minimum (with alternating single and double bonds) and a D_2h transition structure; and on S₁, a D_2h minimum and an adjacent S₁/S₀ conical intersection. A VB exchange density matrix (which is independent of the choice of the spin-coupled basis) was used to rationalize the S₀ and S₁ surface topologies. Craig defined pseudoaromatic molecules to be those with nontotally symmetric electronic ground states. For pentalene, this is true for both CASSCF and MMVB calculations: the CASSCF S₀ transition structure is an open-shell B_1x singlet, and the VB ground state is dominated by a spin-coupling which transforms as B_1g. A C_2v minimum and a D_2h transition structure were located on the CASSCF S₂ potential energy surface. This state cannot be represented by MMVB because of the importance of ionic configurations. The characters of the S₁ S₂ states of pentalene are shown to be reverse of the S₁ and S₂ states of benzene. © 1996 John Wiley & Sons, Inc.     

Molecular electric properties in electronic excited states: multipole moments and polarizabilities of H2O in the lowest1 B 1 and3 B 1 excited states

Summary The dipole and quadrupole moments and the dipole polarizability tensor components are calculated for the¹ B ₁ and³ B ₁ excited states of the water molecule by using the complete active space (CAS) SCF method and an extended basis set of atomic natural orbitals. The dipole moment in the lowest¹ B ₁ (0.640 a.u.) and³ B ₁ (0.416 a.u.) states is found to be antiparallel to that in the ground electronic state of H₂O. The shape of the quadrupole moment ellipsoid is significantly modified by the electronic excitation to both states investigated in this paper. All components of the excited state dipole polarizability tensor increase by about an order of magnitude compared to their values in the ground electronic state. The present results are used to discuss some aspects of intermolecular interactions involving molecules in their excited electronic states.