Electronic nonadiabatic interactions and ultrafast internal conversion in phenylacetylene radical cation

Quantum chemistry and dynamics of the ground X^~ 2B₁ and low-lying excited A^~ 2A₂, B^~ 2B₂, and C^~ 2B₁ electronic states of phenylacetylene radical cation are examined here in striving to understand its photostability, long-lived excited electronic states, and resolved (<10 meV) vibrational energy level spectrum. The electronic potential energy surfaces and their nonadiabatic coupling are computed ab initio. A model Hamiltonian is constructed in a diabatic electronic basis for the nuclear dynamical simulations from first principles. Analysis of electronic structure data reveals the relevance of 24 vibrational degrees of freedom in the quantum dynamics of the X˜-Ã-B˜-C˜ coupled electronic states of the radical cation. The complex vibrational energy level spectrum of this coupled electronic manifold is calculated and assigned. Theoretical results are in excellent accord with the experimental photoelectron spectroscopy data. The agreements and discrepancies of the theoretical results are also recorded and discussed with the mass-analyzed threshold ionization and photoinduced Rydbergionization and photodissociation spectroscopy results of the X˜ and C˜ electronic states, respectively. The lifetimes of the excited electronic states of phenylacetylene radical cation are estimated from the decay of electronic population and are discussed in relation to the available experimental data

Photostability of electronically excited polyacenes: a case study of vibronic coupling in the naphthalene radical cation

Novel issues of electronic nonadiabatic coupling in the excited state dynamics of prototypical naphthalene radical cation of polycyclic aromatic hydrocarbon of the polyacene family are theoretically investigated. A benchmark ab initio quantum dynamical study is performed and its complex vibronic spectra and nonradiative decay are examined. The findings are in very good accord with the experiment, unambiguously establishing the crucial role of intricate electron-nuclear coupling in the photoinduced dynamical processes of this system

Theoretical study of the electronically excited radical cations of naphthalene and anthracene as archetypal models for astrophysical observations. Part II. Dynamics consequences

Nuclear dynamics is investigated theoretically from first principles by employing the ab initio vibronic models of the prototypical naphthalene and anthracene radical cations developed in Part I. This Part is primarily aimed at corroborating a large amount of available experimental data with a specific final goal to establish an unambiguous link with the current observations in astrophysics and astronomy. The detailed analyses presented here perhaps establish that these two prototypical polycyclic aromatic hydrocarbon radical cations are indeed potential carriers of the observed diffuse interstellar bands

First principles quantum dynamical investigation provides evidence for the role of polycyclic aromatic hydrocarbon radical cations in interstellar physics

Inspired by the recent astronomical discovery of new diffused interstellar bands (DIBs) assigned to the electronic transitions in the naphthalene radical cation based on complementary laboratory measurements, we attempt here an ab initio quantum dynamical study to validate this assignment. In addition, the existence and mechanistic details of nonradiative deactivation of electronically excited polycyclic aromatic hydrocarbon (PAH) radical cations in the interstellar medium and their identity as carriers of DIBs are established here focusing on the prototypical naphthalene and anthracene radical cations of the PAH family

(B+E⊗b)⊗e Jahn–Teller and pseudo-Jahn–Teller effects in the spiropentane radical cation

In this paper we examine the Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) effects in the spiropentane radical cation (SP<SUP>+</SUP>) by an ab initio quantum dynamical method. Spiropentane (SP) possesses D<SUB>2d</SUB> symmetry at its equilibrium configuration. The two low-lying electronic states of SP<SUP>+</SUP> belong to X&#732;<SUP>2</SUP>B<SUB>2</SUB> and &#195;<SUP>2</SUP>E symmetry, respectively. SP<SUP>+</SUP> in the degenerate &#195;state is susceptible to JT distortions along the vibrational modes of b symmetry. The &#195; state of SP<SUP>+</SUP> is vertically ∼0.51 eV spaced from its X&#732; state. Symmetry rule allows a coupling of the X&#732; and &#195; states via the degenerate e vibrational modes. This is termed as the (B + E ⊗ b) ⊗ e JT and PJT effects revealing the symmetry of the electronic states and the coupling vibrational modes. The theoretical findings establish significant impact of the JT and PJT coupling in the observed complex structure of the X&#732;–&#195;bands of SP<SUP>+</SUP>

Vibronic interactions in the photodetachment spectroscopy of phenide anion

Photodetachmentspectroscopy of phenide anion C₆H₅^- is theoretically studied with the aid of electronic structure calculations and quantum dynamical simulations of nuclear motion. The theoretical results are compared with the available experimental data. The vibronic structure of the first, second, and third photoelectron bands associated with the ground X^~2A₁ and low-lying excited A^˜2B₁ and B^˜2A₂ electronic states of the phenyl radical C6H5 is examined at length. While the X˜ state of the radical is energetically well separated and its interaction is found to be rather weak with the rest, the A^~ and B^~ electronic states are found to be only ∼0.57eV apart in energy at the vertical configuration. Low-energy conical intersections between the latter two states are discovered and their impact on the nuclear dynamics underlying the second and third photoelectron bands is delineated. The nuclear dynamics in the X˜ state solely proceeds through the adiabatic path and the theoretically calculated vibrational level structure of this state compares well with the experimental result. Two Condon active totally symmetric (a1) vibrational modes of ring deformation type form the most dominant progression in the first photoelectron band. The existing ambiguity in the assignment of these two vibrational modes is resolved here. The A˜-B˜ conical intersections drive the nuclear dynamics via nonadiabatic paths, and as a result the second and third photoelectron bands overlap and particularly the third band due to the B˜ state of C₆H₅ becomes highly diffused and structureless. Experimental photodetachmentspectroscopy results are not available for these bands. However, the second band has been detected in electronic absorption spectroscopy measurements. The present theoretical results are compared with these absorption spectroscopy data to establish the nonadiabatic interactions between the A˜ and B˜ electronic states of C₆H₅

Reaction surface approach to multimode vibronic coupling problems: General framework and application to furan

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Access to Triplet Excited State in Core-Twisted Perylenediimide

Solvent-free crystal structure of <i>N,N</i>-bis­(propylacetyl)-1,6,7,12-tetrabromoperylene-3,4:9,10-bis­(dicarboximide), PDI-Br₄, obtained by X-ray diffraction reveals the core-twisted perylene motif having π–π stacks at an interplanar separation of 3.7 Å. Slip-stacked arrangement of PDI units in PDI-Br₄ arises due to the presence of bulky bromine atoms. Femtosecond pump–probe measurements of monomeric PDI-Br₄ in toluene reveal ultrafast intersystem crossing (τ_ISC < 110 fs) when excited at 400 nm. Triplet quantum yield (Φ_T) of 19 ± 1% and 105 ± 5% for PDI-Br₄ in toluene and vapor-annealed polycrystalline 60 nm thick film respectively are estimated from nanosecond transient absorption measurements. Quantum chemical calculations show that the combined effects of heavy atom and core-twist in PDI-Br₄ can activate the intersystem crossing by altering the singlet–triplet energy gap. Enhanced quantum yield accounts for the singlet fission mediated generation of triplet excited state in the PDI-Br₄ thin film

Access to Triplet Excited State in Core-Twisted Perylenediimide

Solvent-free crystal structure of <i>N,N</i>-bis­(propylacetyl)-1,6,7,12-tetrabromoperylene-3,4:9,10-bis­(dicarboximide), PDI-Br₄, obtained by X-ray diffraction reveals the core-twisted perylene motif having π–π stacks at an interplanar separation of 3.7 Å. Slip-stacked arrangement of PDI units in PDI-Br₄ arises due to the presence of bulky bromine atoms. Femtosecond pump–probe measurements of monomeric PDI-Br₄ in toluene reveal ultrafast intersystem crossing (τ_ISC < 110 fs) when excited at 400 nm. Triplet quantum yield (Φ_T) of 19 ± 1% and 105 ± 5% for PDI-Br₄ in toluene and vapor-annealed polycrystalline 60 nm thick film respectively are estimated from nanosecond transient absorption measurements. Quantum chemical calculations show that the combined effects of heavy atom and core-twist in PDI-Br₄ can activate the intersystem crossing by altering the singlet–triplet energy gap. Enhanced quantum yield accounts for the singlet fission mediated generation of triplet excited state in the PDI-Br₄ thin film

Quantum Dynamics Simulations Reveal Vibronic Effects on the Optical Properties of [<i>n</i>]Cycloparaphenylenes

The size-dependent ultraviolet/visible photophysical property trends of [<i>n</i>]­cycloparaphenylenes ([<i>n</i>]­CPPs, <i>n</i> = 6, 8, and 10) are theoretically investigated using quantum dynamics simulations. For geometry optimizations on the ground- and excited-state Born–Oppenheimer potential energy surfaces (PESs), we employ density functional theory (DFT) and time-dependent DFT calculations. Harmonic normal-mode analyses are carried out for the electronic ground state at Franck–Condon geometries. A diabatic Hamiltonian, comprising four low-lying singlet excited electronic states and 26 vibrational degrees of freedom of CPP, is constructed within the linear vibronic coupling (VC) model to elucidate the absorption spectral features in the range of 300–500 nm. Quantum nuclear dynamics is simulated within the multiconfiguration time-dependent Hartree approach to calculate the vibronic structure of the excited electronic states. The symmetry-forbidden <i>S</i>₀ → <i>S</i>₁ transition appears in the longer wavelength region of the spectrum with weak intensity due to VC. It is found that the Jahn–Teller and pseudo-Jahn–Teller effects in the doubly degenerate <i>S</i>₂ and <i>S</i>₃ electronic states are essential in the quantitative interpretation of the experimental observation of a broad absorption peak around 340 nm. The vibronic mixing of the <i>S</i>₁ state with higher electronic states is responsible for the efficient photoluminescence from the <i>S</i>₁ state. The fluorescence properties are characterized on the basis of the stationary points of the excited-state PESs. The findings reveal that vibronic effects become important in determining the photophysical properties of CPPs with increased ring size