The elusive S2 state, the S1/S2 splitting, and the excimer states of the benzene dimer

We observe the weak S 0 → S 2 transitions of the T-shaped benzene dimers (Bz)2 and (Bz-d 6)2 about 250 cm−1 and 220 cm−1 above their respective S 0 → S 1 electronic origins using two-color resonant two-photon ionization spectroscopy. Spin-component scaled (SCS) second-order approximate coupled-cluster (CC2) calculations predict that for the tipped T-shaped geometry, the S 0 → S 2 electronic oscillator strength f el (S 2) is ∼10 times smaller than f el (S 1) and the S 2 state lies ∼240 cm−1 above S 1, in excellent agreement with experiment. The S 0 → S 1 (ππ ∗) transition is mainly localized on the “stem” benzene, with a minor stem → cap charge-transfer contribution; the S 0 → S 2 transition is mainly localized on the “cap” benzene. The orbitals, electronic oscillator strengths f el (S 1) and f el (S 2), and transition frequencies depend strongly on the tipping angle ω between the two Bz moieties. The SCS-CC2 calculated S 1 and S 2 excitation energies at different T-shaped, stacked-parallel and parallel-displaced stationary points of the (Bz)2 ground-state surface allow to construct approximate S 1 and S 2 potential energy surfaces and reveal their relation to the “excimer” states at the stacked-parallel geometry. The f el (S 1) and f el (S 2) transition dipole moments at the C 2v -symmetric T-shape, parallel-displaced and stacked-parallel geometries are either zero or ∼10 times smaller than at the tipped T-shaped geometry. This unusual property of the S 0 → S 1 and S 0 → S 2 transition-dipole moment surfaces of (Bz)2 restricts its observation by electronic spectroscopy to the tipped and tilted T-shaped geometries; the other ground-state geometries are impossible or extremely difficult to observe. The S 0 → S 1/S 2 spectra of (Bz)2 are compared to those of imidazole ⋅ (Bz)2, which has a rigid triangular structure with a tilted (Bz)2 subunit. The S 0 → S 1/ S 2 transitions of imidazole-(benzene)2 lie at similar energies as those of (Bz)2, confirming our assignment of the (Bz)2 S 0 → S 2 transition

Excitonic Splitting, Delocalization, and Vibronic Quenching in the Benzonitrile Dimer

The excitonic S1/S2 state splitting and the localization/delocalization of the S1 and S2 electronic states are investigated in the benzonitrile dimer (BN)2 and its 13C and d5 isotopomers by mass-resolved two-color resonant two-photon ionization spectroscopy in a supersonic jet, complemented by calculations. The doubly hydrogen-bonded (BN-h5)2 and (BN-d5)2 dimers are C2h symmetric with equivalent BN moieties. Only the S0 → S2 electronic origin is observed, while the S0 → S1 excitonic component is electric-dipole forbidden. A single 12C/13C or 5-fold h5/d5 isotopic substitution reduce the dimer symmetry to Cs, so that the heteroisotopic dimers (BN)2-(h5 – h513C), (BN)2-(h5 – d5), and (BN)2-(h5 – h513C) exhibit both S0 → S1 and S0 → S2 origins. Isotope-dependent contributions Δiso to the excitonic splittings arise from the changes of the BN monomer zero-point vibrational energies; these range from Δiso(12C/13C) = 3.3 cm–1 to Δiso(h5/d5) = 155.6 cm–1. The analysis of the experimental S1/S2 splittings of six different isotopomeric dimers yields the S1/S2 exciton splitting Δexc = 2.1 ± 0.1 cm–1. Since Δiso(h5/d5) ≫ Δexc and Δiso(12C/13C) > Δexc, complete and near-complete exciton localization occurs upon 12C/13C and h5/d5 substitutions, respectively, as diagnosed by the relative S0 → S1 and S0 → S2 origin band intensities. The S1/S2 electronic energy gap of (BN)2 calculated by the spin-component scaled approximate second-order coupled-cluster (SCS-CC2) method is Δelcalc = 10 cm–1. This electronic splitting is reduced by the vibronic quenching factor Γ. The vibronically quenched exciton splitting Δelcalc·Γ = Δvibroncalc = 2.13 cm–1 is in excellent agreement with the observed splitting Δexc = 2.1 cm–1. The excitonic splittings can be converted to semiclassical exciton hopping times; the shortest hopping time is 8 ps for the homodimer (BN-h5)2, the longest is 600 ps for the (BN)2(h5 – d5) heterodimer

Experimental and Calculated Spectra of π‑Stacked Mild Charge-Transfer Complexes: Jet-Cooled Perylene·(Tetrachloroethene)_<i>n</i>, <i>n</i> = 1,2

The S₀ ↔ S₁ spectra of the mild charge-transfer (CT) complexes perylene·tetrachloroethene (P·4ClE) and perylene·(tetrachloroethene)₂ (P·(4ClE)₂) are investigated by two-color resonant two-photon ionization (2C-R2PI) and dispersed fluorescence spectroscopy in supersonic jets. The S₀ → S₁ vibrationless transitions of P·4ClE and P·(4ClE)₂ are shifted by δν = −451 and −858 cm^–1 relative to perylene, translating to excited-state dissociation energy increases of 5.4 and 10.3 kJ/mol, respectively. The red shift is ∼30% larger than that of perylene·<i>trans</i>-1,2-dichloroethene; therefore, the increase in chlorination increases the excited-state stabilization and CT character of the interaction, but the electronic excitation remains largely confined to the perylene moiety. The 2C-R2PI and fluorescence spectra of P·4ClE exhibit strong progressions in the perylene intramolecular twist (1a_u) vibration (42 cm^–1 in S₀ and 55 cm^–1 in S₁), signaling that perylene deforms along its twist coordinate upon electronic excitation. The intermolecular stretching (T_z) and internal rotation (R_c) vibrations are weak; therefore, the P·4ClE intermolecular potential energy surface (IPES) changes little during the S₀ ↔ S₁ transition. The minimum-energy structures and inter- and intramolecular vibrational frequencies of P·4ClE and P·(4ClE)₂ are calculated with the dispersion-corrected density functional theory (DFT) methods B97-D3, ωB97X-D, M06, and M06-2X and the spin-consistent-scaled (SCS) variant of the approximate second-order coupled-cluster method, SCS-CC2. All methods predict the global minima to be π-stacked centered coplanar structures with the long axis of tetrachloroethene rotated by τ ≈ 60° relative to the perylene long axis. The calculated binding energies are in the range of −<i>D</i>₀ = 28–35 kJ/mol. A second minimum is predicted with τ ≈ 25°, with ∼1 kJ/mol smaller binding energy. Although both monomers are achiral, both the P·4ClE and P·(4ClE)₂ complexes are chiral. The best agreement for adiabatic excitation energies and vibrational frequencies is observed for the ωB97X-D and M06-2X DFT methods

Structure and Intermolecular Vibrations of Perylene·<i>trans</i>-1,2-Dichloroethene, a Weak Charge-Transfer Complex

The vibronic spectra of strong charge-transfer complexes are often congested or diffuse and therefore difficult to analyze. We present the spectra of the π-stacked complex perylene <i>trans</i>-1,2-dichloroethene, which is in the limit of weak charge transfer, the electronic excitation remaining largely confined to the perylene moiety. The complex is formed in a supersonic jet, and its S₀ ↔ S₁ spectra are investigated by two-color resonant two-photon ionization (2C-R2PI) and fluorescence spectroscopies. Under optimized conditions, vibrationally cold (<i>T</i>_vib ≈ 9 K) and well resolved spectra are obtained. These are dominated by vibrational progressions in the “hindered-rotation” R_c intermolecular vibration with very low frequencies of 11 (S₀) and 13 cm^–1 (S₁). The intermolecular T_<i>z</i> stretch and the R_a and R_b bend vibrations are also observed. The normally symmetry-forbidden intramolecular 1a_u “twisting” vibration of perylene also appears, showing that the π- stacking interaction deforms the perylene moiety, lowering its local symmetry from <i>D</i>_2<i>h</i> to <i>D</i>₂. We calculate the structure and vibrations of this complex using six different density functional theory (DFT) methods (CAM-B3LYP, BH&HLYP, B97-D3, ωB97X-D, M06, and M06-2X) and compare the results to those calculated by correlated wave function methods (SCS-MP2 and SCS-CC2). The structures and vibrational frequencies predicted with the CAM-B3LYP and BH&HLYP methods disagree with the other calculations and with experiment. The other four DFT and the ab initio methods all predict a π-stacked “centered” structure with nearly coplanar perylene and dichloroethene moieties and intermolecular binding energies of <i>D</i>_e = −20.8 to −26.1 kJ/mol. The 0₀⁰ band of the S₀ → S₁ transition is red-shifted by δν = −301 cm^–1 relative to that of perylene, implying that the <i>D</i>_e increases by 3.6 kJ/mol or ∼15% upon electronic excitation. The intermolecular vibrational frequencies are assigned to the calculated R_c, T_z, R_a, and R_b vibrations by comparing to the observed/calculated frequencies and S₀ ↔ S₁ Franck–Condon factors. Of the three TD-DFT methods tested, the hybrid-meta-GGA functional M06-2X shows the best agreement with the experimental electronic transition energies, spectral shifts, and vibronic spectra, closely followed by the ωB97X-D functional, while the M06 functional gives inferior results

Excitonic Splitting and Vibronic Coupling Analysis of the <i>m</i>‑Cyanophenol Dimer

The <i>S</i>₁/<i>S</i>₂ splitting of the <i>m</i>-cyanophenol dimer, (mCP)₂ and the delocalization of its excitonically coupled <i>S</i>₁/<i>S</i>₂ states are investigated by mass-selective two-color resonant two-photon ionization and dispersed fluorescence spectroscopy, complemented by a theoretical vibronic coupling analysis based on correlated <i>ab initio</i> calculations at the approximate coupled cluster CC2 and SCS-CC2 levels. The calculations predict three close-lying ground-state minima of (mCP)₂: The lowest is slightly <i>Z</i>-shaped (<i>C</i>_<i>i</i>-symmetric); the second-lowest is <5 cm^–1 higher and planar (<i>C</i>_2<i>h</i>). The vibrational ground state is probably delocalized over both minima. The S₀ → S₁ transition of (mCP)₂ is electric-dipole allowed (A_<i>g</i> → A_<i>u</i>), while the S₀ → S₂ transition is forbidden (A_<i>g</i> → A_<i>g</i>). Breaking the inversion symmetry by ¹²C/¹³C- or H/D-substitution renders the S₀ → S₂ transition partially allowed; the excitonic contribution to the S₁/S₂ splitting is Δ_<i>exc</i> = 7.3 cm^–1. Additional isotope-dependent contributions arise from the changes of the <i>m</i>-cyanophenol zero-point vibrational energy upon electronic excitation, which are Δ_<i>iso</i>(¹²C/¹³C) = 3.3 cm^–1 and Δ_<i>iso</i>(H/D) = 6.8 cm^–1. Only partial localization of the exciton occurs in the ¹²C/¹³C and H/D substituted heterodimers. The SCS-CC2 calculated excitonic splitting is Δ_<i>el</i> = 179 cm^–1; when multiplying this with the vibronic quenching factor Γ_{<i>vibron</i>}^<i>exp</i> = 0.043, we obtain an exciton splitting Δ_{<i>vibron</i>}^<i>exp</i> = 7.7 cm^–1, which agrees very well with the experimental Δ_<i>exc</i> = 7.3 cm^–1. The semiclassical exciton hopping times range from 3.2 ps in (mCP)₂ to 5.7 ps in the heterodimer (mCP-<i>h</i>)·(mCP-<i>d</i>). A multimode vibronic coupling analysis is performed encompassing all the vibronic levels of the coupled <i>S</i>₁/<i>S</i>₂ states from the <i>v</i> = 0 level to 600 cm^–1 above. Both linear and quadratic vibronic coupling schemes were investigated to simulate the S₀ → S₁/S₂ vibronic spectra; those calculated with the latter scheme agree better with experiment

A novel locus (DFNA24) for prelingual nonprogressive autosomal dominant nonsyndromic hearing loss maps to 4q35-qter in a large Swiss German kindred

Nonsyndromic hearing loss is one of the most genetically heterogeneous traits known. A total of 30 autosomal dominant nonsyndromic hearing-loss loci have been mapped, and 11 genes have been isolated. In the majority of cases, autosomal dominant nonsyndromic hearing loss is postlingual and progressive, with the exception of hearing impairment in families in which the impairment is linked to DFNA3, DFNA8/12, and DFNA24, the novel locus described in this report. DFNA24 was identified in a large Swiss German kindred with a history of autosomal dominant hearing loss that dates back to the middle of the 19th century. The hearing-impaired individuals in this kindred have prelingual, nonprogressive, bilateral sensorineural hearing loss affecting mainly mid and high frequencies. The DFNA24 locus maps to 4q35-qter. A maximum multipoint LOD score of 11.6 was obtained at 208.1 cM at marker D4S1652. The 3.0-unit support interval for the map position of this locus ranges from 205.8 cM to 211.7 cM (5.9 cM)