Franziska Balmer

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 (13)C 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 (12)C/(13)C or 5-fold h5/d5 isotopic substitution reduce the dimer symmetry to Cs, so that the heteroisotopic dimers (BN)2-(h5 – h5(13)C), (BN)2-(h5 – d5), and (BN)2-(h5 – h5(13)C) 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((12)C/(13)C) = 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((12)C/(13)C) > Δexc, complete and near-complete exciton localization occurs upon (12)C/(13)C 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 Δel(calc) = 10 cm(–1). This electronic splitting is reduced by the vibronic quenching factor Γ. The vibronically quenched exciton splitting Δel(calc)·Γ = Δvibron(calc) = 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.

Structure and intermolecular vibrations of perylene·trans-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 trans-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 (T(vib) ≈ 9 K) and well resolved spectra are obtained. These are dominated by vibrational progressions in the “hindered-rotation” Rc intermolecular vibration with very low frequencies of 11 (S0) and 13 cm(–1) (S1). The intermolecular Tz stretch and the Ra and Rb 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 D2h to D2. 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 D(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 D(e) increases by 3.6 kJ/mol or 15% upon electronic excitation. The intermolecular vibrational frequencies are assigned to the calculated Rc, Tz, Ra, and Rb 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 m-Cyanophenol Dimer.

The S1/S2 splitting of the m-cyanophenol dimer, (mCP)2 and the delocalization of its excitonically coupled S1/S2 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 ab initio 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 Z-shaped (Ci-symmetric); the second-lowest is <5 cm-1 higher and planar (C2h). The vibrational ground state is probably delocalized over both minima. The S0 → S1 transition of (mCP)2 is electric-dipole allowed (Ag → Au), while the S0 → S2 transition is forbidden (Ag → Ag). 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 Δexc = 7.3 cm-1. Additional isotope-dependent contributions arise from the changes of the m-cyanophenol zero-point vibrational energy upon electronic excitation, which are Δiso(12C/13C) = 3.3 cm-1 and Δiso(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 Δel = 179 cm-1; when multiplying this with the vibronic quenching factor Γvibronexp = 0.043, we obtain an exciton splitting Δvibronexp = 7.7 cm-1, which agrees very well with the experimental Δexc = 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-h)·(mCP-d). A multimode vibronic coupling analysis is performed encompassing all the vibronic levels of the coupled S1/S2 states from the v = 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.

Do Hydrogen Bonds Influence Excitonic Splittings

The excitonic splitting and vibronic quenching of the inversion-symmetric homodimers of benzonitrile, (BN)2, and meta-cyanophenol, (mCP)2, are investigated by two-color resonant two-photon ionization spectroscopy. These systems have very different hydrogen bond strengths: the OH···N≡C bonds in (mCP)2 are ∼10 times stronger than the CH···N≡C hydrogen bonds in (BN)2. In (BN)2 the S0((1)Ag) → S1((1)Ag) transition is electric-dipole forbidden, while the S0((1)Ag) → S2((1)Bu) transition is allowed. The opposite holds for (mCP)2 due to the different transition dipole moment vector alignment. The S0 → S1S2 spectra of the dimers are compared and their excitonic splittings and vibronic quenchings are investigated by measuring the (13)C-substituted heterodimer isotopomers, for which the centrosymmetry is broken and both transitions are allowed. The excitonic splittings are determined as Δexc = 2.1 cm(-1) for (BN)2 and Δexc = 7.3 cm(-1) for (mCP)2. The latter exhibits a much stronger vibronic quenching, as the purely electronic splitting resulting from ab initio calculations is determined to be Δcalc = 179 cm-1, while in (BN)2 the calculated splitting is Δcalc = 10 cm(-1). The monomer site-shifts upon dimerization and comparing certain vibrations that deform the hydrogen bonds confirm that the OH···N≡C hydrogen bond is much stronger than the CH···N≡C bond. We show that the H-bonds have large effects on the spectral shifts, but little or no influence on the excitonic splitting.

Experimental and Calculated Spectra of π-Stacked Mild Charge-Transfer Complexes: Jet-Cooled Perylene·(Tetrachloroethene)n, n = 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·trans-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 (1au) 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 (Tz) and internal rotation (Rc) 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 -D0 = 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.