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

Abstract

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.

Analysis of the S2←S0 vibronic spectrum of the ortho-cyanophenol dimer using a multimode vibronic coupling approach

“…The examples chosen were typically doubly H-bonded aromatic and heteroaromatic species, most notably the 2-aminopyridine (2AP) 2 , ortho-cyanophenol (oCP) 2 , 2-pyridone (2PY) 2 , and related molecular dimers. [6][7][8][9][10][11] The systems all show similar excitonic splitting schemes, with a (mostly) forbidden S 1 ← S 0 transition, that becomes slightly allowed upon asymmetric isotopic substitution.…”

Do Hydrogen Bonds Influence Excitonic Splittings?

“…The 0 � � transition energies are indicated next to the corresponding band, namely 36420 cm -1 for (BN) 2 and 33255 cm -1 for (mCP) 2 . [9,13] These energies correspond to spectral shifts of δν =-92 cm -1 for (BN) 2 and δν =-1094 cm -1 for (mCP) 2 relative to the respective monomer S 0 → S 1 transition energies. The much larger shift for (mCP) 2 indicates a stronger hydrogen bond of (mCP) 2 relative to that of (BN) 2 .…”

“…The BN S 0 → S 1 0 � � band is observed at 36513 cm -1 [18] and the average of the dimer S 0 → S 1 /S 2 transitions is 36420 cm -1 . [13] This result corresponds to a small site-shift of ∆ site = -92 cm -1 , which is 5% of the binding energy. The mCP monomer origin lies at 34354 cm for (mCP) 2 .…”

“…1. (BN) 2 is a planar, C 2h -symmetric dimer, [13,14] while (mCP) 2 is C i -symmetric, with the monomers slightly out of plane. [9,15] The calculated intermonomer distance between the centers of mass in (mCP) 2 is R AB = 5.34 Å with an H … N distance of 2.04 Å; the center-of-mass distance in (BN) 2 is R AB = 6.46 Å and the H … N distance is 2.36 Å.…”

“…Only one intermolecular vibration is observed, the shear vibration χ � � ; otherwise the spectrum is very similar to that of BN. [13] By contrast, the spectrum of (mCP) 2 shows many excitations of intermolecular vibrations (δ, β, θ, σ, χ) as well as several intramolecular vibrations. [9,15] …”