Identification of vibrational excitations and optical transitions of the organic electron donor tetraphenyldibenzoperiflanthene (DBP)

Abstract

Tetraphenyldibenzoperiflanthene (DBP) attracts interest as an organic electron donor for photovoltaic applications. In order to assist in the analysis of vibrational and optical spectra measured during the formation of thin films of DBP, we have studied the vibrational modes and the electronic states of this molecule. Information on the vibrational modes of the electronic ground state has been obtained by IR absorption spectroscopy of DBP grains embedded in polyethylene and CsI pellets and by calculations using density functional theory (DFT). Electronic transitions have been measured by UV/vis absorption spectroscopy applied to DBP molecules isolated in rare-gas matrices. These measurements are compared with the results of ab initio and semi-empirical calculations. Particularly, the vibrational pattern observed in the S1 ← S0 transition is interpreted using a theoretical vibronic spectrum computed with an ab initio model. The results of the previous experiments and calculations are employed to analyze the data obtained by high-resolution electron energy loss spectroscopy (HREELS) applied to DBP molecules deposited on a Au(111) surface. They are also used to examine the measurements performed by differential reflectance spectroscopy (DRS) on DBP molecules deposited on a muscovite mica(0001) surface. It is concluded that the DBP molecules in the first monolayer do not show any obvious degree of chemisorption on mica(0001). Regarding the first monolayer of DBP on Au(111), the HREELS data are consistent with a face-on anchoring and the absence of strong electronic coupling.

Exciting vibrons in both frontier orbitals of a single hydrocarbon molecule on graphene

“…According to the ball-and-stick model of free DBP added to figure 1(b) the four phenyl groups appear with the brightest contrast. The molecular backbone is darker and exhibits uniform contrast, which indicates the parallel adsorption of the molecule to the surface, in agreement with previous reports on metal surfaces [52,53]. Evidence for the otherwise enhanced mobility of DBP on graphene is the dominant occurrence of large molecular islands covering extended regions of graphene (figure 1(c)).…”

“…Assuming the vacuum character of DBP is reasonable due to its efficient decoupling from the metal surface via graphene, in agreement with previous work for other molecules adsorbed on graphene [30,39,55,[58][59][60]. Electron energy loss spectra (supporting information, figure S2) reveal energy losses that virtually coincide with vibration energies calculated for the free molecule [52], which corroborates the free-molecule character of DBP on graphene. Our calculations for free DBP yield 12 (8) in-plane deformation modes of the molecular backbone (of the phenyl groups in the normal plane) in an energy range of 194.1 meV ≤ ω ≤ 199.3 meV (192.2 meV ≤ ω ≤ 194.8 meV) ( ω: vibrational energy with the Planck constant, supporting information, table S1).…”

Fano Description of Single-Hydrocarbon Fluorescence Excited by a Scanning Tunneling Microscope

“…The optimized structure was obtained by density functional calculations at the B3LYP/6 − 311G++(d,p) level. 44 The energy of the weak fluorescence peak at 2.28 eV (ε), which was not reported in the absorption experiments, 45 is compatible with light emission from non-thermalized excitons, a photon emission process that is generally referred to as hot electroluminescence where vibrationally excited states of S 1 relax to the vibrational ground state of S 0 , i. e., The Purcell effect (vide supra) entails a strong reduction of the fluorescence lifetime in plasmonic environments. Similar observations of luminescence peaks blue-shifted from the S 1 (ν = 0) −→ S 0 (ν = 0) transition were reported from multilayers of phthalocyanines 21…”

The influence of an interfacial hBN layer on the fluorescence of an organic molecule

“…This can be explained by the fact that DBP is a lander-type molecule. It exhibits four peripheral phenyl groups, which function as spacers between the molecular backbone and the surface [71]. This presumably, in addition to the hBN layer and a weak interaction, supports the suppression of the CT process and allows for a higher FL intensity.…”