Nature of Charge Transfer Effects in Complexes of Dopamine Derivatives Adsorbed on Graphene-Type Nanostructures

Abstract

Continuing the investigation started for dopamine (DA) and dopamine-o-quinone (DoQ) (see, the light absorption and charge transfer properties of the dopamine zwitterion (called dopamine-semiquinone or DsQ) adsorbed on the graphene nanoparticle surface is investigated using the ground state and linear-response time-dependent density functional theories, considering the ωB97X-D3BJ/def2-TZVPP level of theory. In terms of the strength of molecular adsorption on the surface, the DsQ form has 50% higher binding energy than that found in our previous work for the DA or DoQ cases (-20.24 kcal/mol vs. -30.41 kcal/mol). The results obtained for electronically excited states and UV-Vis absorption spectra show that the photochemical behavior of DsQ is more similar to DA than that observed for DoQ. Of the three systems analyzed, the DsQ-based complex shows the most active charge transfer (CT) phenomenon, both in terms of the number of CT-like states and the amount of charge transferred. Of the first thirty electronically excited states computed for the DsQ case, eleven are purely of the CT type, and nine are mixed CT and localized (or Frenkel) excitations. By varying the adsorption distance between the molecule and the surface vertically, the amount of charge transfer obtained for DA decreases significantly as the distance increases: for DoQ it remains stable, for DsQ there are states for which little change is observed, and for others, there is a significant change. Furthermore, the mechanistic compilation of the electron orbital diagrams of the individual components cannot describe in detail the nature of the excitations inside the complex.

Keywords: TDDFT; charge transfer; dopamine; graphene; quinone; zwitterion.

Figure 1

The 2D chemical structure of (a) dopamine (or DA), (b) dopamine-o-quinone (or DoQ) and (c) dopamine-semiquinone (or DsQ).

Figure 2

The geometry configuration of the dopamine-semiquinone (or DsQ) adsorbed on the GrNP surface computed at ωB97X-D3BJ/def2-TZVPP/CPCM level of theory.

Figure 3

Theoretical UV absorption spectra of the graphene–DsQ binary complex and the individual constituents computed at the TDDFT/ωB97X-D3BJ/def2-TZVPP/CPCM level of theory.

Figure 4

The molecular orbital energy scheme (in eV) of the individual, GrNP, and DsQ components and of the mixed GrNP–DsQ binary complex (H = HOMO (or Highest Occupied Molecular Orbital), L = LUMO (or Lowest Unoccupied Molecular Orbital)) based on the fragment orbital contribution analysis.

Figure 5

The transferred charge (between 0 and 1 values of the elementary charge) calculated for different plane distances relative to the equilibrium geometry (z₀ + Δz, Δz between −0.3 Å and +1.0 Å) for DA (a), DoQ (b) and DsQ (c), respectively, adsorbed on the GrNP surface, computed at the TDDFT/ωB97X-D3BJ/def2-TZVPP level of theory.

Figure 6

Molecular orbital energy schemes (in eV) built based on the fragment orbital contribution analysis of the individual, GrNP (1st col.), DA, DoQ, and DsQ (last col.) components and of the mixed (a) GrNP–DA, (b) GrNP–DoQ, and (c) GrNP–DsQ binary complexes (H = HOMO, L = LUMO) computed for Δz = −0.3 (2nd col.), 0.0 (3rd col.), and +1.0 (4th col.) relative stacking distances.