Internal Stark effect of single-molecule fluorescence

Abstract

The optical properties of chromophores can be efficiently tuned by electrostatic fields generated in their close environment, a phenomenon that plays a central role for the optimization of complex functions within living organisms where it is known as internal Stark effect (ISE). Here, we realised an ISE experiment at the lowest possible scale, by monitoring the Stark shift generated by charges confined within a single chromophore on its emission energy. To this end, a scanning tunneling microscope (STM) functioning at cryogenic temperatures is used to sequentially remove the two central protons of a free-base phthalocyanine chromophore deposited on a NaCl-covered Ag(111) surface. STM-induced fluorescence measurements reveal spectral shifts that are associated to the electrostatic field generated by the internal charges remaining in the chromophores upon deprotonation.

Fig. 1. Imaging protonated and deprotonated phthalocyanine molecules and monitoring tautomerization processes.

a Sketch of the experiment where the fluorescence energy (hν) of a phthalocyanine molecule is progressively Stark-shifted (δhν_i) by charges located at the center of the chromophore. b, d, f STM images (2.7 × 2.7 nm²) of a phthalocyanine molecule on 3ML-NaCl/Ag(111). The images of the HOMO were all recorded at I  = 10 pA, with voltage V = −2.5 V for H₂Pc, V = −1.5 V for HPc⁻ and V = −2 V for Pc²⁻. The LUMO were recorded at I = 10 pA, V = 0.5 V for (b) H₂Pc, I = 2.3 pA, V = 1.43 V for (d) HPc⁻ and I = 10 pA, V = 1.65 V for (f) Pc²⁻. The LUMO images of H₂Pc and HPc⁻ reveal two and four different patterns, respectively, in successive images. The dots in the LUMO images of HPc⁻ indicate the side where the remaining hydrogen atom is located. c, e, g Relative tip-sample distance (Δz) time traces and their histograms recorded at a bias voltage of −2.5 V on (c) H₂Pc, (e) HPc⁻, and (g) Pc²⁻ , and at constant current (I_set−point = 10 pA for H₂Pc  and 5 pA for HPc⁻ and Pc²⁻).

Fig. 2. Identifying charges confined in deprotonated phthalocyanines.

a, e, i Constant current dI/dV maps (I = 30 pA, V = 400 mV, 12.5 × 10.8 nm², modulation voltage δV_mod = 50 mV) recorded on (a) H₂Pc, (e) HPc⁻, and (i) Pc²⁻. Images (a), (e), and (i) were acquired on a 2ML-NaCl island where the interface state scattering is stronger than on 3ML for a better visualization of the scattering. The conclusions regarding the charged character of the different species on 2 or 3ML NaCl remain however the same (see Supplementary Fig. 8). The insets show topographic STM images (3.7 × 3.7 nm²) recorded simultaneously that reveal dark areas around HPc⁻ and Pc²⁻, characteristic of charged molecules on NaCl. b, f, j dI/dV spectra recorded on (b) H₂Pc, (f) HPc⁻, and (j) Pc²⁻ on 3ML of NaCl. c, g, k DFT calculations of the electronic structure of the frontier orbitals of (c) H₂Pc, (g) HPc⁻, and (k) Pc²⁻ together with their iso-surface representations and their electronic occupation. d, h, l calculated images of the electrostatic field for (d) H₂Pc, (h) HPc⁻, and (l) Pc²⁻. The potential is displayed 0.21 nm above the plane of the molecule. The NaCl layer is not included in the calculation. Source data are provided as a Source Data file.

Fig. 3. ISE in fluorescence spectra of neutral and charged phthalocyanines.

STML spectra (acquisition time t = 120 s) of (a) H₂Pc (I = 100 pA, V = −2.5 V), (b) HPc⁻ (I = 60 pA, V = −2.5 V), and (c) a Pc²⁻ (I = 100 pA, V = 2.5 V) on 3ML NaCl/Ag(111). The spectrum of HPc⁻ was obtained by resonant energy transfer from a neighboring ZnPc. The vertical dashed lines correspond to TD-DFT calculations of the S₀ → S₁ absorption energies for H₂Pc, HPc⁻, and Pc²⁻, rigidly shifted by −250 meV to fit the experiment. This shift can be rationalized by considering that the dynamical screening linked to the presence of the NaCl/Ag(111) substrate and of the STM tip are not accounted for by our theoretical approach. While this prevents a quantitative comparison between experimental and theoretical energies, this procedure shows that theory accurately predicts the energy shift upon deprotonation. d Comparison between TD-DFT absorption energies calculated for H₂Pc, HPc⁻, and Pc²⁻ (blue dots) and TD-DFT absorption energies calculated for a Pc²⁻ as a function of partial positive charges added artificially in its center (white dots). The line through the white dots is a parabolic fit with the parameters given in inset. Source data are provided as a Source Data file.

Fig. 4. Effect of the deprotonation on the vibronic signature of the phtahlocyanine molecule.

a STML vibronic spectra (I = 100 pA, acquisition time t = 120 s) of H₂Pc (V = −2.5 V) and Pc²⁻ (V = 2.5 V) molecules on 3ML of NaCl compared with (b) theoretical vibronic intensities calculated for the molecules in vacuum. The Q_x-line of H₂Pc and Q-line of Pc²⁻ are used as the origin of the experimental energy scale. α and β indicate vibronic modes for which the four nitrogen atoms of the pyrrole cycles stand nearly still. Source data are provided as a Source Data file.