Tunable Luminescence from a Single Free-Base Porphyrin Molecule by Controlled Access to Optically Active States

Area of Science:

• Surface Science
• Molecular Spectroscopy
• Nanotechnology

Background:

• Scanning tunneling microscopy-induced luminescence (STML) probes individual molecule optical properties via multi-state relaxation.
• Dark molecules in STML result from insufficient charge attachment energies hindering relaxation to optically active states.

Purpose of the Study:

• To control access to optically active states and achieve bright exciton emission in single molecules using STML.
• To overcome limitations preventing intrinsically bright molecules from emitting light in STML experiments.

Main Methods:

• Utilized substrate work function control to tune molecular energy levels.
• Employed tip-induced gating of a double-barrier tunnel junction to manipulate the ionic transition state.
• Applied rate equation and polaron models for theoretical validation, including NaCl decoupling layer relaxation energy.

Main Results:

• Successfully induced an energy shift in the ionic transition state of single free-base tetrabenzoporphyrin (H2TBP).
• Achieved controlled access to optically active states, resulting in bright exciton emission.
• Experimental findings were validated by theoretical models.

Conclusions:

• Substrate work function control and tip-induced gating are effective strategies for managing molecular energy states in STML.
• This approach enables the observation of optical properties of molecules previously appearing dark in STML.
• The study advances the capability to study single-molecule optics with high precision.