Non-planar distortion of terrylene molecules in a naphthalene crystal

Probing the in-plane dipole moment vector between ground and excited state of single molecules by the Stark effect

Single molecules, embedded inside a well-defined insertion site of a single-crystalline host matrix, are sensitive probes of electric field via the induced Stark shift on their lifetime-limited electronic transition. Though the response of molecules to electric field has been shown to be relatively homogeneous, crystal symmetry allows for several, spectroscopically-indistinguishable, orientations of the net permanent dipole moment between the ground and excited state - the dipole vector - and this is problematic for measuring field orientation and magnitude. In this work, we measure for each terrylene molecule, embedded inside a new host matrix, the dipole vector independently by an electric field that we can rotate in the plane of the crystal. This single crystal host matrix, called [1]BenzoThieno[3,2-b]BenzoThiophene, induces a moderate symmetry breaking of the embedded centrosymmetric terrylene molecule, and gives rise to a net dipole moment of 0.28±0.09 Debye. Based on quantum chemistry calculations we propose an insertion site that best matches the experimental findings.

Matrix-induced Linear Stark Effect of Single Dibenzoterrylene Molecules in 2,3-Dibromonaphthalene Crystal

Absorption and fluorescence from single molecules can be tuned by applying an external electric field - a phenomenon known as the Stark effect. A linear Stark effect is associated to a lack of centrosymmetry of the guest in the host matrix. Centrosymmetric guests can display a linear Stark effect in disordered matrices, but the response of individual guest molecules is often relatively weak and non-uniform, with a broad distribution of the Stark coefficients. Here we introduce a novel single-molecule host-guest system, dibenzoterrylene (DBT) in 2,3-dibromonaphthalene (DBN) crystal. Fluorescent DBT molecules show excellent spectral stability with a large linear Stark effect, of the order of 1.5 GHz/kVcm-1 , corresponding to an electric dipole moment change of around 2 D. Remarkably, when the electric field is aligned with the a crystal axis, nearly all DBT molecules show either positive or negative Stark shifts with similar absolute values. These results are consistent with quantum chemistry calculations. Those indicate that DBT substitutes three DBN molecules along the a-axis, giving rise to eight equivalent embedding sites, related by the three glide planes of the orthorhombic crystal. The static dipole moment of DBT molecules is created by host-induced breaking of the inversion symmetry. This new host-guest system is promising for applications that require a high sensitivity of fluorescent emitters to electric fields, for example to probe weak electric fields.