Non-radiative excitation transfer between two molecular entities separated by distances considerably exceeding the sum of their van der Waals radii. It describes the transfer in terms of the interaction between the transition (dipole) moments of the entities in the very weak dipole-dipole coupling limit. It is a Coulombic interaction frequently called a dipole-dipole coupling. The transfer rate constant from donor to acceptor, $k_{\text{T}}$, is given by $$k_{\text{T}}=k_{\text{D}}{\left(\frac{R_0}{r}\right)}^6=\frac{1}{{\tau }_D^0}{\left(\frac{R_0}{r}\right)}^6$$ where $k_{\text{D}}$ and ${\tau }_{\text{D}}^0$ are the emission rate constant and the lifetime of the excited donor in the absence of transfer, respectively, $r$ is the distance between the donor and the acceptor and $R_0$ is the critical quenching radius or Förster radius, i.e., the distance at which transfer and spontaneous decay of the excited donor are equally probable ($k_{\text{T}}=k_{\text{D}}$) (see Note 3). $R_0$ is given by $$R_0=Const.{\left(\frac{{\kappa }^2{\mathit{\Phi }}_D^{0}J}{n^4}\right)}^{1/6}$$ where $\kappa$ is the orientation factor, ${\mathit{\Phi }}_D^0$ is the fluorescence quantum yield of the donor in the absence of transfer, $n$ is the average refractive index of the medium in the wavelength range where spectral overlap is significant, $J$ is the spectral overlap integral reflecting the degree of overlap of the donor emission spectrum with the acceptor absorption spectrum and given by $$J={\int }_{\lambda }{I}_{\lambda }^D(\lambda ){ϵ}_A\left(\lambda \right){\lambda }^4\text{d}\lambda$$ where $I_{\lambda }^D(\lambda )$ is the normalized spectral radiant intensity of the donor so that ${\int }_{\lambda }{I}_{\lambda }^D(\lambda )\text{d}\lambda =1$. ${\epsilon }_{\text{A}}(\lambda )$ is the molar decadic absorption coefficient of the acceptor. See Note 3 for the value of $Const.$.