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,
is given by
are the emission rate constant
and the lifetime
of the excited donor in the absence of transfer, respectively,
is the distance between the donor and the acceptor and
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
(see Note 3).
is a constant in spectrophotometers and spectrofluorometers using gratings. Thus, the scale is linear in wavelength and it is convenient to express and calculate the integrals in wavelengths instead of wavenumbers in order to avoid confusion.
In practical terms, the integral
is the area under the plot of the donor emission intensity versus the emission wavelength.
A practical expression for
The orientation factor
is given by
is the angle between the donor and acceptor moments, and
are the angles between these, respectively, and the separation vector;
is the angle between the projections of the transition moments on a plane perpendicular to the line through the centres.
can in principle take values from 0 (perpendicular transition moments) to 4 (collinear transition moments). When the transition moments are parallel and perpendicular to the separation vector,
When they are in line (i.e., their moments are strictly along the separation vector),
For randomly oriented transition (dipole) moments, e.g., in fluid solutions,
The transfer quantum efficiency is defined as
and can be related to the ratio
or written in the following form :
is the donor excited-state lifetime in the presence of acceptor, and
in the absence of acceptor.
FRET is sometimes inappropriately called fluorescence-resonance energy transfer. This is not correct because there is no fluorescence involved in FRET.
Foerster is an alternative and acceptable spelling for Förster.
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 342
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook