Nonradiative
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 dipoledipole
coupling limit. It is a Coulombic interaction frequently called a dipoledipole
coupling. The transfer
rate constant from donor to acceptor,
,
is given by
where
and
are the
emissionrate 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).
Notes:

The bandpass
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
is:
The orientation factor
is given by
where
is the angle between the donor and acceptor moments, and
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
as follows:
or written in the following form :
where
is the donor excitedstate lifetime in the presence of acceptor, and
in the absence of acceptor.
 FRET is sometimes inappropriately called fluorescenceresonance energy transfer. This is not correct because there is no fluorescence involved in FRET.

Foerster is an alternative and acceptable spelling for Förster.
Source:
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 342
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML online corrected version: http://goldbook.iupac.org (2006) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0967855098.
https://doi.org/10.1351/goldbook.