© Ultrafast Systems LLC /Vernier Software & Technology
34
7.3
Determination of the Activation Energy of the
Thermal Back Reaction of One Spiropyran in
Toluene
Excitation wavelength: <400 nm
Observation wavelength: 600 nm
Chemicals needed
1’,3’-dihydro-1’,3’,3’-trimethyl-6-nitrospiro(2
H
-1-benzopyran-2,2’-2
H
-indole) (6-NO
2
-BIPS)
Toluene
Background
1
6-NO
2
-BIPS is a type of spiropyran molecule that is colorless in its normal form (N isomer) and
undergoes a photochemical ring-opening reaction to yield an isomeric colored merocyanine form (MC
isomer) when irradiated with UV light. It has been proposed that the MC isomer is a zwitterion, as shown
in Figure 19.
Figure 19:
Structure and photochromic reaction of 6-NO
2
-BIPS
The N isomer is the thermodynamically more stable isomer and absorbs in the UV region,
whereas the MC isomer absorbs in both the UV and visible regions. The MC isomer exhibits a strong and
characteristic absorption band between 550 and 650 nm. Upon UV irradiation of 6-NO
2
-BIPS, the
formation of the MC isomer is induced. The increase in the concentration of the MC isomer results in the
increase of absorbance in the visible region of the absorption spectrum. The MC isomer spontaneously
returns to the N isomer once the UV irradiation is stopped. The kinetics of this back reaction process can
be characterized by measuring the visible absorption at 600 nm of the MC isomer as a function of time. In
1
Reference: Piard, Jonathan, “Influence of the Solvent on the Thermal Back Reaction of One Spiropyran”
Journal
of Chemical Education
. 2014,
91
, 2105–2111.
