Center for Exploitation of Solar Energy &
Department of Chemistry, University of Copenhagen
Solar Energy Storage in Photochromic Molecules
Mogens Brøndsted Nielsen
Compact long term heat storage workshop, DTU, Jan. 28, 2016
Energy Storage using Photochromic Molecules
– Closed Energy Cycle with No CO2 Emissions or Other Pollutants
Light absorption – Storage – Energy release on demand
Energy barrier
for back-reaction
Photoinduced
isomerization
Energy stored
light
low-energy
molecule
"trigger"
high-energy
molecule
determines
storage time
… Material with energy density of 1 MJ / kg
• Heat release of 1 MJ can be used to bring 3 L of water
from room temperature to the boiling point
• Harvesting light during the day and releasing heat during the night:
- maintaining 1 m3 at 19 oC with outside temperature of -6 oC requires
ca. 3 kg of solar battery (when using foam insulation)
T. R. Kucharski, Y. Tian, S. Akbulatov, R. Boulatov, Energy Environ. Sci. 2011, 4, 4449-4472.
Challenges
• How do we design photochromic molecules with sufficiently high energy densities?
• How is the energy-releasing back-reaction put on stand-by? … energy storage
• How do we reach high quantum yields
• How do we avoid photodegradation of molecules over many cycles?
…
Norbornadiene – Quadricyclane Couple
light
heat
Energy storage: 1 MJ/kg
✔
Quantum yield: 9%
÷
Only absorbs UV light
÷
Prone to polymerization
÷
Z.-I. Yoshida, J. Photochem. 1985, 29, 27-40.
trans-Azobenzene – cis-Azobenzene Couple
light
N
N
heat
Energy storage: ca. 0.3 MJ/kg
Photostationary states, usually with <75% cis
N
N
Dihydroazulene (DHA) / Vinylheptafulvene (VHF) Couple
Quantum yield = 55%
Dihydroazulene (DHA) / Vinylheptafulvene (VHF) Couple
DHA lmax 353 nm
VHF lmax 470 nm
DHA Synthesis … Easy to do
S. L. Broman, S. L. Brand, C. R. Parker, M. Å. Petersen, C. G. Tortzen,
A. Kadziola, K.Kilså, M. B. Nielsen, ARKIVOC 2011, ix, 51-67.
Energy storage: 0.11 MJ / kg
… We need to modify the molecule to increase this value!
S.T. Olsen, J. Elm, F.E. Storm, A.N. Gejl, A.S. Hansen, M.H. Hansen, J.R. Nikolajsen, M.B. Nielsen,
H.G. Kjaergaard, K.V. Mikkelsen, J. Phys. Chem. A 2015, 119, 896-904.
Energy Storage
NC
CN
Substitute one
CN for a H
NC
Ph
Ph
0.11 MJ / kg
NC
0.25 MJ / kg
CN
NC
Ph
Ph
0.15 MJ / kg
H
H
stored
energy
Ph
Ph
0.23 MJ / kg
By minor structural variations we can make a doubling of the energy storage capacity!
S.T. Olsen, J. Elm, F.E. Storm, A.N. Gejl, A.S. Hansen, M.H. Hansen, J.R. Nikolajsen, M.B. Nielsen,
H.G. Kjaergaard, K.V. Mikkelsen, J. Phys. Chem. A 2015, 119, 896-904.
M. Cacciarini, A.B. Skov, M. Jevric, A.S. Hansen, J. Elm, H.G. Kjaergaard, K.V. Mikkelsen, M.B. Nielsen,
Chem. Eur. J. 2015, 21, 7454-7461.
Synthesis of New Targets by Reductive Decyanations
NC CN
Ph
NC CN
Ph
Ph
VHF of this DHA:
Half-life of 14 s
DIBAL-H
H CN
THF
16%
H
Ph
THF
23%
DIBAL-H
hn
H CN
Ph
hn
H
Ph
Ph
CN
very
slowly
CN
Ph
Ph
TS calculations:
DG‡ = 125.7 kJ/mol
Half-life >10 years at rt
M. Cacciarini, A.B. Skov, M. Jevric, A.S. Hansen, J. Elm, H.G. Kjaergaard, K.V. Mikkelsen, M.B. Nielsen,
Chem. Eur. J. 2015, 21, 7454-7461.
Triggering the Ring-Closure
NC
CN
Ag+
NC CN
Ph
Ph
Effect of Ag+ on the thermal ring-closure reaction of VHF:
min
Solvent: 1,2-dichloroethane
C. R. Parker, C. G. Tortzen, S. L. Broman, M. Schau-Magnussen,
K. Kilså, M. B. Nielsen, Chem. Commun. 2011, 47, 6102-6104.
Ring-Closure Reactions in MeCN at Room Temperature
Me 2N
Me 2N
NC
CN
fast
NC CN
Half-life = 50 min
VHF
DHA
O2N
O2N
NC
CN
slow
NC CN
Half-life = 490 min
VHF
S. L. Broman, M. Jevric, M. B. Nielsen, Chem. Eur. J. 2013, 19, 9542-9548.
DHA
Hammett Substituent Constants
- Measure of electron-withdrawing / donating effect
O
OH
O
+ H 2O
X
O
+ H 3O
X
KX
s X = log
= pKa,H - pKa,X
KH
Electron-withdrawing groups have positive s-values (e.g. NO2)
Electron-donating groups have negative s-values (e.g. NMe2)
KX
VHF Ring-Closure – Hammett Correlation
- Effect of changing the aryl group at the vinylic position
NC
fast
CN
NC CN
X
-8.8
X
p-CN
p-CN
p-NO
2
p-NO
p-CO2Me
-9.0
2
p-CHO
-9.2
m-CN
p-CCH
ln(k)
-9.4
p-I
-9.6
p-F
-9.8
slow
m-SAc
p-Br
m-I
m-CCH
p-OMe
p-OMe
H
-10.0
p-NHAc
p-Me
-10.2
-0.4
-0.2
0.0
sm/p
0.2
sm / por
(meta
0.4
0.6
0.8
para)
Electron-withdrawing group at vinylic position ring enhances the ring-closure
S. L. Broman, M. Jevric, M. B. Nielsen, Chem. Eur. J. 2013, 19, 9542-9548.
VHF Ring-Closure – Hammett Correlation
- Effect of changing the aryl group at the seven-membered ring
X
fast
X
p-NMe2
NC
CN
NC CN
Ph
Ph
through-conjugation
values
p-OMe
p-NO2
slow
☐
sp or

sp+
Electron-donating group at the seven-membered ring enhances the ring-closure
S. L. Broman, M. Jevric, M. B. Nielsen, Chem. Eur. J. 2013, 19, 9542-9548.
Ultrafast Ring-closure of VHF
in Locked s-cis Conformation
NC
1.0
CN
NC CN
DHA
Abs
0.8
Half-life: < 2 s (cyclohexane)
0.6
0.4
VHF
0.2
0.0
300
400
500
600
Wavelength (nm)
700
800
S. L. Broman, O. Kushnir, M. Rosenberg, A. Kadziola, J. Daub,
M. B. Nielsen, Eur. J. Org. Chem. 2015, 4119-4130.
Dihydroazulene / Vinylheptafulvene Switch
Low energy
High energy
Halting the back-reaction:
<1 s
Infinitely
long…
Half-life for VHF to DHA back-reaction
Another Approach – Building in Strain
… Macrocyclic Structures
N N
N N
Calculations predict increased energy storage
and step-wise back-reactions
N
N
E. Durgun, J. C. Grossman, J. Phys. Chem. Lett. 2013, 4, 854-860.
Current Status
Energy storage capacities of 0.5 MJ/kg may soon be within reach
Storage times of days have been reached
– longer storage times have been reached for various molecules
Up to years for the DHA-VHF couple, but triggering the energy release is still a challenge
Center for Exploitation of Solar Energy &
Department of Chemistry, University of Copenhagen
Acknowledgements
Dr. Søren L. Broman
Dr. Martina Cacciarini
Dr. Martyn Jevric
Anders B. Skov
Alexandru Vlasceanu
Prof. Kurt V. Mikkelsen
Prof. Henrik G. Kjaergaard
Dr. Jonas Elm
Stine T. Olsen
Anders Gertsen
Anne Schou
University of Copenhagen & the Carlsberg Foundation