Paper
PCCP
yield of the free base porphyrin is produced, most likely by reaction
with trace impurity water. The free base participates in an unusual,
fully reversible equilibrium in which the monomer dominates at
high temperature and a dimeric J aggregate at low temperature.
The thermodynamic functions characterizing the ground state
equilibrium reveal an unusually large enthalpy change for the
dimerization in the IL, consistent with a strong solute–solvent
interaction.
8 P. Cheng, T. Lan, W. Wang, H. Wua, H. Yang, C. Deng,
X. Dai and S. Guo, Improved dye-sensitized solar cells by
composite ionic liquid electrolyte incorporating layered
titanium phosphate, Sol. Energy, 2010, 84, 854–859.
9 R. Kawano, T. Katakabe, H. Shimosawa, M. K. Nazeeruddin,
M. Gr ¨a tzel, H. Matsui, T. Kitamura, N. Tanabe and M. Watanabe,
Solid-state dye-sensitized solar cells using polymerized ionic
liquid electrolyte with platinum-free counter electrode,
Phys. Chem. Chem. Phys., 2010, 12, 1916–1921.
The absorption and fluorescence emission spectra of both
the monomer and the J aggregate have been analyzed and their 10 T. Stergiopoulos, M. Konstantakou and P. Falaras, Dye solar
excited state kinetics determined via time-dependent fluores-
cence measurements. Both the spectroscopic and kinetic
measurements reveal unusual solvation effects attributable to this
cells combining a TiO surface-blocking organic sensitizer
2
and solvent-free ionic liquid-based redox electrolyte, RSC Adv.,
2013, 3, 15014–15021.
particular ionic liquid and not found in those of the imidazolium 11 L.-L. Li and E. W.-G. Diau, Porphyrin-sensitized solar cells,
series. In particular the strong solvation effects reduce the exci-
Chem. Soc. Rev., 2013, 42, 291–304.
tonic interaction energy in the J aggregate resulting in a red-shift 12 J. Szmytkowski, T. Bond, M. F. Paige, R. W. J. Scott and
of the Soret absorption band that is unusually small and a
radiationless decay rate of the singlet exciton that is unusually
slow. The unusual solvation effects of this IL are thus reflected
R. P. Steer, Spectroscopic and photophysical properties of
ZnTPP in a room temperature ionic liquid, J. Phys. Chem. A,
2010, 114, 11471–11476.
consistently in both the ground state thermodynamics and the 13 V. Ermolaev, V. Miluykov, D. Krivolapov, I. Rizvanov, E. Zvereva,
excited state kinetics of this system.
S. Katsyuba, O. Sinyashin and R. Shmuzler, Phosphonium
ionic liquids based on bulky phosphines: synthesis, structure
and properties, Dalton Trans., 2010, 39, 5564–5571.
Despite many favourable properties, tetraalkylphosphonium
ionic liquids appear not to be suitable for OPVs in which
metalloporphyrins are the primary solar energy absorbers.
14 L. Ferguson and P. Scovazzo, Solubility, diffusivity and
permeability of gases in phosphonium-based room tem-
perature ionic liquids: data and correlations, Ind. Eng.
Chem. Res., 2007, 46, 1369–1374.
5 K. J. Fraser and D. R. MacFarlane, Phosphonium-based
ionic liquids: an overview, Aust. J. Chem., 2009, 62, 309–321.
6 M. van Burgel, D. A. Wiersma and K. Duppen, The dynamics
of one-dimensional excitons in liquids, J. Chem. Phys., 1995,
Acknowledgements
1
The authors are pleased to acknowledge the continuing support
of this research by the Natural Sciences and Engineering
Research Council of Canada.
1
102, 20–33.
1
7 J. Moll, S. Daehne, J. R. Durrant and D. A. Wiersma, Optical
dynamics of excitons in J aggregates of a carbocyanine dye,
J. Chem. Phys., 1995, 102, 6362–6370.
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