Full Citation: Fluorescence Analysis of Thermoresponsive Polymers. A.G. Ryder and C. Morris, Reviews in Fluorescence 2015 ,
Annual Volumes, Vol. 8, pp. 97-126, (2015). ISBN: 978-3-319-24607-9 (Hardcover), 978-3-319-24609-3 (ebook) Springer.
Page 13 of 31
solvents.
Szczupak et al. evaluated the micro-polarity and H-bond donor/acceptor ability for a series of
thermoresponsive N-isopropylacrylamide/N-tert-butylacrylamide (NIPAm/NtBA) copolymer films
using the ET(30), , and * empirical solvatochromic parameters [19,60]. For the dry
NIPAm/NtBA copolymer films it was found that they are strong H-bond acceptors (), moderate Hbond donors () and are strongly dipolar/polarizable (*). It was observed that ET(30), and *
values all decreased linearly on increasing the hydrophobic NtBA fraction in the copolymer films
whereas the parameter remained relatively unchanged. A good correlation was also found between
experimental ET(30) and independently determined , and * values which confirms that the
behavior of the solvatochromic indicators in the NIPAm/NtBA films was similar to that in solvents
[19].
5.3
PHYSICOCHEMICAL PARAMETERS BY FLUORESCENCE:
While the application of solvatochromic methods for the characterization of physicochemical
properties is feasible, and can generate accurate data, there is still a need for a more flexible
measurement methodology. Specifically, for many applications, the concentrations required of the
solvatochromic probes for absorption spectroscopy in these condensed media is relatively high (typical
weight ratios of 36–20 to 1) [99], and it would be preferable to utilize much lower concentrations such
as the levels employed in fluorescence spectroscopy (typical weight ratios of 1900–2230 to 1) [100].
Another need is the requirement for standoff measurements in sealed environments and in such cases,
fluorescence may be the only feasible option.
Pyrene is a common fluorophore used for polarity assessment because the intensities of various
vibronic bands are very sensitive to solvent polarity [101,102]. Karpovich and Blanchard explained
that the relative changes in vibronic band intensities of the pyrene fluorescence spectrum are a result
of vibronic coupling between the first (weakly allowed) and the second (strongly allowed) excited
singlet states [103]. Their observations also suggest that solvent-dipole-solute induced dipole
interactions play a major role in the pyrene scale. The extent to which an induced dipole moment is
generated by vibrational distortions of pyrene is governed by the polarity of the solvent. The polarity
of the pyrene environment may thus be estimated by measuring the ratio of the fluorescence intensities
of the third and first vibronic bands (I1/I3) [101,102]. Dong and Winnik developed the Py scale of
solvent polarities, based on this ratio and the scale is relatively insensitive to the hydrogen bonding
ability of protic solvents [92]. Py scale values range from 0.58 for n-hexane to 1.95 for dimethyl
sulfoxide. Variations in the pyrene lifetime can also be used to monitor changes in thermoresponsive
polymers.
For aqueous solutions of PNIPAm one observes approximately a 60 ns increase in
average lifetime from 115 ns to ~175 ns as the temperature increases from 30 to 40 °C [104].
Szczupak et al. assessed the polarity of poly(NIPAm-co-NtBA) copolymer films by means of
fluorescence based methods, using pyrene and 3-HF derivatives as polarity sensitive fluorescent probes
Annual Volumes, Vol. 8, pp. 97-126, (2015). ISBN: 978-3-319-24607-9 (Hardcover), 978-3-319-24609-3 (ebook) Springer.
Page 13 of 31
solvents.
Szczupak et al. evaluated the micro-polarity and H-bond donor/acceptor ability for a series of
thermoresponsive N-isopropylacrylamide/N-tert-butylacrylamide (NIPAm/NtBA) copolymer films
using the ET(30), , and * empirical solvatochromic parameters [19,60]. For the dry
NIPAm/NtBA copolymer films it was found that they are strong H-bond acceptors (), moderate Hbond donors () and are strongly dipolar/polarizable (*). It was observed that ET(30), and *
values all decreased linearly on increasing the hydrophobic NtBA fraction in the copolymer films
whereas the parameter remained relatively unchanged. A good correlation was also found between
experimental ET(30) and independently determined , and * values which confirms that the
behavior of the solvatochromic indicators in the NIPAm/NtBA films was similar to that in solvents
[19].
5.3
PHYSICOCHEMICAL PARAMETERS BY FLUORESCENCE:
While the application of solvatochromic methods for the characterization of physicochemical
properties is feasible, and can generate accurate data, there is still a need for a more flexible
measurement methodology. Specifically, for many applications, the concentrations required of the
solvatochromic probes for absorption spectroscopy in these condensed media is relatively high (typical
weight ratios of 36–20 to 1) [99], and it would be preferable to utilize much lower concentrations such
as the levels employed in fluorescence spectroscopy (typical weight ratios of 1900–2230 to 1) [100].
Another need is the requirement for standoff measurements in sealed environments and in such cases,
fluorescence may be the only feasible option.
Pyrene is a common fluorophore used for polarity assessment because the intensities of various
vibronic bands are very sensitive to solvent polarity [101,102]. Karpovich and Blanchard explained
that the relative changes in vibronic band intensities of the pyrene fluorescence spectrum are a result
of vibronic coupling between the first (weakly allowed) and the second (strongly allowed) excited
singlet states [103]. Their observations also suggest that solvent-dipole-solute induced dipole
interactions play a major role in the pyrene scale. The extent to which an induced dipole moment is
generated by vibrational distortions of pyrene is governed by the polarity of the solvent. The polarity
of the pyrene environment may thus be estimated by measuring the ratio of the fluorescence intensities
of the third and first vibronic bands (I1/I3) [101,102]. Dong and Winnik developed the Py scale of
solvent polarities, based on this ratio and the scale is relatively insensitive to the hydrogen bonding
ability of protic solvents [92]. Py scale values range from 0.58 for n-hexane to 1.95 for dimethyl
sulfoxide. Variations in the pyrene lifetime can also be used to monitor changes in thermoresponsive
polymers.
For aqueous solutions of PNIPAm one observes approximately a 60 ns increase in
average lifetime from 115 ns to ~175 ns as the temperature increases from 30 to 40 °C [104].
Szczupak et al. assessed the polarity of poly(NIPAm-co-NtBA) copolymer films by means of
fluorescence based methods, using pyrene and 3-HF derivatives as polarity sensitive fluorescent probes