Applications of EPR Spectroscopy to Understanding Carotenoid Radicals - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 162

TABLE 9.1
Various EPR Techniques Used to Study Radical Cations and Neutral Radicals of
Carotenoids

Technique References

SEEPR detection of the transient radical during
electrochemical preparation in solution

Khaled et al. (1991), Jeevarajan et al. (1994)

TREPR measurements Jeevarajan et al. (1993)

Photoinduced electron transfer in frozen solutions Konovalova et al. (1997)

Carotenoid radical cations formed by chemical oxidation
with I 2

Ding et al. (1988)

Carotenoid radical cations formed by chemical oxidation
with FeCl 3

Jeevarajan et al. (1996)

EPR spin trapping methods Polyakov et al. (2001a,b,c, 2006)

EPR studies of host-guest complexes of carotenoids Polyakov et al. (2004, 2006)

Measuring distances between carotenoid radicals and
distant metals in matrices by using ESEEM methods
and pulsed EPR relaxation techniques

Gao et al. (2005)

EPR studies of radical cations on activated alumina
and silica-alumina

Konovalova et al. (1999, 2001a,b), Gao et al. (2002, 2003)

Use of high-frequency/high-magnetic i eld techniques
to resolve g -anisotropy

Konovalova et al. (1999)

Identify high-spin metal complexes of carotenoids Konovalova et al. (2003)

Establish the effect of distant high-spin metals on
π-radicals properties

Konovalova et al. (2004), Gao et al. (2005)

Kevan and Kispert (1976); Goslar et al. (1994), Piekara-
Sady and Kispert (1994), Kispert and Piekara-Sady (2006)

Use of CW ENDOR techniques to detect β-proton
hyperi ne couplings and matrix nuclei

Konovalova et al. (2001), Focsan et al. (2008), Lawrence
et al. (2008)

Pulsed ENDOR techniques to detect β-proton hyperi ne
couplings and matrix nuclei

HYSCORE techniques to detect α -proton anisotropic Konovalova et al. (2001), Focsan et al. (2008)

coupling tensors

Density functional theory (DFT) calculations to interpret
the powder ENDOR and HYSCORE spectra

Gao et al. (2006), Focsan et al. (2008)

Establish the use of the g -tensor parameters to detect the
presence of dimers

Petrenko et al. (2005)

9.3 TIME-RESOLVED EPR (TREPR)
Time-resolved X- (Jeevarajan et al. 1993a) and Q-band (Jeevarajan et al. 1996b) EPR measurements
have demonstrated that upon excitation at 300 K with a 308 nm pulsed excimer laser (200 mJ/pulse,
17 ns full width at half maximum (fwhm) of a carbon tetrachloride solution of carotenoid in an EPR
l at cell, an electron is transferred from the carotenoid to the solvent. It was estimated that at least
10% of the 200 mJ/pulse laser light entered the cavity, giving rise to a broad emissive high-i eld line
and a broad low-i eld absorption line, 0.1-0.5

s after the laser pulse. The g -values of each species
were determined and a large difference in radical stability as a function of solvent was found. The
EPR signals decayed below detectability after several

μ

s. Analysis of the polarized chemically
induced dynamic electron polarized (CIDEP) 35 GHz spectra showed that a solvent-separated radi-
cal ion pair CCl 4 •− ••CCl 4 ••Car •+ was formed by electron transfer from the excited singlet state of the
carotenoids to the solvent (CCl 4 ). The low-i eld line was due to CCl 4 •− ( g

μ

= 2.009) and the high-i eld

line was due to Car •+ ( g 2.002). The TREPR study showed the photophysics and photochemistry

of carotenoid in CCl 4 is given in Scheme 9.1.

=