Circular Dichroism Spectrometers

George E Tranter, Chiralabs Limited, Oxfordshire, UK

& 2010 Elsevier Ltd. All rights reserved.

AbbreviationsACS ammonium d-10-camphor sulfonate

CD circular dichroism

CPL circularly polarized light

ECD electronic circular dichroism

LPL linearly polarized light

ORD optical rotatory dispersion

PEM photoelastic modulator

PMT photomultiplier tube

ROA Raman optical activity

UV ultraviolet

VCD vibrational circular dichroism

Historical Perspective

Circular dichroism (CD) is the differential absorption ofleft and right circularly polarized light (CPL) by a chiralsubstance. The phenomenon can occur for all wavelengthsof light and results in ‘vibrational’ CD (VCD) in the in-frared region and ‘electronic’ CD (ECD) in the ultraviolet(UV) and visible regions, corresponding to the differentexcitations induced in a molecule by the light; ‘RamanOptical Activity’ (ROA) is the analogous phenomenon forRaman scattering. The VCD and ROA techniques aredescribed elsewhere in this encyclopedia; the section hereconcentrates on the instrumentation for ECD.

Of these different forms of chiroptical spectroscopy,ECD is by far the most commonly employed techniquefor investigating chiral substances and is, therefore, oftenreferred to simply as ‘CD.’ Not least this choice is aidedby the relatively small sample requirements (typicallyless than 1 mg) and wide variety of solution conditionsthat can be accommodated by ECD.

As early as 1811, Arago had discovered that quartzplates cause plane-polarized light to be rotated by dif-ferent degrees with varying wavelength of light. Thiswavelength dependence of optical rotation, the basis ofoptical rotatory dispersion (ORD), was further elucidatedby Biot and Fresnel in the following two decades.However, ORD subsequently received little experimentalattention, with the primary experimental work beingconstrained to optical rotations of a single wavelength,the sodium D-line (589.3 nm). Apart from work byTschugaefi and Rupe (early 1900s), the phenomenon ofORD remained a theoretical concern until the studies ofKuhn, Lowry, Mitchell, Levene, and Rothen in the 1930s.

Nonetheless, the technical difficulties of measuringORD, primarily through photographic means, hamperedthe science until the development of photoelectric spec-tropolarimeters in the mid-1950s. Brand and Rudolphintroduced the first photoelectric spectropolarimeterc.1956. During the subsequent 10 years various manu-facturers, including Roussel-Uclaf, Jasco (1961 onwards,the ORD-UV-5 instrument being notable), and Bellinghamand Stanley (otherwise known as Bendix-Ericsson orBendix-Gillham-King), introduced new designs and auto-mated spectropolarimeters with extended wavelengthranges, often taking advantage of a Faraday cell to elec-trically generate a compensating optical rotation.

The phenomenon of CD was discovered relatively soonafter Arago’s initial observations of ORD, with Hidingerreporting differential absorption of CPL by amethystcrystals in 1847. CD of copper and chromium tartratesolutions was later observed by Cotton in 1895–96. Itsapplication to organic molecules awaited the pioneeringwork of Mitchell, Lowry, and Kuhn in the late 1920s and1930s. The development of CD instrumentation broadlyfollowed that for ORD and in the late 1950s the Roussel-Juoan dichograph was produced, based around the Pockelcell, a tetragonal crystal of monoammonium phosphatesubjected to an electric field to generate circularly polar-ited light from linearly polarized light (LPL), a forefatherof the photoelastic modulators (PEMs) used today. Jascosubsequently made an attachment for its ORD-UV-5spectropolarimeter in order that it may measure CD; tothis day, Jasco CD instruments are described by the com-pany as ‘spectropolarimeters’ despite their now being de-signed to primarily measure CD not ORD.

Linearly, Elliptically, and CircularlyPolarized Light

The properties of polarized light are of crucial import-ance to the operation of CD instruments and thus themain features are briefly recapped here.

LPL of a given wavelength l and frequency n is de-fined as having its electric field vector transverse to thedirection of propagation, lying in a single plane andvarying sinusoidally with time and space as depicted inFigure 1(a). The equation for the electric field vector at aparticular position z along the direction of propagation attime t can be written as follows:

Aðz; tÞ ¼ A o sinoz

cm� ot

� �i

325

where i is the unit vector in the direction of polarization,o is the angular frequency (2pn), and cm is the velocity oflight in the medium, with the scalar amplitude being Ao.

This LPL can be considered as composed of eithertwo mutually orthogonal linearly polarized components,as in Figure 1(b), or two oppositely handed circularlypolarized components, as in Figure 5(b). Both de-scriptions are equally valid and by modifying thesecomponents in various ways, a range of possible polar-ization states of light can be obtained.

Linearly Polarized Components

First (LPL), can be considered as being composed of twomutually orthogonal, in-phase, equal magnitude linearlypolarized components with planes of polarization lying at� 451 and þ 451 to the original plane, as in Figure 1(b).In this figure, the directions of the component polar-izations are denoted as u and v.

If the v component were to be retarded relative to theu component by a distance d¼ l/2 and the componentsrecombined, as depicted in Figure 2, then LPL would beregained, but the plane of polarization would be rotatedthrough 901. A similar result is obtained if the u com-ponent is retarded instead. This is the basis of a

‘half-wave plate’ for rotating plane-polarized light andapplies for all retardations that are odd integer multiplesof l/2, the even integer multiples simply retaining thelinear polarization at its initial state.

Alternatively, if the v component were to be retardedrelative to the u component by a distance of d¼ l/4 andthe components recombined, then left CPL would begenerated, as depicted in Figure 3. (By convention, aviewer looking at the oncoming light, back along thedirection of propagation toward the source, sees theelectric field describe an anticlockwise motion with timefor left CPL and, correspondingly, a clockwise motion forright CPL; thus, at any point in time, left CPL spatiallydescribes a left-handed helix and right CPL spatiallydescribes a right helix.) An analogous result but gener-ating right CPL occurs if the u component is retardedinstead (which can alternatively be described as a nega-tive retardation), as depicted in Figure 4. This is the basisof a ‘quarter-wave plate’ for converting plane-polarizedlight to circular polarization.

For retardations that are not integer multiples of l/4or l/2, the situation is rather more complex and gener-ally ellipsoidally-polarized light results. The degree ofellipticity can be described by the ratio of its minor tomajor axes, Pminor and Pmajor , respectively (as electric

�

u

(a)

(b)

v

Line

ar p

olar

izat

ion

Direction of propagation

Figure 1 (a) (Top) linearly polarized light of wavelength l, polarized in the vertical plane, showing the electric field vectors

perpendicular to the direction of propagation through space; (b) (bottom) the light decomposed into two in-phase components with

mutually orthogonal plane polarizations at 7451 to the original polarization. From Chiralabs Ltd., with permission.

326 Circular Dichroism Spectrometers

field vector amplitudes), to give an angular ellipticityof y:

tan y ¼ Pminor

Pmajor

Now suppose, instead of retardation, the v polarizedcomponent underwent a reduction in magnitude (such aswhen it is absorbed). In this case, on combining back withits u polarized complement, plane-polarized light is re-gained albeit with a loss in amplitude, but with the planerotated clockwise (following the viewing convention

uv

L

�/4 Retardation

Figure 3 Retardation of v component by l/4 relative to u component gives left circularly polarized light. From Chiralabs Ltd., with

permission.

u

v �/2 Retardation

Figure 2 Retardation of one component by l/2 gives plane-polarized light with its plane of polarization rotated through 901. From

Chiralabs Ltd., with permission.

Circular Dichroism Spectrometers 327

described earlier). Correspondingly, a reduction inmagnitude of the u polarized component gives rise to ananticlockwise rotation. This forms the basis of lineardichroism spectroscopy, details of which can be foundelsewhere in this encyclopedia.

Circularly Polarized Components

LPL can also be considered as being composed of two in-phase equal magnitude CPL components, with theirelectric field vectors describing left- and right-handedhelices, respectively, as in Figure 5.

If the left circularly polarized component were to beretarded relative to the right by a distance d and then thecomponents recombined, LPL would be regained butwith a rotated plane of polarization; the angle or rotation(j) being given by the relationship:

j ¼ d � pl

Similarly, if the right CPL were to be retarded (a negatived) a rotation of the plane in the opposite direction wouldbe apparent, corresponding to an inversion in the sign ofrotation. This is the basis of optical rotation, more ofwhich can be found in the related sections in the en-cyclopedia, the retardation distance being given by thedifferential refractive indices of a chiral sample to CPL(nL� nR). Likewise, it is the basis of ‘half-wave plates’

that rotate plane-polarized light by 901, but simply de-scribed from a different perspective.

However, suppose instead of retardation, the left cir-cularly polarized component underwent a reduction inmagnitude (such as when it is absorbed). Combining backwith its right circular polarized complement, elliptically

polarized light is obtained instead of the original linearlypolarization. In terms of the amplitudes of the two cir-cularly polarized components (PL and PR, respectively),the ellipticity defined earlier in the text is now given by

tan y ¼ PR � PL

PR þ PL

and, in terms of their intensities IL and IR:

tan y ¼ I1=2R � IL

1=2

I1=2R þ IL

1=2

Differential Absorption and LightIntensities

To measure CD, a CD spectrometer invariably generatesboth hands of CPL from plane-polarized light and, insome way, measures the difference in absorption of eachby a sample. By scanning or delineating the wavelengthsof light, a CD spectrum is acquired, that is,

DAl ¼ ALl � ARl

u

v

R

�/4 Retardation

Figure 4 Retardation of u component by l/4 relative to v component gives right circularly polarized light. From Chiralabs Ltd., with

permission.

328 Circular Dichroism Spectrometers