Resonance Enhancement of Raman Spectroscopy: Friend Or Foe?

www.spectroscopyonline.com ® Electronically reprinted from June 2013 Volume 28 Number 6 Molecular Spectroscopy Workbench Resonance Enhancement of Raman Spectroscopy: Friend or Foe? The presence of electronic transitions in the visible part of the spectrum can provide enor- mous enhancement of the Raman signals, if these electronic states are not luminescent. In some cases, the signals can increase by as much as six orders of magnitude. How much of an enhancement is possible depends on several factors, such as the width of the excited state, the proximity of the laser to that state, and the enhancement mechanism. The good part of this phenomenon is the increased sensitivity, but the downside is the nonlinearity of the signal, making it difficult to exploit for analytical purposes. Several systems exhibiting enhancement, such as carotenoids and hemeproteins, are discussed here. Fran Adar he physical basis for the Raman effect is the vibra- bound will be more easily modulated. So, because tional modulation of the electronic polarizability. electrons are more loosely bound than electrons, the T In a given molecule, the electronic distribution is polarizability of any unsaturated chemical functional determined by the atoms of the molecule and the electrons group will be larger than that of a chemically saturated that bind them together. When the molecule is exposed to group. Figure 1 shows the spectra of stearic acid (18:0) and electromagnetic radiation in the visible part of the spec- oleic acid (18:1). These two free fatty acids are both con- trum (in our case, the laser photons), its electronic dis- structed from a chain of 18 carbon atoms, in one case fully tribution will respond to the electric field of the photons. saturated and, in the other, with one carbon double bond Then, as the atoms move during a vibration, the electronic between the ninth and 10th carbon atom from the methyl polarizability will be modulated. This modulation of the end. In addition to this chemical difference, the saturated electronic polarizability is what gives rise to the Stokes fatty acid is solid, whereas oleic acid is liquid. The liquid and anti-Stokes Raman spectrum. form accounts for the relative broader lines and some dif- If you think about it, electrons that are more loosely ferences in the band ratios. However, the most important Spectroscopy 28(6) June 2013 www.spectroscopyonline.com 400 1298 1439 350 1063 1129 300 1461 1466 1423 250 1104 1413 1370 200 150 1656 1440 100 1302 1459 Intensity (arbitrary units) 1086 1264 1064 50 1119 0 600 800 1000 1200 1400 1600 Raman shift (cm-1) 600 2880 500 3 284 400 2899 1 2923 300 2935 285 7 2 2962 289 287 200 2930 Intensity (arbitrary units) 100 3009 0 2600 2700 2800 2900 3000 3000 3000 3300 3400 Raman shift (cm-1) Figure 1: Raman spectra of stearic acid (red) and oleic acid (blue). difference for this discussion is the clear. There is no electronic transition represent resonant conditions. The presence of the analytical bands for in the visible part of the spectrum resonance condition for Raman scat- the -C=C- stretch at 1655 cm-1, and where the Raman effect is being ex- tering occurs when there is a transi- the CH stretch on the –C=C- bond cited and transmitted. When there tion in the vicinity of the energy at 3009 cm-1. Assuming the signal are such transitions things become of the laser photon. Because most strengths reflect the number of func- much more complicated. Aside from work, especially analytical work, is tional groups, one would expect the needing to worry about the sample done with visible lasers, this means –C=C- band to be weaker than the self-absorption (similar to the issue in a visible transition, which will be an C-C bands. In fact, the integrated the near infrared [NIR]), there is the electronic transition because typical intensity between 1600 and 1700 cm-1 enhancement issue that we want to visible photon energies (1.5–3 eV) are (C=C) is twice as strong as the inte- explore in this column. in the range of electronic transitions. grated intensity between 1030 and For the purposes of this article we 1140 cm-1 (C-C) even though there Resonant Raman Scattering will discuss molecular systems with are 16 single bonds versus 1 double The previous example illustrates electrons, although there are certainly bond! The spectrum of an unsatu- how the presence of electrons can other classes of electronic transitions rated fatty acid is dominated by the increase the polarizability of a mol- in the visible part of the spectrum. -C=C- stretch. ecule, thus increasing the Raman For unsaturated molecular systems These compounds are still optically cross-section. But this does not yet to produce visible optical transitions, www.spectroscopyonline.com June 2013 Spectroscopy 28(6) the electrons must be “conjugated,” at about 200 nm (~6 eV). As double ment of alternating single and double which means the molecule must con- bonds are added to a conjugated bonds. The result is that degraded tain multiple carbon double bonds chain, the energy of the lowest energy PVC has polyenes with a population separated by single bonds. These mol- absorption occurs at decreasing ener- of sequence lengths. If you examine ecules can be linear (called polyenes) gies (or increasing wavelengths). The such materials visibly they may be or they can be represented by closed free electron model has been used to tinged orange or there may be no rings that are called “aromatic,” cer- describe these states: The electrons visible coloration. But if you record a tainly because of the characteristics of are approximated as particles in Raman spectrum the polyene can be some of the earliest known examples a box defined by the length of the observed with very high sensitivity, such as benzene. Toward the end of chain (1). As the box becomes longer, even when there is no evidence for its the 19th century and into the begin- the energy of the states become lower, presence optically. What is addition- ning of the 20th century, there was with an asymptotic long wavelength ally interesting is that the observed development of synthetic textile dyes value of about 600 nm. Raman frequency depends on the based on conjugated molecules, often There is an analogous phenom- excitation wavelength because each containing N atoms; this industry enon in describing the vibrational wavelength excites a different part actually represents the beginnings states (2). An isolated double bond of the population with its own reso- of organic chemistry. Very often the has a frequency of about 1650 cm-1 nance values. excited electronic states of dye mol- and a single bond has a frequency The second surprising instance ecules are long-lived and result in near 1000 cm-1. But in a conjugated where I observed a carotenoid oc- decay via radiative emission, which system the double bond frequency curred while I was working with is known as fluorescence and phos- is lowered, and the single bond Professor Gene Hall of Rutgers Uni- phorescence. Fluorescence tends to be frequency is raised (because of the versity. Professor Hall was studying a an intense process (quantum yields averaging of the bond order). As the series of fish oil supplements. When of the order of 0.5). Because Raman chain is lengthened, these shifts will he examined a krill oil, he found that quantum yields are of the order of increase until the frequencies hit the spectrum was overwhelmed by 10-6, any molecule that fluoresces will “limiting” values of about 1495 and the spectrum of a carotenoid, even probably not be a good candidate for 1150 cm-1. though the sample was colorless. Raman measurements; the Raman All of this means that there is a A more careful examination of the signal will be overwhelmed by its correlation between the conjuga- spectrum, while referring to the Mer- “fluorescence background.” tion length, the optical absorption lin publication that we cited earlier Examples that exhibit beautiful res- wavelength, and the Raman vibra- (2), indicated the presence of astax- onance-enhanced spectra are taken tional frequencies. However, there anthin, a well known natural carot- from biomolecules — carotenoids and is actually more than a correlation. enoid that differs from -carotene hemes. They illustrate the sensitivity The presence of the optical absorp- in that there are carbonyl and OH of the technique, as well as the infor- tion enables the resonance Raman functionalities on each of the ring mation that can be derived. They also phenomenon, and the resonance is end groups. illustrate that the symmetry of the quite sensitive to the excitation wave- It is useful to review briefly the molecule as well as the electronic and length; that is, the important param- mechanism by which the spectra of vibrational states determine the mode eter in the resonance phenomenon is polyenes is enhanced. The to * of enhancement and the types of vi- the proximity of the laser wavelength electronic transition is an “allowed” brations that are enhanced. to the absorption maximum. So, if transition that is polarized along the you are able to tune the laser wave- axis of the molecule, so vibrations Carotenoids length, you will see a variation in the that couple to it most effectively have Carotenoids are chains of alternating resonance enhancement that follows symmetric character. But not all vi- double and single bonds. -Carotene, the visible absorption spectrum. brations will be resonance enhanced. the orange pigment in carrot root, The resonance enhancement of A vibration will be resonance Raman is the quintessential carotenoid, al- the Raman signals of polyenes shows active if there is a change in bond though there are many others, such up in surprising circumstances.

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