UV/Vis-NIR absorption spectroscopy

The Electromagnetic Spectrum

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103 102 101 100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 1011 10-12

Longer wavelength

Shorter wavelength

H2OViru

s

Tennis ballBuilding

Wavewlength(metres)

Generic name

Sources

Frequency (Hz)

Energy ofa photon

(ev)

FM

rad

io

10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 5 105 106

106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018 10201019AM

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X-r

ay m

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Radio waves

Microwaves

Infrared ultraviolet

Soft x-rays

Hard x-rays

Gamma rays

IR Spectroscopy

UV/Vis Spectroscopy

UV/Vis-NIR spectroscopy

What region of the EM spectrum are we interested in?

UVC 210-280 nmUVB 280-320 nmUVA 320-400 nmVisible 400-700 nm

Far Red 700-1100 nmNear-Infra red 1100-2500 nm (9000 cm-1- 4000 cm-1)

(increasingly important for materials science)Mid Infrared 2500-6000 nm (4000 - 60 cm-1)

(unusual for electronic absorptions)

UV/Vis spectrum

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Wavelength /nm

•Plot of the absorbance (unitless dimension) or molar absorptivity (M-1 cm-1) against wavelength•Expressed in nanometres (nm, 10-9 m)•In older books and papers Angstroms (Å, 10-10 m) are used•In the near infrared region (from 900-2500 nm) microns or micrometres (µm) are used

UV region visible region far red region

Wavelength vs wavenumber

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Wavelength /nm

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wavenumber /cm-1

Electronic Absorption E = hν

λ

EM radiation has a magentic and an electronic vector

If electrons move from one orbital to another then there is a change in the charge distribution in a molecule and hence a change in the molecules electric dipole.

Selection rules

-Resonance condition-Change in principle quantum number-Change in orbital angular momentum-No change in spin: (2S+1) must not change-Laporte’s rule: transitions between orbitals of the same parity are forbidden

singlet triplet

singlet singlet

Electronic Absorption

• Total internal energy Sum of the electronic, vibrational and rotational energy

• Etotal = Eelec + Evib + Erot

• For an electronic transition:

∆Etotal= ∆Eelec+∆Evib+∆Erot

Molar Absorptivity

• Probability that a photon of light will be absorbed upon passing through an optically dilute solution

• Proportional to the square of the transition moment (change in electron density)

• Forbidden transitions

Chromophores & Conjugation

Chromophore – brings colourThe absorption of light is quantisedMixing of electronic, vibrational and rotational energy levels leads to broad spectra instead of lines

C = O C = C

Pigments & Dyes, I

• Retinal – cis/trans isomerisation is the first step in sensing light

• Chlorophylls absorb light for photosynthesis

• Types of transitionsn – π* (auxochromes have free electron pairs e.g C=ö: )

n – σ*σ –σ*π – π*

Vibrational progression

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Nor

mai

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d ab

sorp

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emis

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wavenumber /cm-1

9,10-di-phenyl-anthracene

Frank-Condon state

0�10�0

0�2

The Bougier-Beer-Lambert Law

An Empirical law – i.e. based on observation not theoretical

derivation

Transmittance

• The light passing through a sample• Solvents and optical material are

transparent (100% transmittance) only in certain regions of the EM spectrum

• Solvent cut-off (see table 7.6, Hesse):– Water – 190 – CH3CN – 190– Acetone -350

Transmittance

εεεε

T = I1/I0

T = 10-A

The Beer-Lambert-Bouguer Law(empirical law)

• The intensity of light passing through a sample decreases exponentially and the absorbance of light is proportional to the concentration of the chromophore

• The absorbance of light is proportional to the pathlength through which the light travels

A = εclA is absorbance, c is concentration (mol L-1), l is pathlength

(in cm) and ε is the molar absorptivity (L mol-1 cm-1)

N.B. Assumption that all particles behave independently

(no shadow effect)

Justification of BLB lawdI = - κ[J]I dl

where [J] is the molar concentration of the absorbing species, I is the incident intensity,

dI is the reduction in intensity of I, κ is the proportionality coefficient and dl is the

pathlength or thickness of the layer.

Rearranged the equation is

dI = - κ[J] dl

I

And for a series of pathlengths dl and integral is used:

I dI = - κ l [J] dl

I0 I 0

ln (I/I0) = - κ[J] l and since ln (I/I0) = ln 10 * log (I/I0)

Then - log (I/I0) = (1/2.303)κ[J] l = ε c l

This is describes the absorption at a single wavelength however sometimes the

absorption of an entire band is being treated and in this case the integrated

absorption coefficient is used which corresponds to the area uinder the absorption

band Α = ε (v) dv

∫∫

~~∫

Bicomponent systems

• Two species present in solution• Species behave independentlyA = A1 + A2 = ε1c1l + ε2c2l = (ε1c1 + ε2c2)l

pH dependence of fluorescein

Isosbestic pointNo change in abosrbance

Instrumentation

How do we record spectra?

Single beam diode array spectrometer

Halogen lamp for visible& deuterium lamp for UV

Diode array detector

grating

Single beam scanningspectrometer

From Halogen lamp for visible& deuterium lamp for UV

To sample and then PMT Detector

grating

Czerny-Turner monochromator

Dual beam scanningspectrometer

From Halogen lamp for visible& deuterium lamp for UV

Two PMT (190-1100 nm) or PbS (1100-2500 nm) Detectors

From: http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/spectrum.htm

Wavelength vs wavenumber

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Wavelength /nm

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wavenumber /cm-1

In converting from wavelength to wavenumber you have to correct for band pass. The slit width is constant in nm over the whole spectrum but varies in cm-1.

5 nm, 1112 cm-1 5 nm, 156 cm-1

Sample Handling• Spectroscopic grade solvents• Concentration should be known• Cuvette – generally quartz glass (QS), also OS or plastic

cuvettes can be used• Typically 1 cm pathlength cuvettes (cells) are used• Cleaning - hot nitric acid• Cleaning – do not use of strong hydroxide bases or

sources of fluoride as these etch the glass surface• UV/Vis light used to measure can also result in

photochemistry – do not expose the sample to light any more than is necessary.

Measuring spectra

Single Beam vs. Dual beam spectrophotometers Single beam (diode arrays) are fast but often less stableFirst measure a solution not containing the compound of interest

(Spectrum 1)Second measure a solution containing the compound of interest

(spectrum 2)Subtract the reference spectrum from the spectrum with the sample

Dual beam (PMT) are slower but very stableFirst measure a baseline using two solutions that do not contain the

compound of interest (baseline/background)Second measure a solution containing the compound of interest

together with a blank solution Subtract the reference spectrum from the spectrum with the sample

Measuring Molar Absorptivity

Measure absorbance over a range of concentrations

Use different pathlength cells 1 mm to 10 cm to increase dynamic range

Overlap data points i.e. 1 mM with a 1 mm pathlength cell and 0.1 mM with a 1 cm cell

Deviations from linearity

• Assumption of independently absorbing species– Inner filter effect

• Aggregation of samples at higher concentration– Scattering– Intermolecular interactions (H and J aggregates in perylenes)

• Photochemistry• Change in gas content in solution• Reaction with solvent

• Scatter – the wall!

Micellaneous topics

Photochromism

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Wavelength /nm

S

S

∆

hv

R

R

Anti-foldedSyn-folded

S

S

R

R

Pigments and Dyes II

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Wavelength /nm

S

hv

S

HH -H2

S

H H

hv

HH

-H2

H H

Solvatochroism

Interaction of a part of the chromophoric group, e.g. a ketone, or amine, result in a change in the energy of the frontier orbitals and hence a shift in the absorption spectrum

pH dependence of fluorescein

Isosbestic pointNo change in abosrbance

Computational methods

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ZINDO/S 1 ZINDO/S H

21

300 400 500 600 700 800 900

TD-DFT 1 TD-DFT H

21

300 400 500 600 700 800 900

MeCN 1 MeCN H

21

300 400 500 600 700 800 900

H2O 1

H2O H

21

N

NNNN

N NNMe Me

Ru

RuN

N

NN

NN

N

N

N N

2+

__

= 2,2'-bipyridine

Predicted UV.Vis electronic

spectra is done on the basis of

an isolated molecule (in

vacuo)

However solvent is not

innocent in determining the

absorptivity of chromophores

Band GapsSemiconductor materials: TiO2 (titanium dioxide –

white paint, some suncreams, solar cells), ITO (Indium doped SnO2), NiO (nickel oxide)

VB (NiO)

CB (NiO)

hν

Self cleaning windows generate active oxygen and charges

Diffuse Reflectance

Uses an integrating sphere – used to look at rough surfacesAlso used for opaque solutions samples can be prepared as dilute‘solutions’ in KBr

Specular reflectance

Brewster’s AnglePolarisation angle (angle at which light is fully polarised)

Applications: Following Reactions

• Isosbestic points• Multiple reaction steps

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Wavelength /nm

S

S

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hv

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R

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S

S

R

R

Applications: Analysis

• Structural assignments• Quantitative analysis

– HPLC– Following reactions