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Optical Electronic Spectroscopy 2
Lecture Date: January 28th, 2008
Molecular UV-Visible Spectroscopy
Molecular UV-Visible
spectroscopy can:
– Enable structural analysis
– Detect molecular chromophore
– Analyze light-absorbing properties
(e.g. for photochemistry)
Figures from http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/uvspec.htm#uv1
Basic UV-Vis spectrophotometers acquire data in the 190-
800 nm range and can be designed as “flow” systems.
Molecular UV-Visible spectroscopy is driven by electronic
absorption of UV-Vis radiation.
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Molecular UV-Vis Spectroscopy: Terminology
UV-Vis Terminology
– Chromophore: a UV-Visible absorbing functional group
– Bathochromic shift (red shift): to longer wavelengths
– Auxochrome: a substituent on a chromophore that
causes a red shift
– Hypsochromic shift (blue shift): to shorter wavelengths
– Hyperchromic shift: to greater absorbance
– Hypochromic shift: to lesser absorbance
Molecular UV-Vis Spectroscopy: Transitions
Classes of Electron transitions
– HOMO: highest occupied molecular orbital
– LUMO: lowest unoccupied molecular orbital
– Types of electron transitions:
(1) , and n electrons (mostly organics)
(2) d and felectrons (inorganics/organometallics)
(3) charge-transfer (CT) electrons
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Molecular UV-Vis Spectroscopy: Theory
Molecular energy levels and absorbance wavelength:
* and * transitions: high-energy, accessible in vacuum
UV (max <150 nm). Not usually observed in molecular UV-Vis.
n * and * transitions: non-bonding electrons (lone pairs),
wavelength (max) in the 150-250 nm region.
n * and * transitions: most common transitions observed in
organic molecular UV-Vis, observed in compounds with lone pairs
and multiple bonds with max = 200-600 nm.
Figure from http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/spectrum.htm
Molecular UV-Vis Spectroscopy: Theory
d/f orbitals – transition metal complexes
– UV-Vis spectra of lanthanides/actinides are particularly sharp, due
to screening of the 4f and 5f orbitals by lower shells.
– Can measure ligand field strength, and transitions between d-
orbitals made non-equivalent by the formation of a complex
Charge transfer (CT) – occurs when electron-donor and
electron-acceptor properties are in the same complex –
electron transfer occurs as an “excitation step”
– MLCT (metal-to-ligand charge transfer)
– LMCT (ligand-to-metal charge transfer)
– Ex: tri(bipyridyl)iron(II), which is red – an electron is exicted from
the d-orbital of the metal into a * orbital on the ligand
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Molecular UV-Vis Spectroscopy: Absorption
max is the wavelength(s) of maximum absorption (i.e. the
peak position)
The strength of a UV-Visible absorption is given by the
molar absorptivity ():
= 8.7 x 1019 P a
where Pis the transition probability (0 to 1) – governed by selection
rules and orbital overlap,
and ais the chromophore area in cm2
Again, the Beer-Lambert Law:
A = ebc
Molecular UV-Vis Spectroscopy: Quantum Theory
UV-Visible spectra and the states involved in electronic transitions
can be calculated with theories ranging from Huckel to ab initio/DFT.
Example: * transitions responsible for ethylene UV absorption
at ~170 nm calculated with ZINDO semi-empirical excited-states
methods (Gaussian 03W):
HOMO ubonding molecular orbital LUMO gantibonding molecular orbital
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Molecular UV-Visible Spectrophotometers
Continuum UV-
Vis sources – the
2H lamp:
Tungsten lamps
used for longer
wavelengths.
The traditional
UV-Vis design –
double-beam
grating systems
Figure from http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/uvspec.htm#uv1
Hamamatsu
L2D2 lamps
Molecular UV-Visible Spectrophotometers
Diode array detectors can acquire all UV-Visible
wavelengths at once.
Advantages:
– Sensitivity
(multiplex)
– Speed
Disadvantages:
– Resolution
Figure from Skoog, et al., Chapter 13