UV-Vis. MolecularAbsorption SpectroscopyProf. Tarek A. Fayed

UV-Vis. Electronic Spectroscopy The interaction of molecules with ultraviolet and visiblelight may results in absorption of photons. This resultsin electronic transition, involving valance electrons, fromground state to higher electronic states (called excitedstates). The promoted electrons are electrons of thehighest molecular orbitals HOMO.Absorption of ultraviolet and visible radiation in organicmolecules is restricted to certain functional groups(known as chromophores) that contain valenceelectrons of low excitation energy.A chromophore is a chemical entity embedded within amolecule that absorbs radiation at the same wavelengthin different molecules.Examples of Chromophores are dienes, aromatics,polyenes and conjugated ketones, etc.

Types of electronic transitionsElectronic transitions that can take place are of threetypes which can be considered as; Transitions involving p-, s-, and n-electrons. Transitions involving charge-transfer electrons. Transitions involving d- and f-electrons in metalcomplexes.Most absorption spectroscopy of organic molecules isbased on transitions of n- or -electrons to the *-excitedstate. These transitions fall in an experimentallyconvenient region of the spectrum (200 - 700 nm), andneed an unsaturated group in the molecule to provide the -electrons.

In vacuum UV or far UV(λ 190 nm )In UV/VIS

For formaldehyde molecule;

Selection Rules of electronic transitionsElectronic transitions may be allowed or forbiddentransitions, as reflected by appearance of an intense orweak band according to the magnitude of εmax, and isgoverned by the following selection rules :1. Spin selection rule ( S 0 for the transition to beallowed): there should be no change in spin orientationi. e no spin inversion takes place during thesetransitions. Thus, S S, T T are allowed, but S T,T S are forbidden transitions.2. Laporte (orbital) selection rule (Δl 1): If the molecule has a centre of symmetry, transitionswithin a given set of p- or d-orbitals (i.e. those which onlyinvolve a redistribution of electrons within a given subshell)are forbidden.

There must be a change in the parity (symmetry), i.e transition can occur only between states of oppositeparity.Laporte - allowed transitions: g u or u gLaporte - forbidden transitions: g g or u ug stands for gerade – compound with a center ofsymmetry. u stands for ungerade – compound withouta center of symmetry.Thus, s p, p d are allowed, but s s, p pare forbidden.

3. h E transition energy (photon matches energygap between ground and excited state)4. μ 0 (anisotropic charge displacement)This selection rule tells us transitions which give nochange in dipole moment are forbidden and hence willhave almost zero intensity.Selection rules can be relaxed due to:1. Spin-forbidden transitions may be relaxed by spinorbit coupling effects that make spin a poor quantumnumber (heavy atom effects).2. Symmetry-forbidden transitions may be relaxed bycoupling of electronic transitions to vibrational transitions(vibronic coupling effect).

Features of an Electronic Spectrum1. The frequency, wavelength or energy of a transition; itis related to the energy required to excite an electron(determines the colours of molecules).2. The band width; it is related to the vibrational excitationthat accompanies the electronic transition. narrow bands: excited state has similar geometry as theground state broad bands: excited state has different geometry to theground state3. The band intensity (height or area of a band); it isrelated to the number of photons absorbed, and dependson: concentration, optical path length and the transitionprobability.

UV-Visible Light Absorption in Polyatomic Molecules(a)(b)(c)The intensity of absorption for (a) an atom, (b) a diatomic molecule,and (c) a polyatomic molecule.Spectral lines: individual transitions between two energy levels.Spectral bands: simultaneous transitions between very closeenergy levels, thus, represent the sum of many spectral lines.

Origin of bands in molecular spectraMolecules have chemical bonds which are electronsin molecular orbitals. Absorption of light photons causes electronictransitions between HOMO and LUMO Molecules undergo bond rotations and vibrations.Since different energy sub-levels are occupied at RTand accessible through absorption, many transitionsare possible: A band is the sum of many lines LUMOVibrational sub - levelsHOMOrotational sub - states

Examples of electronic transitions for; a diatomic (I2) molecule,and a polyatomic (chlorophyll) molecule.

The vibrational fine structure of electronic transitions indiatomic moleculesBorn–Oppenheimer (BO) approximationIn quantum mechanics, the Born–Oppenheimer (BO)approximation is the assumption that the motion of theatomic nuclei and electrons in a molecule can beseparated. In mathematical terms, it allows the wavefunction of a molecule to be broken into its electronic(vibrational, rotational) and nuclear components.Etotal Eelec Evib Erot EnuclThat is to say:Where;Eelec: electronic; Evib: vibrational ; Erot: rotational; andEnucl: nucleus spin, transitional energies.


Frank – Condon (FC) PrincipleIt states that transitions betweenelectronic states correspond tovertical lines on an energy versusinter-nuclear distance diagram. Electronic transitions occur on atimescale that is very shortcompared to the vibrationalperiod of a molecule. The electronic transition is fast (10-15s) with respect to nuclear motions(10-13 s).I. e transitions where the position andmomentum of the nuclei don’t changeare favored.

(highly favored transition is 0-0) (highly favored transition is 0-2)

Progressions(v ) Represents a series of bandswithacommonlowervibrational level (have the samev, for absorption) or a commonupper vibrational level (have thesame v , for emission). Bands are separated by v for absorption or v foremission.((vv)) Anabsorptionspectrumreflects the vibrational levels ofthe electronically excited state. Anemissionspectrumreflects the vibrational levels ofthe electronic ground stateElectronic transitions involvingvibrational levels with quantumnumbers v and v

Sequences(Vv ’)Represents a series of bandsseparated by (v – v) with v isconstant.More commonly observed inemission spectra.(v”)V

What do the intensity patterns tell us about theallowed transitions?

Transitions among the ground and excited states(The Jablonski Diagram)

Factors affecting electronic transitions:1. Molecular structure: π-Conjugated Molecules- Nmerous examples from organic and biological chemistry(e.g. β-carotene, 11 conjugated double bonds;chromophore responsible for color vision; etc.- Molecules composed of alternating single and double (ortripl)e) bonds e.g.ethylene(not a conjugated molecule)All will have differentelectronicspectradue to differences inmolecular e

2. General solvent effects *Solvents affect electronicspectra due to changesof their polarity (dielectricconstant,f( ),andrefractive index, f(n)) aswellashydrogenbonding, by changing theprobabilityandtheenergy of both absorptionand emission.bc0-0 transition0-0 transitionh Ah Fda

Solvents affect electronic spectra through their ability tostabilize ground and excited states differently, therebychanging the probability and the energy of bothabsorption and emission( ab)g( f)g( ab)s( f)s


l1Diagramshowing whythe transitionsdo not exactlyoverlap andthere will be aStokes shift.STOKES SHIFTl2Stokes Shiftl3

Effects on * and n * transition *Hydrogen bondSolvent relaxationn Intensify solvent effectRed shiftBlueshiftIn n * transitions as the solvent polarity increases. Solvationstabilizes the nonbonding pair (blue shift ).In * transitions as the solvent polarity increases. Solvationstabilizes *, which is often more polar than (red shift).

3- Viscosity and pH Effects: increasing viscosity leads to increasing of bandintensity and formation of vibrational structures ofbands due to decreasing of molecular collisions. Electronic transitions are pH dependentcompounds with acidic or basic substituents.forChanges in pH influence the degree of ionization, which, in turn, mayaffect the extent of conjugation or the aromaticity of the compound.HHNHHNHHNresonance forms of aniline more resonance forms stabilize excited state

Terms describing UV-Vis. absorptions1. Chromophores: functional groups that is responsiblefor electronic transitions (Group will have acharacteristic lmax and max). Molecular structure orenvironment can influence lmax and .2. Auxochromes: substituents with unshared pair ofelectrons like OH, NH, SH ., when attached to πchromophore they generally move the absorption longer λ.3. Bathochromic shift: shift to longer λ, (red shift).4. Hysochromic shift: shift to shorter λ, (blue shift).5. Hyperchromic effect: increase in εmax of a band.6. Hypochromic effect: decrease in εmax of a band.

Electronic transitions that can take place are of three types which can be considered as; Transitions involving p-, s-, and n-electrons. Transitions involving charge-transfer electrons. Transitions involving d- and f-electrons in metal complexes. Most absorption spectroscopy of organic molecules is