UV-Vis SPECTROSCOPY, CHEMOMETRICS AND NON- BONDING INTERACTIONS IN SOLUTION: qualitative conclusions and quantitative estimations

UV-Vis Spectroscopy background (structural information, quantitative analysis) UV-Vis Spectroscopy background (structural information, quantitative analysis) Crown ethers complex formation Crown ethers complex formation Self-association of dyes Self-association of dyes Difficult cases for quantitative analysis of equilibria in solution (chemometrics) Difficult cases for quantitative analysis of equilibria in solution (chemometrics) Lecture Outline:

What is Optical Spectroscopy? The study of molecular structure and dynamics through the absorption, emission and scattering of light. UV-Vis Spectroscopy: Maxwell: The light is an electromagnetic field characterized by a frequency, and wavelength. Light obeys the relationship c =. With energy of photons E = h.

The Electromagnetic Spectrum: UV-Vis Spectroscopy: = c / E = h.

The Electromagnetic Spectrum: UV-Vis Spectroscopy: = c / E = h.  Hz 

Molecular Spectroscopy: UV-Vis Spectroscopy:

Optical Spectroscopy and Non-Bonding Interactions (IR vs UV-Vis): IR Spectroscopy: Vibrational motion ( > 1000 nm); More suitable for structural identification; High concentration range (> mol/l); Limited number of solvents; UV-Vis Spectroscopy: Optical Spectroscopy and Non-Bonding Interactions (IR vs UV-Vis): UV-Vis Spectroscopy: Electronic transitions (200 – 1000 nm) More suitable for quantitative analysis; Low concentration range (< mol/l) Large number of solvents, including water; Simple instrumentation, low running costs

Single beam Beckman B UV-Vis spectrometer Dual beam scheme Cary & Beckman Beckman DU-2 UV-Vis spectrometer ( , units produced) UV-Vis Spectroscopy:

Frank-Condon principle: Describes the intensities of vibronic transitions, or the absorption or emission of a photon. When a molecule is undergoing an electronic transition the nuclear configuration experiences no significant change. This is due in fact that nuclei are much more massive than electrons and the electronic transition takes place faster than the nuclei can respond. When the nucleus realigns itself with the new electronic configuration, the theory states that it must undergo a vibration.

UV-Vis Spectroscopy: A max A max /2  1/2 max position of the band – energy of the transition integral intensity (area) – oscillator strength (not molar absorptivity) Spectral shape: - position of the band - intensity - half-band width

Structural information: -chromophore is needed (conjugation and/or auxochrome(s)) -broad bands (usefulness for identification is limited) UV-Vis Spectroscopy:

Quantitative analysis: Single compound: Beer-Lambert’s law Mixture: Additivity principle UV-Vis Spectroscopy:

Fathers of Modern Supramolecules: Charles J. Donald J. Jean-Marie Pedersen Cram Lehn Nobel Prize in Chemistry 1987 Crown Ethers:

Modern Supramolecules: crown container cryptands ethers molecules Charles J. Donald J. Jean-Marie Pedersen Cram Lehn Crown Ethers:

Charles J. 12-Crown-4 Pedersen JACS 89, 7017 (1967) Crown Ethers:

-flexible ring structure, containing several ether groups; -size fit effect of complex formation with metals (very strong complexes: 18-crown- 6 has high affinity for potassium cation, 15-crown-5 for sodium cation, and 12- crown-4 for lithium cation); Crown Ethers:

-the oxygen atoms are well situated to coordinate with a cation located at the interior of the ring, whereas the exterior of the ring is hydrophobic. The resulting cations often form salts that are soluble in nonpolar solvents, and for this reason crown ethers are useful in phase transfer catalysis. Crown Ethers:

-contain electron-rich atoms (O,S,N) crown aza-crown cyclen Crown Ethers:

BUT: no spectra, no possibilities for optical sensors SOLUTIONS: -crown ether becomes part of a chromophore -aza-crown ether linked to a chromophore through N-atom Crown Ethers:

-crown ether becomes part of a chromophore, but reduced flexibility; reduced electron density at O-atoms Crown Ethers:

-a distinct and specific interaction between the nitrogen(s) and cations may improve the selectivity of ionophores based on size-fit effect; -the ionophore nitrogen may be a part of the conjugated system improving the potential of the whole molecule to act as a sensor; -the nitrogen may facilitate the synthesizing of three-dimensional cavities, which improve the receptor selectivity. Aza Crown Ethers:

UV-Vis spectroscopy: - quantitative (spectral shift upon complexation or/and change in the quantum yield); - qualitative (stoichiometry of the complex in solution; binding constant, which might be used as measure for sensitivity (one ion) and selectivity (set of ions)); Ligand + Metal Salt  Complex Crown Ethers:

Spectrophotometric titration: - quantitative - qualitative Ligand + Metal Salt  Complex Crown Ethers:

Aza Crown Ethers: direct part of conjugated donor- acceptor system Crown Ethers:

Aza Crown Ethers: linked to conjugated donor-acceptor system via spacer Crown Ethers:

Aza Crown Ethers: non-conjugated donor-acceptor system Crown Ethers:

Crown Ethers & UV-Vis spectroscopy: Relatively simple case for study IF: -The binding constant is large enough; -The complex stoichiometry is simple; -The process of complexation is simple as a mechanism; -The binding site is linked to a donor- acceptor system and the complexation influence either donor or acceptor; Crown Ethers:

Beer-Lambert’s law: Aggregation: A x A A A C 1 C C Concentration Absorbance C 4 1 A Deviations: in the textbooks is written – at high concentrations WRONG!

Beer-Lambert’s law: Aggregation: path length from 0.01 mm to 100 mm

Aggregation of dyes: - affecting their colouristic and spectral properties - increases with an increase of dye concentration or ionic strength; - decreases with temperature rising or organic solvents adding; - addition to the dye structure of ionic solulilizing groups (as sulphonate group) decreases aggregation; - inclusion of long alkyl chains increases aggregation because of higher hydrophobic interaction in solution Aggregation:

Exciton theory: spectral changes observed upon aggregation are caused by electronic interactions between the dye molecules in the aggregate. Simple Dimer Model: Aggregation:

Exciton theory: transition dipols parallel (sandwich)in-line (head-to-tail) Aggregation:

Exciton theory: transition dipols parallel (sandwich)in-line (head-to-tail) Aggregation:

J-Aggregates: - red (bathochromic) shift in the absorption with increased intensity; -red shift in the emission; -aggregation caused by concentration, solvent or salt addition; -typical example: cyanine dyes (non-planar structures) forming helix patern

H-Aggregates: Aggregation: - blue (hypsochromic) shift in the absorption; -weak or no emission; -aggregation caused by concentration, solvent or salt addition; -typical example: ionic planar dyes forming rods (rarely, mainly dimers)

UV-Vis Spectroscopy and Dimerization: Aggregation: - determination of the type of aggregate; -estimation of the distance between monomer molecules in the dimer; -estimation of the angle of transition dipols in the dimer

Structural Parameters of The Dimers: Aggregation: - require finding the constant of dimerization and the spectra of the monomer and dimer

Difficult cases for quantitative UV-Vis spectral analysis: Chemometrics: -complexation with low stability constant of the complex, which does not allow to obtain experimentally the pure spectrum of the complex with addition of metal salt; -dimerization with large dimeric constant, which does not allow to obtain experimentally the pure monomer spectrum upon dilution -dimerization with low dimeric constant, which does not allow to obtain the pure dimer spectrum

Soft & Hard Modeling: Chemometrics:

Soft Modeling (dimerization): strongly bound to the model Chemometrics:

Hard Modeling (complexation): gives flexibility and might be used for complicated equilibria Chemometrics:

UV-Vis spectroscopy is very suitable for study of equilibria in solution, but has some limitations in both quantitative and qualitative analysis. Always use in combination with other instrumental methods for analysis. Do not forget theoretical approaches. Conclusions: