Transducer influence of polycyclic aromatic hydrocarbons (PAHs) on photoisomerisation of 1-alkyl-2-(arylazo)imidazoles

The UV light irradiation to a solution of 1-alkyl-2-(arylazo)imidazole (L) in toluene shows transformation of trans to cis configuration about NN bond. The rate of photo-isomerisation and quantum yields are influenced by internal parameters like – nature of substituents, coordination to metal ions, steric and electronic effect, protonation etc. and the external factors like solvent (polarity, viscosity, dipole moment etc.), presence of innocent (to be chemically noninteracting) and noninnocent (chemically interacting) ions or molecules. In this work the influence of polycyclic aromatic hydrocarbons (PAH) such as – napththalene, anthracene, phenanthrene, pyrene – an innocent system, on the photochromic efficiency of L has been examined. The PAHs are π-donor and may form a π–π continuum with L which can significantly affect the motional (vibration, rotation) properties of the analyte and hence the photochromic activity. The rate and quantum yield of trans-to-cis isomerisation of L decrease with increasing [PAH] and also with increasing number of the fused phenyl ring(s) in PAH. Thus the photoisomerisation follows the rate sequence: no PAH > napththalene > anthracene–phenanthrene > pyrene. The reverse change, cis-to-trans is very slow upon light irradiation while appreciably fast in thermal process at dark. The activation energy (Ea) also decreases with [PAH] and number of fused phenyl rings.

Graphical abstractPolycyclic aromatic hydrocarbons (PAHs like naphthalene, anthracene, phenathrene and pyrene) are good transducers and may encapsulate 1-alkyl-2-(arylazo)imidazoles (L) and restrict their motion (rotation and vibration). The photoisomerisation of L, trans to cis configuration has been affected in the strenuous environment. The rate and quantum yields of trans-to-cis isomerisation of L decreases with increasing [PAH] and also with increasing number of fused phenyl ring(s) in PAH. Thus the photoisomerisation follows the rate sequence: no PAH > napththalene > anthracene > phenanthrene > pyrene. The reverse change, cis-to-trans has been carried out by thermal process at dark. The activation energy (Ea) also decreases with [PAH] and number of fused phenyl rings.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Photoisomerisation of azoimidazoles, trans-to-cis, has been carried out in presence of PAHs. ► The rate and quantum yields decrease with increasing [PAH] and number of fused phenyl ring(s). ► The rate sequence follows no-PAH > napththalene > anthracene > phenanthrene > pyrene. ► The cis-to-trans is considerably fast under thermal process at dark than light. ► The activation energy (Ea) decreases with [PAH] and number of fused phenyl rings.