Transducer influence of polycyclic aromatic hydrocarbons (PAHs) on photoisomerisation of 1-alkyl-2-(arylazo)imidazoles Pallab Gayen 1 , Chittaranjan Sinha ⇑ Department of Chemistry, Jadavpur University, Kolkata 700 032, India highlights " 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 (E a ) decreases with [PAH] and number of fused phenyl rings. graphical abstract Polycyclic 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 fol- lows 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 (E a ) also decreases with [PAH] and number of fused phenyl rings. article info Article history: Received 25 September 2012 Received in revised form 6 November 2012 Accepted 8 November 2012 Available online 24 November 2012 Keywords: Arylazoimidazole Photochromism PAH Activation energy Supramolecular interaction abstract The UV light irradiation to a solution of 1-alkyl-2-(arylazo)imidazole (L) in toluene shows transformation of trans to cis configuration about AN@NA 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 p-donor and may form a p–p continuum with L which can signif- icantly 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, 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.11.036 ⇑ Corresponding author. Fax: +91 033 2413 7121. E-mail address: c_r_sinha@yahoo.com (C. Sinha). 1 Present address: Raja Peary Mohan College, Uttarpara, Hooghly 712 258, India. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 477–485 Contents lists available at SciVerse ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa