Density functional theory study of linear and non-linear optical properties of dihydroazulene-vinylheptafulvene photoswitches Riffat Un Nisa, Nasir Shahzad, Khurshid Ayub ⇑ Department of Chemistry, COMSATS Institute of Information Technology, University Road, Tobe Camp, Abbottabad 22060, Pakistan article info Article history: Received 11 May 2016 Received in revised form 4 September 2016 Accepted 8 September 2016 Available online 10 September 2016 Keywords: Density functional theory Photoswitches Linear and non-linear optical properties Structure property relationship abstract The UV–vis absorption spectra and hyperpolarizability of a series of substituted dihydroazulenes (DHAs) and vinylheptafulvenes (VHFs) photochrome are studied at B3LYP/6-31G(d,p) level of theory. Electronic properties strongly depended on the nature and position of substituents on DHA-VHF photoswitch. Electron withdrawing groups (NO 2 , CHO) and halogens at position 8a of DHA cause red shift of the k max , whereas blue shift of k max was observed when these functional groups were present at positions 8, 7, 5 and 4. Mesomerically electron donating groups (NH 2 , OH, and SH) on DHA cause hypsochromic shift of the k max except amino and thiol at positions 4 and 6 (Red shift of k max ). The absorptions maximum of VHF, on the other hand, was red shifted for any functionalization at the seven membered ring of VHF. The maximum red shift of k max (VHF) was observed for substituents at positions 8a, followed by position 6 and 4. Electron donating substituents are very effective at decreasing the spectral overlap of DHA and VHF. In general, hyperpolarizabilities of VHFs are higher than their corresponding DHAs. For electron withdrawing groups, the largest b (8.80 esu) was obtained for DHA bearing NO 2 at position 6. Among electron donors, NH 2 substituted DHAs showed the higher b values compared to the molecules bearing OH and CH 3 . For VHF, the highest value of b (16.58 esu) was observed for molecule bearing bromine at position 6. Ó 2016 Published by Elsevier B.V. 1. Introduction Molecular photoswitches interconvert between two (or more) isomeric states by light of different wavelengths. Photoswitches find application in memory [1], electronic switches, magnetic switches [2–4], non-linear optics (NLO) [5,6], synthetic ion chan- nels [7,8], biological systems, liquid crystals [1,9–19], and even more complex devices including logic gates, half adder [20–28] encoder-decoder [29]. For practical application in memory and other related non-linear optical devices, a photoswitch must have minimum overlap of absorption spectra of isomeric species, but the polarizability difference between isomeric states should be high. Overlap of absorption spectra of isomeric species leads to photostationary state formation where substantial amounts of both isomers exist, and the process is not complete on either side. Some other requirements for practical applications include; high thermal stability, fatigue resistance, non-destructive readout and high quantum yield of photochemical conversions [7]. The most commonly studied classes of photoswitches are dithienylethene [7] and fulgides [34], mainly because of the associated thermal stabilities. A less explored, but potentially important class of photochromic compounds is dihydroazulene-vi nylheptafulvene (Fig. 1). The colorless dihydroazulene (DHA) is thermodynamically more stable than the photogenerated colored vinylheptafulvene (VHF) [30–38]. The s-cis conformer of vinylhep- tafulvene, generated from photochemical process, is in equilibrium with relatively more stable s-trans conformer. The vinylheptaful- vene returns back to dihydroazulene [31,33,34] either photochem- ically, or thermally (T-type photoswitch) [39]. Besides thermal instability of VHF form, synthetic challenges for this class of photo- switch provide another obstacle for application in practical devices. The focus of research in DHA-VHF photoswitch had been two fold; (a) design of multimode photoswitches by fusing DHA-VHF photoswitch with dithienylethene [40], and other photochromic molecules (b) exploration of variation in physical properties through structural modifications [31–34,37,38]. The latter has been limited to functionalization at position 2 and 3 of DHA (five membered ring, Fig. 2), mainly because of the associated synthetic challenges for functionalization at other position. With the advent of modern synthetic strategies, it is now possible to functionalize [41–43] the seven membered ring of DHA. http://dx.doi.org/10.1016/j.comptc.2016.09.013 2210-271X/Ó 2016 Published by Elsevier B.V. ⇑ Corresponding author. E-mail address: khurshid@ciit.net.pk (K. Ayub). Computational and Theoretical Chemistry 1095 (2016) 1–8 Contents lists available at ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc