Structural and optical properties of Purpurin for dye-sensitized solar cells S. Ranjitha a,⇑ , G. Rajarajan b , T.S. Gnanendra c , P.M. Anbarasan d , V. Aroulmoji c a Velalar College of Engineering and Technology, Thindal, Erode, Tamilnadu, India b Selvam College of Technology, Namakkal, Tamil Nadu, India c Center for Research and Development, Mahendra Educational Institutions, Mahendhirapuri, Mallasamudram, Namakkal District, Tamil Nadu, India d Department of Physics, Periyar University, Salem, Tamil Nadu, India highlights  The experimental and theoretical analysis shows that Purpurin act as a good photo sensitizer.  In the absorption band n to p/ transitions of the electrons are observed.  Adding a strong p-conjugation bridge to the molecule is a promising approach.  To enhance photovoltaic properties for Purpurin. graphical abstract article info Article history: Received 27 November 2014 Received in revised form 20 March 2015 Accepted 16 April 2015 Available online 24 April 2015 Keywords: Electronic structure Absorption spectrum Dye sensitizer Density functional theory Vibrational spectra Purpurin abstract In this work, we reported a combined experimental and theoretical study on molecular structure, vibra- tional spectra and Homo–Lumo analysis of Purpurin and TiO 2 /Purpurin. The geometries, electronic struc- tures, molecular orbital analysis of natural dye sensitizer Purpurin were studied based on density functional theory (DFT) using the hybrid functional B3LYP. Fourier transform infrared (FT-IR) and FT- Raman spectra have been recorded and extensive spectroscopic investigations have been carried out on Purpurin. The optimized geometries, wave number and intensity of the vibrational bands of Purpurin have been calculated using density functional level of theory (DFT/B3LYP) employing 6-311G (d, p) basis set. Based on the comparison between calculated and experimental results, assignments of the fundamental vibrational modes are examined. Features of the electronic absorption spectrum in the visible and near-UV regions were assigned based on TD-DFT calculations. The calculated results sug- gest that the three excited states with the lowest excited energies in 1,2,4, trihydroxy 9-10 anthraqui- none was due to photo-induced electron transfer processes. Frontier molecular orbitals (FMO), LUMO, HOMO, and energy gap, of these dyes have been analyzed to show their effect on the process of electron injection and dye regeneration. Interaction between HOMO and LUMO of Purpurin are investigated to understand the recombination process and charge transfer process involving these dyes. We also per- formed analysis of I–V characteristics to investigate the role of charge transfer and the stability of the dye molecule. Ó 2015 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2015.04.046 1386-1425/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +91 9842903086. E-mail address: ranjilotus31@gmail.com (S. Ranjitha). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 997–1008 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa