Photophysics of Soret-excited free base tetraphenylporphyrin and its zinc analog in solution Mihir Ghosh a , Aruna K. Mora b , Sukhendu Nath b , Asit Kumar Chandra c , Alakananda Hajra d , Subrata Sinha a,⇑ a Integrated Science Education and Research Centre, Siksha-Bhavana, Visva-Bharati, Santiniketan 731 235, India b Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India c Department of Chemistry, North Eastern Hill University, Shillong 793 022, India d Department of Chemistry, Siksha-Bhavana, Visva-Bharati, Santiniketan 731 235, India highlights  Spectral characteristics of porphyrins are affected by solvent polarizability.  Ground state dipole moments of porphyrins are found to be very low.  Solvatochromic Stokes’ shift method is used to find excited state dipole moment.  The first excited singlet state of porphyrins possesses moderate dipole moments.  Other properties of porphyrins are nearly insensitive to solvent polarity. graphical abstract 0.0 0.1 0.2 0.3 0 30 60 90 120 NH HN N N Stokes' shift Orientation polarizability of solvent μ E (S 1 ) = 3.65 D article info Article history: Received 20 June 2013 Accepted 22 July 2013 Available online 1 August 2013 Keywords: Porphyrin Solvent polarity Stokes’ shift Lippert–Mataga equation Dipole moment abstract Photophysical properties of free base tetraphenylporphyrin and its zinc analog are investigated in detail in solvents of varying polarity by using steady state and time-resolved techniques. Both the porphyrins are excited at the Soret band to have better signal-to-noise ratio. Also, the fluorescence emission measurements are carried out by using dilute solutions (10 7 mol/L) of the fluorophores in order to minimize the self-quenching effect. It is observed that the steady state absorption and emission charac- teristics of the porphyrin molecules are mainly affected by polarizability (via refractive index) rather than polarity (via dielectric constant) of the moderate to highly polar solvents. As the molecules are highly symmetric in the ground state, the associated dipole moments are found to be very low from quantum chemical calculations performed by density functional theory method by using Gaussian 03 package. The dipole moments associated with the first excited singlet state of the porphyrins are computed by applying solvatochromic Stokes’ shift method. To the best of our knowledge, this is the first attempt to calculate the excited state dipole moments of the porphyrins used in the present investigations. Also, fluorescence quantum yield, fluorescence lifetime of the first excited singlet state, radiative and non-radi- ative rate constants of the porphyrins are reported in solvents of varying polarity. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. Introduction Various porphyrin compounds play important roles in bio- physics [1], solar energy harvesting and artificial photosynthesis [2–12], industrial processes as catalysts [13–20], photodynamic 1386-1425/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.07.047 ⇑ Corresponding author. Tel./fax: +91 3463 261029. E-mail address: subratasinha67@rediffmail.com (S. Sinha). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 116 (2013) 466–472 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa