TY - JOUR
T1 - A theoretical study of the low-lying excited states and the photophysics of dimethoxy curcumin in cyclohexane and acetonitrile
AU - Ramos, Tárcius N.
AU - Canuto, Sylvio
N1 - Funding Information:
We thank Dr. Y. Orozco-Gonzales and Dr. Marcelo H. Cardenuto for discussions. This work is partially supported by grant 2013/01,871-5 from São Paulo Research Foundation (FAPESP), CNPq, CAPES and INCT-FCx.
Publisher Copyright:
© 2017, Springer-Verlag GmbH Germany.
PY - 2017/7/1
Y1 - 2017/7/1
N2 - Two important aspects of the photophysics of dimethoxy curcumin in solvents are theoretically analysed. First, the large Stokes shift in cyclohexane and in acetonitrile is calculated using time-dependent density functional theory with large basis set. Second, a possible explanation is given for the considerable decrease of the nonradiative decay rate in changing from the nonpolar cyclohexane to polar acetonitrile. The solvent is treated using the polarizable continuum model. We analyse different isomers and obtain the absorption transition energies. Similarly, after obtaining the relaxed geometry of the excited state, the emission transition is obtained. In both cases, a red shift of the singlet transition is obtained with increasing solvent polarity. In contrast, the low-lying triplet states are essentially solvent independent. The calculated solvent shift in the emission is in very good agreement with experiment. The Stokes shifts are also in fair agreement with experiment when long-range or dispersion-corrected functionals are used. There are three calculated triplet states lying below the S1 state. As they are π → π* states, they cannot participate in the intersystem crossing by the El-Sayed rule. However, a fourth triplet state of n → π* nature is found only slightly above the Franck–Condon position of the S1 state, and the S1 → T4 separation increases with the solvent polarity, this being a possible explanation for the decrease of the nonradiative transition.
AB - Two important aspects of the photophysics of dimethoxy curcumin in solvents are theoretically analysed. First, the large Stokes shift in cyclohexane and in acetonitrile is calculated using time-dependent density functional theory with large basis set. Second, a possible explanation is given for the considerable decrease of the nonradiative decay rate in changing from the nonpolar cyclohexane to polar acetonitrile. The solvent is treated using the polarizable continuum model. We analyse different isomers and obtain the absorption transition energies. Similarly, after obtaining the relaxed geometry of the excited state, the emission transition is obtained. In both cases, a red shift of the singlet transition is obtained with increasing solvent polarity. In contrast, the low-lying triplet states are essentially solvent independent. The calculated solvent shift in the emission is in very good agreement with experiment. The Stokes shifts are also in fair agreement with experiment when long-range or dispersion-corrected functionals are used. There are three calculated triplet states lying below the S1 state. As they are π → π* states, they cannot participate in the intersystem crossing by the El-Sayed rule. However, a fourth triplet state of n → π* nature is found only slightly above the Franck–Condon position of the S1 state, and the S1 → T4 separation increases with the solvent polarity, this being a possible explanation for the decrease of the nonradiative transition.
KW - Dimethoxy curcumin
KW - Excited state
KW - Photophysics
KW - Solvent effects
KW - Stokes shift
UR - http://www.scopus.com/inward/record.url?scp=85021150327&partnerID=8YFLogxK
U2 - 10.1007/s00214-017-2108-9
DO - 10.1007/s00214-017-2108-9
M3 - Article
AN - SCOPUS:85021150327
VL - 136
JO - Theoretical Chemistry Accounts: Theory, Computation, and Modeling
JF - Theoretical Chemistry Accounts: Theory, Computation, and Modeling
SN - 1432-881X
IS - 7
M1 - 78
ER -