Igor Bondarev1 Adrian Popescu1 2

1, North Carolina Central University, Durham, North Carolina, United States
2, University of South Florida, Tampa, Florida, United States

We develop a theory for the intra-intermolecular exciton intermixing and polarization dynamics in periodic 1D chains of planar organic molecules with two isolated low-lying Frenkel exciton states [1,2], typical of transition metal phthalocyanines [3]. We formulate the Hamiltonian and use the exact Bogoliubov diagonalization procedure to derive the eigen energy spectrum for the two lowest intramolecular Frenkel excitons coupled to the intermolecular charge transfer (CT) exciton state [1]. By comparing our theoretical spectrum with available experimental absorption spectra of crystalline copper phthalocyanine (CuPc) thin films, we obtain the parameters of the Frenkel-CT exciton intermixing for this organic semiconductor material. The two Frenkel exciton states here are spaced apart by 0.26 eV, and the charge transfer exciton state is 50 meV above the lowest Frenkel exciton. Both Frenkel excitons are strongly mixed with the CT exciton, showing the coupling constant 0.17 eV in agreement with earlier electron transport experiments [4]. The third-order nonlinear polarization response function we derive [2] shows dynamical reorientation of the exciton transition dipole polarization (initially excited in the molecular plane) towards the axis of the 1D molecular chain. Such a dynamical reorientation pinpoints the direction of the charge separation process, whereby an originally excited intramolecular (Frenkel) electron−hole configuration gives one of its charges over to a neighboring molecule of the 1D molecular chain, to form a stationary eigen state of the 1D periodic molecular crystal lattice. Our results can be used for the proper interpretation of the physical properties of crystalline transition metal phthalocyanines – next generation organic semiconductors for advanced optoelectronics.

Acknowledgements: DOE-DE-SC0007117 (I.B.), NSF-ECCS-1306871 (A.P.)

[1] I.V.Bondarev, A.Popescu, R.A.Younts, B.Hoffman, T.McAfee, D.B.Dougherty, K.Gundogdu, and H.W.Ade, Appl. Phys. Lett. 109, 213302 (2016).
[2] A.Popescu, R.A.Younts, B.Hoffman, T.McAfee, D.B.Dougherty, H.W. Ade, K.Gundogdu, and I.V.Bondarev, Nano Lett. 17, 6056 (2017).
[3] L.Edwards and M.Gouterman, J. Mol. Spectr. 33, 292 (1970).
[4] I.G.Hill, A.Kahn, Z.G.Soos, and R.A.Pascal, Jr., Chem. Phys. Lett. 327, 181 (2000).