Figure 1

Molecular and crystal structure of picene and pentacene. We also indicate the shorter $x$ and longer $y$ molecular axes and their orientation with respect to the lattice vectors $a$, $b$, and $c$.Reuse & Permissions

Figure 2

Absorption spectra of pentacene molecular crystal for different polarization directions: $a^{*}$, $b^{*}$, and $c^{*}$. The spectra are calculated with the following approximations: RPA without crystal local fields (RPA-NLFE) (black dashed lines), RPA with crystal local fields (RPA-LFE) (solid black lines), and full solution of the BSE (BSE) (red lines).Reuse & Permissions

Figure 3

Same as Fig. 2 for picene.Reuse & Permissions

Figure 4

Bound excitons in pentacene (a) and picene (b) (the band gap in picene is 4.08 eV and in pentacene is 2.02 eV). The arrows indicate the Davydov splitting (DS). Note that the various structures in the spectra all have a pure electronic origin and do not derive from a coupling with vibrational excitations.Reuse & Permissions

Figure 5

Electronic charge distribution $|{\Psi }^{\lambda }({\mathbf{r}}_h,{\mathbf{r}}_e){|}^2$ for a fixed position ${\mathbf{r}}_h$ of the hole (blue ball) for the lowest-energy singlet excitons in picene [panel (a)] and pentacene [panel (b)]. While in picene it is a Frenkel exciton, in pentacene it is a charge-transfer exciton. In ionic crystals for example, charge-transfer excitons arise from excitation from valence states localized around the anion to conduction states localized around the cation. On the contrary, in the present case note that valence and conduction wave functions are localized on both inequivalent molecules in the unit cell. Therefore, an exchange of the position of the hole between the two molecules would correspondingly exchange also the localization of the electronic charge distribution.Reuse & Permissions

Figure 6

Same as Fig. 5 for the lowest-energy triplet excitons in picene [panel (a)] and pentacene [panel (b)]. It is a Frenkel exciton for both systems.Reuse & Permissions

Figure 7

Energy-level diagram for the singlet channel showing the interaction between Frenkel (FR) and charge-transfer (CT) state when the lowest excitation is a Frenkel (a) or a charge-transfer (b) exciton. For simplicity, we assume that the hopping integrals for valence and conduction bands are the same ($t^v=t^c$). Thus antisymmetric states are not modified by the perturbation being in this case $\Delta E_{ex}^{-}\propto {(t^c-t^v)}^2=0$. FR${}^{\pm }$ and CT${}^{\pm }$ are the Frenkel and charge-transfer states in the absence of the hopping terms, while (FR+CT)${}^{+}$ and (CT+FR)${}^{+}$ are the mixture of Frenkel and charge-transfer states arising from the action of the hopping term on the FR${}^{+}$ and CT${}^{+}$ excitons, respectively. DS and DS${}^{\prime }$ are the Davydov splittings with and without the hopping term. Note that DS is zero in a pure charge-transfer state [panel (b)]. When the lowest excitation is a Frenkel exciton [panel (a)] mixing with CT exciton of the same symmetry can in principle (for large values of the hopping term) cause a shift of the symmetric state (FR+CT)${}^{+}$ below the antisymmetric state FR${}^{-}$, as shown in panel (a).Reuse & Permissions