The present document discusses a thorough investigation of N,N'-dimethyl perylene tetracarboxylic acid diimide (Me-PTCDI), as a prototype for the study of organic thin films. These films exhibit semiconducting properties, with direct bandgap transitions, large visible extinction coefficients, photostability and low cost of fabrication [1-8]. The latter properties make these semiconductors potentially useful for a number of photoelectronic applications.
Preliminary optical measurements of the aforementioned materials in solution showed that the PTCDI derivatives' absorption and photoluminescence (PL) spectra are determined by the individual molecules [9]. On the contrary, the optical spectra of the PTCDI bulk or thin films show strong dependence on the substituent groups, relative orientation of the molecules and a crystalline order of the sample [10]. This indicates that intermolecular interactions and crystallinity play a major role in determining the electronic and optical properties of PTCDI solid samples.
The described investigation studied the correlation between the alignment of the molecular units within the films, and their magneto-optical properties. The above correlation was characterized by utilizing absorption, PL, and optically detected magnetic resonance (ODMR) spectroscopy.
Me-PTCDI thin films were deposited on a glass substrate by physical vapor technique, according to the procedure given in reference [3]. The typical thickness of the films were about 300 nm. Representative absorption and PL spectra of the deposited Me-PTCDI molecules, recorded at 1.4K, are shown in Figure 1 (solid lines). The absorption spectrum consists of two progressions, starting at 2.12 eV and 2.52 eV, respectively. Each subgroup is accompanied by a vibration side- band, separated by 0.14-0.15 eV from its corresponding origin. The latter vibration energy is in close proximity to that of the benzene ring stretching mode [11]. It should be noted that absorption spectrum of the corresponding solution shows only one progression, centered around 2.36 eV [9]. The splitting of the absorption spectrum, in thin films, suggests the existence of intermolecular interactions, and creation of aggregates within the films [10,12]. The latter phenomenon can be represented for the smallest aggregate, the dimer.
Photo-excitation of organic thin films usually results in the formation of Frenkel exciton. According to Kasha et.al., the dimer singlet exciton state splits into two states and the splitting energy depends on the relative orientation of the molecular units, according to the following equation [13,14]:

Equation (1)

where M is the transition moment for the singlet-singlet absorption in the monomer, r is the line connecting the centers of the molecular units, α is the angle between the long axes of the monomers, while θ is the angle made between long axes and the line r. When α unequal 0 and θ unequal 0, the molecular units are tilted with respect to each other. When α=0 and θ=90ø, the molecules are completely parallel to each other, while when α=0 and θ=0ø, they are in a head-to-tail configuration. Kasha et.al. [13] have showed that the tilt configuration creates optically allowed split states. However, the parallel and head-to-tail configurations create upper(lower) allowed state and lower (upper) forbidden state, respectively.
The PL spectrum consists of three bands centered at 1.89 eV (HE), 1.75 eV (ME) and 1.61 eV (LE). The energy interval between adjacent emission bands is 0.14 eV. This spectrum is red shifted from the corresponding one in solutions, further supporting the existence of dimers or larger aggregates. The PL line shape simulation (shown by the dashed lines in figure 1) suggests that the spectrum is comprised of two progressions, associated with emission from the split exciton state with head-to-tail configuration.

Fig.1 Absorption and photoluminescence spectra of Me-PTCDI thin films, recorded at 1.4K.

The ODMR spectra of Me-PTCDI thin films were observed by measuring the change in luminescence intensity of a PL band, due to a magnetic resonance event at the excited state. A typical ODMR spectrum of the above material is shown in figure 2.
Fig.2 ODMR spectrum of Me-PTCDI, recorded at 1.4K.

The high magnetic field region, in the latter spectrum, consists of a sharp resonance, typical to an unpaired electron such as free radical or polaron. The side shoulders in the high field region resemble a triplet exciton resonance, associated with the ΔM_s = +/- 1 transitions. Moreover, the low field region corresponds to the triplet exciton ΔM_s= +/- 2 transition.
The observation of a triplet resonance with unresolved hyperfine structure suggest that the spin system of Me-PTCDI is mainly influenced by the Zeeman (geff) and dipole-dipole interaction (D, E). Thus, this spin system can be described by the following spin Hamiltonian:
Equation (2)

when S_i correspond to the spin component and B corresponds to the strength of the external magnetic field components. The dipole-dipole interaction of the Me-PTCDI assumed to be non-axial symmetric (E uneqaul 0), with the z principal axis perpendicular to the molecular plan. Simulation of the triplet exciton line shape, using the above Hamiltonian, excluded the possibility of random orientation or perfect alignment of the dimers within the films and with respect to the substrate. However, this simulation suggested the possibility of partial orientation. The dashed line in figure 2 represent a theoretical triplet resonance of the ΔM_s = +/-1 transition, where the dimers are distributed around an orientation parallel to the substrate, with a tilt angle of +/-36ø.
The zero field parameter, D, (extracted from the above simulation) supplies rough estimation of the triplet exciton radius, based on the following relation:
Equation (3)

Considering g_eff = 2.0, the calculated R is 3.0 Angtsrom. This radius is slightly smaller than the perylene molecule. The latter may indicates that the exciton is localized within a monomer.
It is well known that split excitons in dimers show enhancement of the intersystem crossing into the triplet state. The so-called forbidden split state has a longer lifetime, providing the means for intersystem crossing into the triplet state. It should be noted that the ODMR spectrum showed enhancement of the singlet exciton emission intensity, due to magnetic resonance event at the excited state among the triplet sub-levels. Such an enhancement can take place when the excited singlet and triplet channels are coupled via the ground state.

Acknowledgment
The authors thank Prof. S. Speiser and Prof. T. Schaafsma for the stimulating discussions. This work partly supported by the BSF construct no.94256. A. K. expresses his gratitude to the Ministry of Science for the student fellowship.

References

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