Asymmetry between absorption and photoluminescence line shapes of TPD: Spectroscopic fingerprint of the twisted biphenyl core

For TPD, absorption, photoluminescence, and resonant Raman spectra are interpreted with model calculations based on density functional theory (DFT) and time-dependent DFT. The peculiarities of the potential energy surfaces related to a twisting around the cen-tral bond in the biphenyl core of TPD allow a quantitative interpretation of the asymmetry between the line shapes observed in absorption and emission. During the last decade, N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD, see Fig. 1) was mainly attracting attention due to its transport properties, so that it became a prototypical hole-conducting compound used in light emitting diodes. Re-cent applications of TPD in UV detectors have revealed that the photo-response is deter-mined by the absorption coefficient of the material, even in blends with Alq3. Moreover, due to the very large Stokes shift of about 0.5 eV, TPD is transparent to its photo-luminescence (PL), so that it has a promising potential for laser applications, showing sti-mulated emission even in the form of neat films. This finding is quite surprising, as most molecules with this property need to be diluted in a host matrix or in a solvent in order to show laser activity.

Absorption and PL line shapes of molecules are directly related to the elongations of internal vibrations in their relaxed excited geometry. These elongations can be quantified with resonant Raman spectroscopy, and different variants of density functional theory (DFT) can be used for a calculation of the deformation in the relaxed excited state [1]. A projection of the vibrational eigenvectors onto this deformation pattern reveals the Huang-Rhys factor of each vibration, defining in turn its Raman activity and the vibronic progressions observed in absorption and photoluminescence.

Fig. 1 shows the excited state deformation obtained with time-dependent DFT. The deformation of the various phenyl rings results in the elongation of high frequency internal modes, and the peculiarities of the potential energy surfaces along the central dihedral angle reveal a substantial Stokes shift. The elongation of high-frequency internal modes can be summarized in terms of an effective vibration at 158 meV with an effective Huang-Rhys factor of 0.87. The Boltz-mann distribution in the ground and excited potential energy surfaces (PES) can be transformed into a distribution over transition energies, defining in turn the line shape of each subband of the internal effective mode. The shape of the two PES along the central dihedral angle in the biphenyl core results directly in a large asymmetry between the densities of states and broadenings involved in absorption and PL. This is also reflected in different contributions to the respective reorganization energies. Further low-frequency modes involving torsional motion around other bonds connecting the various phenyl rings contribute to the large Stokes shift and to the broadening of the vibronic subbands of the effective mode.

In conclusion, the complementary information contained in absorption, photolumines-cence, and resonant Raman spectroscopy can be rationalized by a comprehensive analysis of the photophysics with time-dependent DFT calculations. It was found that the twisted shape of the central biphenyl group allows a quantitative understanding of the different linewidths observed in absorption and PL together with an asymmetry between the reor-ganization energies on the two potential energy surfaces involved. The torsional modes at low frequencies contribute substantially to the different broadenings of absorption and PL spectra. As a result, the vibronic subbands of high-frequency internal modes remain visible in PL, but in absorption, they are washed out by the larger broadening arising from the flat ground state potential along the twisting around the central bond. Our detailed model cal-culations have demonstrated that the photophysics of TPD can be understood from the properties of the molecule itself. The influence of intermolecular interactions is restricted to a small dependence of the Stokes shift on the solvent or matrix material, but the contri-bution of the surroundings to the broadening remains much smaller than the influence of the torsional modes of TPD at very low frequency.

[1] R. Scholz, L. Gisslén, C. Himcinschi, I. Vragovic, E. M. Calzado, E. Louis, E. S. Maroto, and M. A. Díaz-García, J. Phys. Chem. A 113, 315 (2009).