Asymmetry between Absorption and Photoluminescence Line Shapes of TPD: Spectroscopic Fingerprint of the Twisted Biphenyl Core

Abstract

We analyze absorption, photoluminescence (PL), and resonant Raman spectra of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD), with the aim of providing a microscopic interpretation of a significant Stokes shift of about 0.5 eV that makes this material suitable for stimulated emission. The optical spectra were measured for TPD dissolved in toluene and chloroform, as well as for polystyrene films doped with varying amounts of TPD. In addition, we measured preresonant and resonant Raman spectra, giving direct access to the vibrational modes elongated in the relaxed excited geometry of the molecule. The experimental data are interpreted with calculations of the molecular geometry in the electronic ground state and the optically excited state using density functional theory. Several strongly elongated high-frequency modes within the carbon rings results in a vibronic progression with a calculated spacing of 158 meV, corroborated by the observation of vibrational sidebands in the PL spectra. The peculiarities of the potential energy surfaces related to a twisting around the central bond in the biphenyl core of TPD allow to quantify the asymmetry between the line shapes observed in absorption and emission.

Amplified Spontaneous Emission Properties of Semiconducting Organic Materials

“…On the other hand, in other materials, such as TPD, both absorption and PL spectra remain unchanged with concentration. Recent calculations based on density functional theory, together with resonant Raman spectroscopy, have been used to propose a model to explain this behavior [44]. Results have shown that the influence of molecular interactions is much smaller than that of the torsional modes of TPD at very low frequency.…”

Theoretical insight into the deep-blue amplified spontaneous emission of new organic semiconductor molecules

“…The inner reorganization energies k play a crucial role in the interpretation of the Stokes shift [13,14]. The total reorganization energies of the three compounds are 0.566 eV, 0.714 eV and 0.861 eV respectively (Table 4).…”

“…The elongation of high-frequency modes contribute to the higher first vibronic peak in PL spectra and the elongation of lowfrequency modes results in a broadening of the vibronic subbands defined by the high-frequency modes [13][14][15][16]. Relative small number of low-frequency modes would bring out a distinct energy level system between two energy levels which root in the main peak and the shoulder peak.…”

“…It was considered that the factors of stokes shift, the radiative decay rate, the fluorescence lifetime and the fluorescence quantum yield are correlated with the stimulated emission phenomenon [10][11][12]. There are some density functional theoretical studies about the geometric structure and energetic of TPD [13,14], but as for the lasing mechanism of small molecules, up to now, few theoretical explanations were proposed in the molecuhttp://dx.doi.org/10.1016/j.orgel.2014.08.024 1566-1199/Ó 2014 Elsevier B.V. All rights reserved. lar structure and energy level point of view.…”

Optical spectra obtained from amorphous films of rubrene: Evidence for predominance of twisted isomer

“…We use two kinds of relaxed excited geometries: First, the result of the TD-DFT geometry optimization, and second, a modified excited geometry where changes of the twisting angle of each phenyl group around the bond connecting it to the tetracene core are eliminated. From similar investigations on other molecules containing phenyl groups we expect that this procedure minimizes artifacts arising from finite angle effects related to the relatively large changes of these angles in the relaxed excited geometry [25].…”