Perylene derivative films: Emission from higher singlet excited state
Introduction
According to the rule proposed by M. Kasha, the fluorescence of a polyatomic molecule occurs only from the first singlet electronic excited state (S1) [1]. This rule can be denominated as Kasha's fluorescence (KF) and describes the behavior of many complex molecules [2,3]. However, evidences have shown that the fluorescence can also occur from singlet electronic excited states higher than S1 level, and this process are expressed as anti-Kasha's fluorescence (AKF) [2]. J. B. Birks predicted the AKF process in 1954 and has reviewed this behavior for different hydrocarbons [3]. Complementary, P. A. Geoldof et al. also verified AKF for vapors of aromatic hydrocarbons and, according to the authors, transitions were observed from a higher singlet electronic excited state (S2) to the ground state [4]. These transitions (S2→S0) are relatively hard to be confirmed because the non-radiative transition S2→S1, named internal conversion (IC), is very fast (typically 10−14 – 10−11 s) and highly efficient, which lead to fluorescence from the lowest singlet electronic excited state (S1) [[5], [6], [7]]. According to A. Nakajima et al. [6], the fluorescence emission from S2 is possible for shorter lifetime, with the process happening before IC. Therefore, although the emission of S2 is less intense due to the fast IC [ [[8], [9], [10]], there is a significant group of molecules presenting this type of emission, such as pyrene [11], 3,4-benzpyrene [4,12], 1,2-benzanthracene [12], 1,12-benzperylene [13] and perylene [6], which is the focus of our study.
Perylene derivatives are polycyclic aromatic organic molecules that have received considerable attention due to their properties and multiple applications, such as the use of perylene derivatives thin films for the development of optic and electronic devices [14]. An important feature of perylene derivative thin films is their supramolecular arrangement, which is directly related to the molecular structure, leading to modifications of the thin film optical and electrical properties [[14], [15], [16], [17]]. In addition, one may take into account the thermal stability of the supramolecular arrangement of the thin film, since changes in the molecular organization after thermal treatment has been reported [[18], [19], [20]]. The latter may be a change in the preferential orientation of a molecule in the film [18], leading them either to disorder [20] or reach a new preferential organization [19].
The correlation between thermal treatment and supramolecular arrangement was studied for bis(butylimido) (BuPTCD) and bis(phenetylimido) (PhPTCD) perylenes by vacuum thermal evaporation (PVD, physical vapor deposition) [21,22]. The aim of this study is to explore possible changes on optical properties (internal conversion) of both perylene derivatives forming PVD films after heating them up to 200 °C for 2 h and cooling down to room temperature. BuPTCD and PhPTCD have the same chromophore as the main molecular structure, however, BuPTCD has two alkyl side chains with four carbons each one in its structure, while PhPTCD has two alkyl side chains with two carbons linked with a benzene ring (insets in Fig. 1). Despite the difference between the chemical structures of BuPTCD and PhPTCD, their optical properties (absorption and emission) are similar when both are in the monomeric form [14]. However, such difference in their molecular structure was responsible for the growth of PVD films with distinct supramolecular arrangements (molecular organization, crystallinity, and molecular aggregation) [14], which lead to changes of the photoluminescent properties of both BuPTCD and PhPTCD PVD films.
Section snippets
Experimental
The compounds BuPTCD (MW 502.56 g/mol) [23] and PhPTCD [24] (MW 602.15 g/mol) were synthetized by Dr. J. Duff, from Xerox Resource Center, Canada, and provided to us by Prof. Ricardo Aroca, from University of Windsor, Canada. Dr. Jim Duff has been performed the synthesis and purification (>99.9%) of a series of perylene derivatives [25,26], including BuPTCD and PhPTCD, which have been applied in experiments using the surface-enhanced Raman scattering (SERS) technique toward single molecule
Photoluminescence spectra of BuPTCD and PhPTCD in solution
The UV–Vis absorption spectra in Fig. 1 were acquired from solutions of BuPTCD and PhPTCD in dichloremethane:TFA (90:10 v/v), at 10−6 mol/L. It is observed two bands, which are attributed to the electronic transition S0→S1 (visible region, with maxima at 623, 486, 456 and 427 nm) [42], as well as bands corresponding to the transition S0→Sn (ultraviolet region, with maxima at 261 and 240 nm), where Sn could be attributed to the higher excited state [42].
From the excitation at 261 nm, it was
Concluding remarks
Photoluminescence spectra of BuPTCD and PhPTCD solubilized in dichloremethane:TFA (90:10 v/v) have shown that both perylene derivatives present an emission band that may be attributed to the radiative decay S2→S0. The emission relative intensity (RI) between S1→S0 and S2→S0 is around 20 for both BuPTCD and PhPTCD solutions, suggesting their distinct lateral groups linked to the chromophore do not affect the internal conversion process. On the other hand, the relative intensity (RI = IS1→S0/IS2→S
CRediT authorship contribution statement
José Diego Fernandes: Conceptualization, Data curation, Formal analysis, Methodology, Writing - original draft, Writing - review & editing. Wallance Moreira Pazin: Conceptualization, Formal analysis, Methodology, Writing - original draft, Writing - review & editing. Wagner Costa Macedo: Conceptualization, Data curation, Formal analysis. Silvio Rainho Teixeira: Data curation, Resources. Sergio Antonio Marques Lima: Conceptualization, Formal analysis, Resources, Writing - original draft. Augusto
Declaration of competing interest
The authors declare no competing financial interests.
Acknowledgments
CAPES, CNPq (grants 448310/2014–7 and 420449/2018–3), INEO and FAPESP (processes 2013/14262–7 and 2016/09633–4) for the financial support. Dr. Ricardo Flavio Aroca, Professor Emeritus, University of Windsor, Canada, for fruitful discussions. This research was also supported by resources supplied by the Center for Scientific Computing (NCC/Grid-UNESP) of the São Paulo State University (UNESP).
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