Abstract
Polycyclic aromatic hydrocarbon compound, 1,2 : 3,4-dibenzanthracene, is spectroscopically analyzed in ethanol. Ultraviolet absorption spectra were taken and fluorescence measurements were performed. From absorption and emission spectra, Stokes' lines were clearly discernible and these shifts were recorded. Being a carcinogenic compound, the detection of 1,2 : 3,4-dibenzanthracene presence in the environment as a pollutant with adverse genotoxic effects is vital.
1. Introduction
Absorption and emission spectroscopy of dibenzanthracene (DBA) isomers such as 1,2,5,6-DBA, 1,2,7,8-DBA have been analyzed in polymer matrices [1, 2]. Amorphous solids such as polymethlymethracrylate remain relatively rigid even at high temperatures and DBA molecules cannot move freely in such solids, thus making them ideal for time resolved absorption and fluorescence spectra studies of DBA isomers. Singlet and triplet electronic absorption and prompt fluorescence can be readily observed. However, DBA in solvents such as ethanol is much more complicated for absorption and fluorescence spectroscopy. We aim to fill this gap by taking ultraviolet (UV) absorption spectroscopy, fluorescence emission spectroscopy, and Fourier Transform infra-red spectroscopy of 1,2 : 3,4-dibenzanthracene in ethanol. We also show Stokes’ shifts in the fluorescence spectroscopy by measuring the quantum efficiency of the 1,2 : 3,4-dibenzanthracene and ethanol solution. The molecular structure of 1,2 : 3,4-dibenzanthracene is shown in Figure 1.
Absorption of light by the molecule excites the molecule to one of the upper electronic singlet states from ground state. Most of the molecules reside in the lowest vibrational state at room temperature and absorption of light initiates upward transitions from this level to higher levels. When the molecule reaches the first excited singlet state, the molecule can return to any one of the vibrational levels of the ground state by emitting fluorescence. Part of the excited molecules can also return to ground state following other mechanisms such as through quenching processes, photochemical changes, or conversion to triplet state. Hence, fluorescence efficiency becomes less than one [3–12]. In particular, fluorescence spectroscopy of 1,2 : 3,4-dibenzanthracene reveals clear Stokes’ shifts when the molecule is excited in UV.
2. Materials and Methods
1,2 : 3,4-dibenzanthracene, was of AccuStandard reagent grade, used as received without any further purification and was mixed with ethanol at atmospheric pressure. The concentration of dibenzanthracene was selected as 2.0 × 10−4 M. Two Princeton Instruments Acton Advanced SP2300 model monochromators were used in all optical measurements. These devices have a 600 g/mm grating and a focal length of 300 mm. The measurement setup is illustrated in Figure 2. A 500 W Xenon bulb was mounted in front of the entrance slit of monochromator 1 to obtain the maximum amplitude and the entrance and exit slits were adjusted at 700 μm openings. With the help of a computer controlled software, monochromatic beams of 300–380 nm wavelength with a 10 nm step were emitted from the exit of monochromator 1. The emitted beam was focused on the sample using a quartz lens to excite the sample. The excited sample was emitting a beam which was perpendicular to the excitation beam. This beam was further focused on the slit of monochromator 2 with the help of sequential lenses where the slit was set again at 700 μm. The alignment was carried out using microstage. The excited beam was scattered from the quartz tube and the liquid interface, however the luminescence originated from the liquid itself. Since the focusing was oriented on the center of the liquid where the luminescence was dense, the excited beam did not arrive at monochromator 2. A computer controlled scan was performed at monochromator 2 between 300 and 550 nm with a step of 1 nm and 500 ms periods. The emitted beam entering through the 700 μm slit from monochromator 2 was converted to electrical signal through the photomultiplier tube (PMT) of the instrument depending on luminescence amplitude. These signals were digitally recorded to a computer through a data scan software. All measurements were performed in a dark room at room temperature and atmospheric pressure. The emitted beam arriving to the PMT was posed with 500 ms time average. Since the Xenon bulb wavelength was filtered at 300–380 nm and the PMT was sensitive to the emission wavelength, a normalization had to be carried out. Because of this, the sample was replaced with a quartz mirror in the measurement setup and the measurement was repeated. The peaks of the excitation were recorded. The maximum amplitude obtained at 360 nm was used to calculate the normalization coefficient for each wavelength. These coefficients were multiplied with the whole spectrum to perform the required normalization.
The Fourier Transform infrared measurement was carried out with Thermo Scientific and Nicolet FT-IR spectrometer 6700 model. The sample was placed in a quartz tube and placed in the optical path to get a scan starting from 2000 to 11000 cm−1. Before the measurement was started, a background scan was performed and background subtraction was carried out from the sample measurement. All measurements were performed using a computer controlled software with little to no user intervention. The measurements were carried out at room temperature and at atmospheric pressure.
UV spectrophotometer measurements were performed with Thermo Lab Multiskan 1500 model. The liquid mixture, 1,2 : 3,4-dibenzanthracene in ethanol, and ethanol only solution were measured. Using background subtraction, the spectrum of the mixture was obtained with a computer controlled software. All measurements were performed at room temperature.
3. Results and Discussion
1,2 : 3,4-dibenzanthracene, a polycyclic aromatic hydrocarbon, has an interesting shape of fluorescence emission spectrum because the spectrum is almost an invariant of the wavelength used to excite the molecule. This can be attributed to the fact that the emissions always occur from the level. The triplet states have lower energies compared to those of corresponding singlet states. The solvent may lead to a breakdown of the spin conservation rule for the molecules, which in turn permit weak absorption.
The UV absorption spectrum of 1,2 : 3,4-dibenzanthracene is shown in Figure 3. Although the absorption band is relatively broad, it is strongest around 315 nm. The broad spectrum can be attributed to the onset of π- and σ-bands, strong vibronic coupling, and ionization energies [13, 14].
The fluorescence measurements were carried out as described in Section 2 and the results are displayed in Figure 4. Three sharp peaks, all coincident with the same emission wavelengths are observed. Excitation wavelengths starting from 300 to 360 consistently pointed the same peaks with the highest level at 360 nm of excitation. These are Stokes’ shifts of the molecule with relative to the highest absorption wavelength of 315 nm. The absorption and emission spectra are shown on the same graph in Figure 5. The three Stokes’ shifts were also identified and labeled on the same figure.
4. Conclusions
1,2 : 3,4-dibenzanthracene is a polycyclic aromatic compound which is carcinogenic and requires careful spectroscopic analysis for monitoring environmental pollutants and genotoxic effects [15–19]. Raman spectroscopy can be a powerful probe for determining chemical composition, but Raman signals can be very weak and easily supressed by fluorescence (prompt and delayed). Thus, our analysis provides methods in the identification of 1,2 : 3,4-dibenzanthracene and its isomers.