A Study of the Electronic Absorption and Emission Spectra of DBDMA Dye: Solvent Effect, Energy Transfer, and Fluorescence Quenching

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Introduction
Boron dipyrromethene (BODIPY) complexes have drawn great attention in the last two decades because they are promising for applications in molecular probes [1], photodynamic therapy [2], laser dyes [3], nonlinear optical materials [4], and solar cells [5][6][7].Tese applications are primarily based on their interesting photophysical properties, such as high chemical stability, high photoluminescence quantum yield, ground-state solid absorption, and intense fuorescent emission.In the past decade, the core structure of BODIPY has been adjusted and developed into its azaderivatives, such as aza-boron dipyrromethene, aza-boron-diindolmethene, aza-boron-dipyridomethene, and azaboron-diquinomethene [7,8].Tese compounds show good electron-transporting properties, enhanced photoluminescence quantum yields, and unusual thermal stability that can be exploited in organic light-emitting devices (OLEDs) [9][10][11].Sathyamoorthi et al. synthesized pyridoamine-based BODIPYs for the frst time in 1993 [12], and then Bañuelos et al. discovered their photophysical properties by 2011 [13].Several quinoline-based complexes showed better electron mobility than pyrrole and pyridine counterparts [14], indicating that the quinoline structure is also a promising candidate for constructing boron-fuorine complexes.Lately, an aza-boron-diquinomethene complex was reported by Kondakova et al. which was exploited as a deep blue fuorescent emitting material to fabricate white OLEDs [15].Despite the obtained promising results, the investigation of the aza-boron-diquinomethene is still lagging.To improve the optical properties through structural modifcation, investigation of the structure-property relationship is critically important.
Both static and dynamic fuorescence quenching originate from fuorophore interactions with another molecule in its surrounding environment.It is well known that the solvent efects play a signifcant role in photochemical reactions and energy transfer processes in the solution system of BODIPY compounds.Te type and extent of interaction between the solute and the solvent depend on the polarity of the solvent and the possibility of the formation of hydrogen bonds with the solute [16].Besides, the infuence of the solvent on the electronic absorption and emission spectra has been the subject of interest and was considered as a method to calculate the dipole moment of the solute in the excited state.
In this work, an attempt was devoted to investigating the photophysical properties of one of the difouroboron BODIPY complexes, namely 2- acetonitrile (DBDMA).DBDMA has been reported as a potential indicator for low c-ray doses [17].Te absorption and fuorescence properties of the dye in sol-gel glass matrices were investigated and found that the photosol-gel glass matrix of the immobilized dye is the best compared to other diferent photosol-gel and organicinorganic matrices [18].
Te structure of the DBDMA dyes is shown in Scheme 1.As can be seen in the structure, the molecule contains four nitrogen atoms of three types of hybridizations and hence is expected to exhibit interesting spectral properties.Te absorption and emission spectra of the molecule have been calculated using the TD-DFT method and compared with the measured ones.Te efect of the solvent on both absorption and emission spectra has been discussed in relation to the polarity of the solvents.Also, the photostability of the molecule and the infuence of quenchers were investigated.

Experimental Setup
DBDMA was gifted by Professor Dr. Ewald Daltrozzo of Konstanz University, Germany, and used as supplied.Organic solvents (Fluka and Puriss) were of spectroscopic grade, and it was found free from impurities that could afect the absorption spectra or fuorescence within spectral ranges 250-600 nm.UV-Vis electronic absorption spectra were recorded using a Shimadzu UV-Vis 1650-PC spectrophotometer, where steady-state fuorescence spectra were presented employing a quartz cuvette of 1 cm path length; the emission was monitored at 90 °geometry using a Jasco FP-8200 spectrofuorometer with excitation bandwidth of 5 nm and emission bandwidth of 5 nm, using Xenon Lamp light source of 5 nm excitation and emission bandwidth.
Photochemical quantum yields of DBDMA (ɸ c ) were measured using a modifed A. J. Lee's method that takes into consideration the decrease in absorbance at the excitation wavelength as photo-irradiation proceeds.Te following equation ( 1) was applied to calculate the photochemical quantum yields: where c is the concentration of DBDMA, t is the time in min, ɸ c is the photochemical quantum yield, I 0 is the intensity of excitation light, D is the absorbance at the irradiation wavelength, ε is the extinction coefcient at irradiation wavelength, and b is the path length of the cell [19].Typically, the samples were irradiated in a photoreactor equipped with a 400 W UV lamp set to 254 nm with a 250 microwatts/cm 2 irradiation intensity in an air atmosphere at ambient temperature.
Fluorescence quantum yields in liquid were estimated using the optically dilute solution relative method with either 9,10-diphenyl-anthracene or quinine sulfate solutions, depending on the emission wavelength range.Te intensity of light was determined using ferrioxalate actinometry [20,21].Te following equation (2) was applied to calculate the fuorescence quantum yields: Te integrals refer to the corrected fuorescence peak areas, A is the absorbance at the excitation wavelength, and n is the solvent's refractive index.Te subscripts s and r refer to the sample and reference, respectively.
Te picosecond fuorescence decay patterns of DBDMA were determined by a picosecond pulse of (470 ± 10 nm) single-photon counting method using the FluoTime 300 (PicoQuant, Germany) in conjunction with the LDH-P-C-470 laser head.Te equipment's lifetimes were determined using the FluoFit software.A Peltier-cooled photomultiplier (PMT) was utilized to detect photons emitted in the 300−900 nm region.[23] and were compared to the ligand at the same level of theory.

Results and Discussion
Te spectroscopic and photophysical characteristics, solvent relative polarity, absorption, emission, and excitation maximum wavelengths, together with fuorescence and photochemical quantum yields of DBDMA in diferent solvents, are summarized in Table 1.Solvents were selected based on their ability to dissolve DBDMA and the diference in polarity and relative viscosity.
Figures 1S and 2S show the measured UV-visible absorption and emission spectra of DBDMA obtained at room temperature for a concentration of 1.4 × 10 −5 mol•dm −3 in solvents of diferent polarities.Te increase in solvent polarity slightly infuences the position of electronic absorption and emission maxima.Te blue shifts (hypsochromic shifts) in the absorption and emission maxima with increasing polarity of the medium indicate a decrease in the ground-state dipole moment of the dye molecule upon excitation with a concomitant increase in its ground-state dipole moment as a result of solvent polarization [24].
Te experimental absorption spectra show narrow absorption bands with three absorption maxima similar in shape to boron dipyrromethene dyes [25]; for example, in CH 3 CN, the band of a strong S 0 -S 1 transition with a maximum at 508 nm, the shorter wavelength centered at about 478 nm together with that at 449 nm.Efect on the absorption maxima wavelength position was observed, where a shift in the direction of a shorter wavelength of around 10 nm is noticed by changing the solvent from ethanol, toluene, and THF at 521, 514, and 512 nm, respectively (S 0 ⟶ S 1 transition) to 508 nm in acetonitrile, as shown in Table 1 and Figure 1S.Figures 1 and 2 for the investigated dye demonstrate a strongly allowed 1(π-π * ) transition with a small geometry change between electronic ground and excited states.Tis transition is consistent with the dye's high molar absorptivity and the mirror image relationship between excitation and fuorescence spectra in methylene chloride and carbon tetrachloride, respectively [26].Tis is not the same as for the dye in butanol, as shown in Figure 3S, which shows a change in the geometry of the dye molecule when the photons were absorbed, as indicated by the disappearance of a mirror image relationship between the fuorescence spectra and electronic absorption.
Te ability of butanol to make efcient hydrogen bonds with DBDMA might cause the relative loss of structured absorption and emission bands.
A more resolved Frank-Condon fuorescence peak is obtained at 560 nm.Tis transition appears as a shoulder with butanol and DMSO solvents.Te small size and high polarity of acetonitrile lead to better solvation of excited BDBMA molecules with subsequent deactivation to a similar ground-state confguration [27].Te high purity of DBDMA is indicated by the coincidence between the absorption and excitation spectra in DMSO solvents, as shown in Figure 4S.Purity is also confrmed by the congruence between the two emission spectra of the dye using three diferent excitation wavelengths (Figure 5S).Tis congruence also indicates the absence of tautomers [28].
Te Frank-Codon peak of the 0-0 transition in both fuorescence and absorption spectra occurs due to molecular rigidity.Te very high polarity in the ground state due to the presence of fuorine and cyano-groups allows its rapid solvation, leading to a less vibrational resolution in the fuorescence spectra compared to the absorption spectra of DBDMA [29].
A minor Stokes shift of 8 nm is observed for the studied dye in the CCl 4 solvent, showing weak solvation in the excited state as illustrated in Figure 2. Besides, the emission spectrum shows a vibrational structure due to a lack of ground-state solvation.In ethanol, the lack of the vibrational structure of emission spectra in various solvents indicates higher ground-state solvation and the possibility of hydrogen bond formation that results in diferent excited state geometry from that of the ground state.
It is observed that there is an increase in Stokes shift with an increase in solvent relative polarity (Figure 3), indicating the increase in the dipole moment on excitation [30].Te resonance structures can explain the observed variations in the dipole moments.Indeed, DBDMA has been found to have a greater excited state dipole moment (μ e ) than that of the ground state (μ g ) [18].

Molecular Modeling and Quantum Chemical Parameters.
Te frontier molecular orbital (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) of DBDMA was studied by using the B3LYB/6-31G level of theory as shown in Figure 4. π-bonding orbitals characterize the HOMO, and the LUMO is characterized by π * anti-bonding orbitals.In the HOMO energy level (Figure 4), the π-bonding orbitals and lone pairs of electrons for all nitrogen atoms and two fuorine atoms are delocalized over the whole DBDMA molecule.Whereas in the LUMO energy level, the π * anti-bonding orbitals and lone pairs of electrons for three nitrogen atoms are distributed over the whole probe dye, while the lone pairs of electrons for nitrogen atom in the cyanide (-CN) group and two fuoride atoms do not take part in the distribution over the whole molecule.
Upon using the orbital energy level at the B3LYB/6-31G level of theory, the efect of diferent solvents on HOMO and LUMO energy orbitals, HOMO-LUMO energy gap (E g ), zero-point energy (ZPE), and dipole moment (μ) are produced and given in Table 2.When the magnitude of (E g ) increases, the kinetic stability of the molecule increases, and chemical reactivity decreases.Hence, the kinetic stability of the DBDMA molecule in dimethyl sulfoxide (DMSO) is the highest (Table 1) compared to other solvents due to the higher polarity of DMSO.As shown in Table 1, the value of E g for the titled molecule increases with increasing the solvent polarity indicating a blue shift in absorption spectra for probe dye.Te value of zero-point energy (ZPE) for DBDMA is decreased upon increasing solvent polarity, as shown in Table 2, indicating the high stability of the titled molecule in DMSO compared to other solvents.Te increased dipole moment reveals large intramolecular charge transfer (ICT) in the molecule.Hence, the ICT for probe dye in DMSO is highest compared to other solvents.
Using the B3LYB/6-31G level of theory and the DFT method, the optimized structure for DBDMA is obtained and presented in Figure 5     optimized structure are illustrated in Figure 5(a).From the DBDMA optimized structure, boron (30B) and nitrogen (35N) atoms have the highest positive (+1.005) and negative (−0.852) charges, respectively.Te calculated HOMO-LUMO gap for most of the selected solvents shows values between 3.092 and 3.093 except for CCl 4 which shows a smaller value of 3.077 and thus a higher wavelength.Tis is in agreement with experimental results which show that λ abs max for the selected solvents are between 510 and 513 nm except for CCl 4 which is 517 nm.Te higher HOMO energy value when the molecule is surrounded by CCl 4 contributes to this smaller energy gap.

Efect of the Solvent on the Absorption and Emission
Spectra.Te UV-Vis absorption and emission spectra for the DBDMA molecule in diferent solvents such as CCl 4 , CH 3 Cl, CH 2 Cl 2 , acetone, and DMSO upon using the TD-DFT method were obtained and are illustrated in Figures 6(a) and 6(b).According to the molecular orbital theory (MOT), the allowed electronic transitions for the DBDMA molecule are σ − σ * , π − π * , n − σ * , and n − π * .For the DBDMA molecule in the gas phase, the UV-Vis absorption spectra showed two maximum absorption wavelengths at 291 and 448 nm.Te maximum absorption wavelength for probe dye at 291 nm disappeared in polar and nonpolar solvents.Depending on the polarity of the solvent, the maximum absorption wavelength for DBDMA is shifted to longer wavelengths (red-shift) when the polarity of the solvents is decreased, as shown in Figure 6 and Table 3. Te calculated spectra in Figure 6 simulate the intensities, which are based on the oscillator strength found in Table 3.
Te emission spectra of DBDMA dye in diferent solvents show two bands of maximum emission wavelengths depending on the polarity of the solvents.Due to the high polarity of DMSO compared to other solvents and the gas phase, the λ max emission for the titled molecule in DMSO is shifted to red (536 nm) compared to other solvents and the gas phase [31].
TD-DFT calculations show qualitative agreement with experimental results.First, it confrms what we have deduced from energy gaps that the molecule in CCl 4 has the highest absorption value.Second, it shows that the highest emission wavelength for the dye occurs in DMSO, which agrees with the experimental results.Among the selected solvents, DMSO has the highest polarity, and the TD-DFT calculations show that the DMSO solvent stabilizes the energy of the excited state to a much greater extent than the ground state and thus gives the highest emission wavelength.

Efect of Medium Acidity.
No change in the absorption spectral pattern of DBDMA was observed due to the addition of acid, as shown in Figure 7. Te vibronic peaks occur nearly at the same wavelengths, either in neutral, acidic, or basic media.Te broadening in some peaks in basic media is probably due to more hydrogen bonding.It seems that the lone pair electron transition (n − π) plays a minor role in modifying electronic spectra compared with the (π − π) transitions.Also, Figure 7 shows that neither protonation nor deprotonation has happened in the ground state.
On the other hand, a slight red shift in the emission spectrum of DBDMA occurs upon the addition of sulfuric acid, as shown in Figure 8.

Photostability.
Te photochemical quantum yield (v c ) values (∼0.003±) of DBDMA in four solvents are summarized in Table 4. Te value of fuorescence quantum yield in ethanol surpassed that of BNTVB [32] and that of the dye itself when was impeded in a sol-gel matrix [18].Te low values of v c indicate a photostability against irradiations, which is attributed to the rigidity of the molecules, which has also been confrmed by the small shifts in the emission and absorption wavelengths as the solvent's polarities change.Te molecule's rigidity has also been indicated by the theoretical calculations, which produced a high fuorescence quantum yield.DBDMA dye has promising applications in many felds, including dye lasers and solar cell collectors, due to its apparent photochemical stability in low chemical quantum yield (ɸ f ) and high fuorescence quantum yield.
DBDMA has short excited state lifetime values, as shown in Table 5. Te short-excited state lifetime values indicate no infuence of solvent polarity on the excited state duration.Tis short-excited state lifetime, together with high fuorescence quantum yield, confrms the ability of the dye to give laser emissions in the studied solvents.Fig. 6S shows the fuorescence decay profle of DBDMA in DMF, their exponential ft, and IRF.A quiet agreement between the timeresolved fuorescence curve and the ftted exponential data is observed, which indicates the applicability of the selected model.Te plot of regular residuals (on the bottom of the curve confrms) confrms the successful application of the exponential model.

Quenching by Oxygen.
Upon aeration of a solution of BDBMA in ethanol with O 2 for 20 mins, no efect on the photophysical deactivation pathway of BDBMA was observed for the excited state, which was an explanation of the stability of peak emission intensity of the dye after exposure to oxygen gas as shown in Figure 7S.Tis behavior is confrmed by measurement of the fuorescence quantum yield of the aerated solution of BDBMA, ϕ f , which was found to be 0.83, confrming that molecular oxygen did not enhance the electronic transition from the singlet state to the excited triplet states through intersystem crossing (ISC) via its paramagnetic ground state.A prohibited singlet to triplet transition is crucial in laser media as the triplet population always has a deteriorating efect in laser media.

Energy Transfer.
DBDMA is used as an energy acceptor from fuorescein dye.Low concentrations of the acceptor, which have low molar absorptivity at 459 nm, were used to avoid reabsorption.A mixture of DBDMA and fuorescein gave fuorescence emission spectra in the range 490 to 600 nm with an emission maximum of 530 nm upon excitation using λ ex 459 nm light, as shown in Figure 9.

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Journal of Spectroscopy Te critical transfer distance R o has been calculated for the fuorescein/DBDMAs energy transfer system by applying the relation: where R o is the distance at which the energy transfer and emission processes are equally probable, ɸ D is the emission quantum yield of the donor in the absence of the acceptor, n is the solvent refractive index, and the integral is the overlap integral for the fuorescence spectrum of donor normalized to unity (F D ) and the absorption spectrum of the acceptor (Ɛ A ) divided by the fourth power of the wavenumber (]).
Te signifcant diference in the value of the critical transfer distance R o calculated from the donor-acceptor spectral overlap region (see Figure 10), 47 Å, is ∼10 times greater than those characterizing collisional energy transfer,    for which R o is in the range 4-6 Å, indicating that the underlying mechanism of energy transfer is that of resonance energy transfer due to long-range dipole-dipole interaction between the excited donor and the ground-state acceptor [33].

3.7.
Quenching by CuSO 4 .5H 2 O. DBDMA has the advantages of a high extinction coefcient (11586 M −1 •cm −1 ) at 514 nm and a strongly fuorescent dye with a high fuorescence quantum yield of 83% in ethanol at emission maxima around 530 nm.Te addition of Cu 2+ ions causes fuorescence quenching because of the heavy atom efect and the paramagnetic nature of Cu 2+ ions.Successive addition of Cu 2+ in the form of copper sulfate pentahydrate results in a gradual decrease in the emission at the wavelength 530 nm as shown in Figure 11, accompanied by a slight bathochromic shift of ca. 3 nm in ethanol.
Te Stern-Volmer plot of the quenching process of the dye using Cu +2 metal ion is shown in Figure 8S.A linear plot is obtained down to CuSO 4 .5H 2 O concentration of 4 × 10 −7 M, indicating an efcient quenching efect of Cu +2 ions [34].Journal of Spectroscopy Te rate constant (k ET ) of energy transfer in the DBDMA/fuorescein system in ethanol was calculated using the Stern-Volmer equation and the Stern-Volmer plot (Figure 8S).Excited state lifetime in DMF was measured as 4.5 ns.Te Stern-Volmer relation in the following form was applied [35]: where I o and I are fuorescence intensities in the absence and the presence of the quencher of concentration (Q), respectively, and τ is the e excited state lifetime of the donor in ethanol (4 ns).K ET has been calculated as 3.70 × 10 7 M −1 •S −1 .Te optimized structure of the complex with Cu in the presence of ethanol molecules is shown in Figure 12.Tis study was conducted to compare the theoretical results to those obtained experimentally in ethanol.
Te coordinates for the ligand and the Cu complex are found in the supplementary material (Tables 1S and 2S).Te copper center is fve coordinated and bonded to the ligand through one nitrogen atom.Te bond length of Cu-N is 1.98 Å.Table 1 shows the TDDFT results of selected electronic transitions, their absorption energies, and their oscillator strengths.Te electron density in the ligand alone is mainly located on the rings for the HOMO and the LUMO (Figure 13).In the Cu complex, the transition from the HOMO to LUMO shows an intramolecular charge transfer (ICT): in the HOMO, the electron density is mainly distributed on the rings, but in the LUMO it is mainly located on the metal (Figure 13).We may suggest that the ICT from ligand to metal leads the quenching of the fuorescence in this complex.Te small value of the oscillator strength of this transition (Table 6) supports this result.

Conclusion
(1) Te low photochemical quantum yield and high fuorescent quantum yield, together with the shortexcited state lifetime value confrm that the studied dye is a highly recommended efcient laser dye.(2) Tere is a correspondence between the measured results and the theoretically calculated results using the TD-DFT method.(3) DBDMA dye in solutions undergoes energy transfer to fuorescein dye and fuorescence energy quenching by copper and cadmium ions.
(4) Te studied dye is not considered an efective sensor for identifying the types of solvents or determining the medium's acidity.

Figure 4 :
Figure 4: Te graphical presentation of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of DBDMA dye in gas at the B3LYB/6-31G level of theory.

Figure 5 :
Figure 5: Te optimized structure for probe dye at the B3LYB/6-31G level of theory (b) and the charge distribution and the direction of dipole moments (a).

Figure 6 :
Figure 6: Calculated absorption (a) and emission (b) spectra for the DBDMA molecule in diferent solvents.

Figure 12 :
Figure 12: Te DFT optimized structure of the Cu(II) complex model in the presence of ethanol molecules.

Table 6 :
TDDFT results of selected electronic transitions, their absorption energies, and their oscillator strengths.

Table 5 :
Excited state lifetime of DBDMA in diferent solvents at room temperature.