Photophysics of Anthril in Fluids and Glassy Matrixes - ACS Publications

Subscriber access provided by Kaohsiung Medical University

A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Photophysics of Anthril in Fluids and Glassy Matrices Pronab Kundu, and Nitin Chattopadhyay J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b00988 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Photophysics of Anthril in Fluids and Glassy Matrices

Pronab Kundu and Nitin Chattopadhyay* Department of Chemistry, Jadavpur University, Kolkata - 700 032, India *Corresponding author: Fax: 91-33-2414-6584 E-mail: [email protected]

ABSTRACT Photophysics of 9,9′-anthril have been investigated at room temperature and cryogen phase (77 K) exploiting steady state and time resolved emission techniques together with quantum chemical calculations. Absorption spectra, emission spectra and emission lifetimes of anthril have been recorded and analyzed in polar ethanol and non-polar methylcyclohexane media. Both room temperature and cryogenic experiments reveal single emission band upon excitation at n* absorption band whereas on exciting the system at * band, dual emission bands have been observed. Characterization of these two fluorescence bands to be originating from near-trans and relaxed skew conformers have been made by monitoring their differential effect on varying the polarity of solvents. Similarly, two phosphorescence bands have been assigned to trans and cis geometries by looking at the change in the emission spectra in the two rigid matrices of different polarity. Observation of single isoemissive point in the time resolved area normalized emission spectroscopy (TRANES) for both fluorescence and phosphorescence emissions unambiguously validate the coexistence of the two conformers in the excited singlet and triplet states respectively. Qualitative quantum chemical calculations indicate that the S 1 and T1 states are responsible for the dual fluorescence and phosphorescence bands. Effortless transitions from the higher excited singlet states (S3 or S2) to the lowest energy excited singlet (S 1) state because of their energy proximity, 1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

discards any possibility of S2 emission, consistent with two other 1,2-dicarbonyl compounds like furil, 2,2′-pyridil, while going in contrast to the observation of S2 emissions from benzil and naphthil. Based on the vivid photophysical studies on five probes in fluid media and 77 K glassy matrices, we conclude that exhibition of the S 2 emission for aromatic 1,2-dicarbonyl compounds is truly system dependent and not a general phenomenon for all the molecular systems in the series.

1. INTRODUCTION The photophysical studies of nonconjugated aromatic 1,2-dicarbonyl compounds have been the subject of extensive research because of their structural flexibility with regard to the intercarbonyl dihedral angle and easiness of attaining various conformations in the excited states. 1-8 The 1,2-dicarbonyl compounds possess several useful applications in organic synthesis, photosensitization and these compounds are also extensively used as precursors for advanced glycation endproducts (AGEs) in foods.9,10 The stabilized geometry of this series of compounds are mainly driven by the two prime factors; (i) steric interaction between the carbonyl group and the ortho substitution on the adjacent aromatic ring, and (ii) resonance stabilization (highest for the planar conformer) of the molecule.7 Combination of these two effects divulge that various stable and/or metastable conformations such as skew, trans, relaxed skew, near-trans etc. can exist in both ground and excited states. The spectroscopic and quantum chemical investigations of numerous aromatic 1,2-dicarbonyl compounds like benzil, -naphthil, -furil, 2,2′-pyridil, mesitil etc. are the topic of numerous investigations due to the photoisomerization in their photoexcited states and possibility of emission from the higher excited electronic state (i.e., S2 emission).1-8,11-14 The most studied 1,2-dicarbonyl compound, benzil, upon photoexcitation undergoes geometry change from the skew form in the 2 ACS Paragon Plus Environment

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ground state to the trans-planar geometry in the excited singlet and triplet states. 3,11,15 In recent times, our group has also studied the detail photophysical changes of benzil, -naphthil, -furil and 2,2′pyridil in the excited singlet and triplet states at room temperature solution phase and in glassy matrices at 77 K using steady state and time resolved emission studies together with the quantum chemical calculations.11-14 Apart from the photorotamarism in the photoexcited state, an additional important observation for some of the 1,2-dicarbonyl compounds is the higher energy emission originating from the second excited singlet state (S2 emission). Our exhaustive fluorometric studies suggest that the noteworthy observation of S2 emission is observed for benzil and -naphthil.11,12 However, -furil and 2,2′-pyridil do not show any emission from the higher excited state due to the energy proximity between the S2 and S1 states and thus the molecules can effortlessly move from the S2 state to the S1 state.13,14 In the present work, we have performed exhaustive spectroscopic studies of another 1,2-dicarbonyl compound, namely, anthril, in both polar and nonpolar solvents at room temperature and liquid nitrogen temperature (77 K) to assign the multiple emissions. Anthril (di-9-anthrylethanedione) (Scheme 1) shows significant curing effects on the E. coli K12 strain carrying an F’lac plasmid.16 The photophysical investigations of anthril in ethanol and methylcyclohexane have been performed at room temperature solution phases and cryogenic temperature glassy matrices to explore the multiple emissions originating from the singlet and triplet states. For the assignment of the fluorescence and phosphorescence emission bands of the fluorophore, we have selected different time windows (depending on the fluorescence and phosphorescence lifetimes) at 77 K frozen matrices to segregate both these bands precisely. 11-14,17 Importantly, coexistence of the two isomeric fluorescences and phosphorescences from the same electronic states (S1 and T1 respectively) are confirmed by time resolved emission spectroscopy (TRES) and its extension, namely, time resolved area normalized emission spectroscopy (TRANES). TRES study provides an useful information on the heterogeneity of the emissive species and the 3 ACS Paragon Plus Environment


Page 4 of 30

excited state kinetics.18,19 However, TRANES is a rather novel and model-free technique as developed by Periasamy et al., is exploited for the analysis of extrinsic and intrinsic fluorophores in chemical and biophysical systems. 19,20 A unique and valuable feature of TRANES is the possible observation of an isoemissive point in the spectra, signifying the coexistence of the two (and only two) emitting species in the emission spectra of the probe. 19-22 Quantum chemical calculations of anthril have also been performed using density functional theory (DFT) and time dependent density functional theory (TDDFT) to construct the potential energy curves (PECs) in different ground and excited electronic states. The constructed PECs provide a qualitative theoretical support for the assignment of different emissions originating from anthril and also rationalize the non observation of the S2 emission from anthril, in a similar way to the cases of -furil and 2,2′-pyridil.

Scheme 1. Schematic and global energy minimized structures of anthril. Atomic notations are grey: carbon, off-white: hydrogen and red: oxygen.

2. MATERIALS AND METHODS The fluorophore, 9,9′-anthril (Scheme 1), was synthesized in the laboratory using the standard method of Becker et. al.23 The compound was purified using silica gel column chromatography followed by repeated recrystallization with minimum amounts of ethanol. The sample was characterized using traditional spectroscopic techniques like IR (KBr), 1H-NMR (500 MHz, CDCl3), 4 ACS Paragon Plus Environment

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13

C-NMR (500 MHz, CDCl3) and UV-vis absorption. The position of the >C=O signal in the IR and

the 1H-NMR and UV-vis absorption spectral patterns were consistent with the ones reported by Becker et al..23

13

C-NMR showed two close lying signature peaks of the two carbonyl groups at

around 190 ppm. The purity of anthril was also checked from thin layer chromatography (TLC). Spectroscopic grade solvents ethanol (Merck, Germany) and methylcyclohexane (Sigma-Aldrich, USA) were used as received. Absorption spectra were obtained by scanning the sample solution on a Shimadzu UV-2450 spectrophotometer (Shimadzu Corporation, Japan) against solvent blank reference in the wavelength range 230 - 550 nm. Fluorescence and phosphorescence spectra of the probe at room temperature (RT) and liquid nitrogen temperature (77 K) were recorded on a Horiba Jobin Yvon Fluoromax-4P spectrofluorometer (Horiba Jobin Yvon, USA) equipped with phosphorescence accessories. Phosphorescence lifetime measurements at 77 K were done using the same instrument. Fluorescence quantum yields of the fluorophore were calculated using quinine sulfate in aqueous 0.1 (N) H 2SO4 as standard with quantum yield 0.54.18 Fluorescence lifetime measurements were carried out using a Horiba Jobin Yvon FluoroCube set-up with a TBX photon detection module as the detector exploiting the time correlated single photon counting (TCSPC) technique. Based on the availability, we have used 370 nm and 283 nm light emitting diodes (IBH UK) for the excitation to the n* and * states respectively. The system response time (IRF) was measured from the scatterer (dilute micellar solution of sodium dodecyl sulfate) in position of the sample. The decays were deconvoluted and analyzed using IBH DAS-6 decay analysis software. The quality of the fitted data was judged from 2 criterion and visual inception of the residuals. For room temperature time resolved emission spectra (TRES) of anthril in ethanol and methylcyclohexane, the sample was excited at 283 nm and the emission spectra are recorded for a wide spectral range (300580 nm) using various time windows. Whereas, TRES for the phosphorescence bands at 77 K were obtained 5 ACS Paragon Plus Environment


Page 6 of 30

by collecting the emissions at various phosphorescence time windows using the Fluoromax-4P spectrofluorometer. The detailed technical methodology for TRES and TRANES studies at room temperature and 77 K have already been illustrated in one of our recent publications.14 The quantum chemical calculations on the ground and excited states of anthril were performed using the Gaussian 09 program.26 For the global ground state energy minimized geometry of the probe, we have opted density functional theory (DFT) employing Becke’s three-parameter hybrid functional B3 with the nonlocal correlation of Lee–Yang–Parr (B3LYP) functional using 6311++G** basis set.27,28 DFT was exploited for its simplicity; viability and correlation with the experimental results.29-30 The energies of the different excited states were obtained by vertical transition of the ground state geometries using time dependent density functional theory (TDDFT) at B3LYP/6-311++G** level of calculations.29-31 Since the rotation about the 1,2-intercarbonyl bond gives the various rotameric conformers of 1,2-dicarbonyl compounds, the simulated potential energy curves (PECs) in the ground (S0) and different excited states (S1, S2, S3 and T1) were constructed using the optimized energies at the ground state and the energies of the corresponding Franck Condon (FC) states at different pre-determined 1,2-dicarbonyl dihedral angles (C=O-C=O) over a range of 0 to 180. The details of construction of such PECs in the ground state (S0), excited singlet states (S1, S2, S3) and the first excited triplet state (T1) are available in several of our publications.11,13,14,31 The constructed PECs of anthril in the excited singlet and triplet states provide a qualitative theoretical support for the assignment of the multiple emissions originating from various singlet and triplet state geometries.

6 ACS Paragon Plus Environment

Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3. RESULTS AND DISCUSSION 3.1. Absorption Studies The room temperature absorption spectra of anthril in polar ethanol (EtOH) and nonpolar methylcyclohexane (MCH) media exhibit two distinct bands in the entire UV-Vis spectral range as depicted in Figure 1. In both the solvents the fluorophore shows a lower energy weak structured absorption band in the wavelength range 300-450 nm along with a relatively much intense band in the higher energy region (220-300 nm) having peak maxima at 257 nm in EtOH and at 256 nm in MCH. The spectral shifts of the absorption band maxima with increasing the polarity of solvents attribute the lower and the higher energy absorption bands to the n* (showing blue shift) and * (showing red shift) transitions of the fluorophore respectively.18 Based on the absorption studies, Becker et. al.23 proposed that the two carbonyl groups are twisted out of the planes of the anthracene, consistence with our theoretical studies. The nπ* absorption band maxima of the series of 1,2-dicarbonyl compounds show a good correlation with its dihedral angle between the two carbonyl groups (). Molecule approaching towards the trans conformation (i.e.,  = 180) leads to the red shift of the nπ* absorption band maximum due to the greater conjugation between the two aromatic rings leading to a reduction in the dipole moment and consequently solvent stabilization of the nπ* band.7,13,14,32 The quantum chemical calculations give the intercarbonyl dihedral angles to be 126.6, 113, 156.1 and 80.48 for benzil, α-naphthil, α-furil and 2,2′-pyridil respectively.11,13,14 Figure 1B depicts the linear correlation between the intercarbonyl dihedral angle and the nπ* absorption band maxima for a series of 1,2-dicarbonyl compounds in MCH solvent. A comparative look at these values imply that the intercarbonyl dihedral angle of anthril is much lower than that of α-furil or benzil or α-naphthil 7 ACS Paragon Plus Environment


and is close to that of 2,2′-pyridil (80.48). The energy minimized ground state optimized structure of anthril reveals the intercarbonyl dihedral angle to be 81.0, consistent with the experimental trend.

28000

1.0

Anthril in EtOH Anthril in MCH

(A)

0.8

2,2'-Pyridil

(B)

MCH solvent

0.08

-1 max abs (n (cm )

Absorbance

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 30

0.06

0.6

0.04 0.02

0.4

0.00

300

350

400

450

500

Wavelength (nm)

0.2

550

0.0 250

300

350

400

450

500

Wavelength (nm)

550

26000

Anthril Benzil

24000

-Furil

-Naphthil

22000 70

80

90 100 110 120 130 140 150 160 0

Dihedral angle ( C)

Figure 1. (A) Room temperature absorption spectra of anthril in EtOH and MCH. (B) Variation of nπ* absorption band maxima against intercarbonyl dihedral angles of a series of 1,2-dicarbonyl compounds in MCH. Inset of (A) shows the magnified spectra at lower energy region.

3.2. Emission Studies At room temperature, upon photoexcitation at the lower energy n* absorption band at 380 nm, anthril exhibits a structured fluorescence band ranging from 390 nm to 570 nm in both ethanol and methylcyclohexane. Figure 2A represents the emission and fluorescence excitation spectra of anthril in both the solvents. In nonpolar MCH, the position of the emission maximum of anthril is found to be little blue shifted compared to that in polar EtOH, which is rationalized from the excited state dipole-dipole induced probe-solvent stabilization.18 Indifference of the excitation spectra in terms of band position and band pattern (figure not shown) upon monitoring at different wavelengths within the entire emission spectral range in both the solvents suggest that the emission originates from a single emissive singlet species. Moreover, the structured excitation and emission spectra bearing the mirror relationship in both the solvents discard the possibility of formation of dimer or 8 ACS Paragon Plus Environment

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


aggregation.18 Therefore, this structured emission is assigned to the fluorescence coming from the excited singlet state. The quantum chemical calculations divulge that this emission originates from the near-trans species in the excited S1 state of anthril having intercarbonyl dihedral angle 169 (see later) which is off-planar. Slight spectral shift in the emission spectra upon variation of the solvent polarity also implies that the emitting species has low but non-zero dipole moment supporting the near-trans geometry of anthril (see below). The emission spectral pattern of anthril is modified significantly upon excitation at its * absorption band. Upon photoexcitation at 290 nm (* band), anthril shows two distinct structured emission bands in both the solvents as depicted in Figure 2B. In EtOH and MCH, these two bands appear at wavelength ranges 320 - 390 nm and 390 - 570 nm with band maxima at ~ 357 nm and ~ 440 nm in EtOH and at ~ 347 nm and ~ 426 nm in MCH. Among these two emission bands, the lower energy structured emission band (ranging between 390-570 nm in both the solvents), similar to the one obtained upon excitation at the n* absorption band of the probe, is ascribed to the S1 fluorescence of anthril having near-trans conformation, as already discussed. Room temperature emission studies of 1,2-dicarbonyl compounds benzil and α-naphthil gives emission from S2 state, apart from the emissions for two differently emitting rotameric species in the S1 state.11,12 On the contrary, α-furil and 2,2′-pyridil gives only two fluorescence bands for two different emissive species in the S1 state. Arguably, the observation of multiple fluorescence for the 1,2-dicarbonyl compounds, the higher energy fluorescence of anthril having maximum at 357 nm in EtOH and at 347 nm in MCH may originate either from the higher excited electronic state (S2) or from a different conformer in the S1 state. Our quantum chemical calculations on the various excited singlet states suggest the energy proximity of the S1, S2 and S3 states in the entire range of possible torsions; and hence the possibility of emission from any higher excited singlet state other than the S1 state can be rejected because of the barrierless smooth transition of the molecule from the higher S3/S2 states to 9 ACS Paragon Plus Environment


Page 10 of 30

the S1 state (see later). Therefore, this higher energy fluorescence band of anthril is ascribed to originate from another geometry in the S1 state having energy higher than the stable near-trans species. We ascribe this emission to correspond to the relaxed skew conformer. DFT calculation gives the dipole moment of anthril in the ground state to be 4.55 D. TDDFT calculations imply that the near-trans and the relaxed skew conformers in the S1 state have different dipole moments (μ = 0.98 D and 6.1 D for the two conformers respectively), consistent with the normal expectation, considering the respective geometries of the probe. A relatively higher red-shift of the fluorescence of the relaxed skew form (810 cm-1) than the near-trans form (740 cm-1) as we move from MCH to EtOH solvent justifies the relative values of the dipole moments of the two conformers of anthril in the S1 state. Existence of two fluorescence bands of anthril from co-existing two species in the S1 state, is further confirmed by the time resolved area normalized emission spectroscopy (TRANES) studies (see later). Fluorescence quantum yields of the two conformers (near-trans and relaxed skew) in EtOH and MCH have been determined by resolving the composite emission spectra for the two species (Figure 2B). The determined quantum yields, however, should not be considered as precise since resolution of the dual emission bands does not incorporate the aspect of vibrational structures. The fluorescence quantum yields for the near-trans and the relaxed skew conformers are thus found to be 0.01 and 0.096 in ethanol and 0.076 and 0.064 in MCH, respectively. Although a structured pattern is evident in both the absorption (on zooming) and the excitation bands, there is, of course, a visual discrepancy between the * absorption band and the fluorescence excitation band for the higher energy emission band, particularly in terms of the relative intensities on the different vibrational peaks (Figures 1 and 2B). That excitation over the entire range of this absorption band gives rise to the same two fluorescence emissions (only the relative intensities of the two bands changes, figure not shown) establishes that this * absorption band contains a single band. The visual difference in the relative intensities of the different 10 ACS Paragon Plus Environment

Page 11 of 30

vibrational peaks between the absorption and the excitation spectra might be due to the fact that there is a change in the molecular geometry in the excited state compared to the ground state

(A)

EtOH (ex = 380 nm) EtOH (em = 440 nm) MCH (ex = 380 nm) MCH (em = 430 nm)

300

350

400

450

500

Wavelength (nm)

550

Norm. Fl. Intensity (a.u.)

geometry since fluorescence excitation spectra are collected monitoring the emission only.

Norm. Fl. Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(B)

EtOH (ex = 290 nm) EtOH (em = 360 nm) MCH (ex = 290 nm) MCH (em = 350 nm)

300

350

400

450

500

Wavelength (nm)

550

Figure 2. Room temperature emission and excitation spectra of anthril in EtOH and MCH for the (A) n* and (B) * transitions. Excitation and emission wavelengths are depicted in the legends.

3.3. Room Temperature Fluorescence Lifetime Measurements Time resolved fluorescence lifetime measurement serves as an excellent experimental tool to recognize the existence of various emitting species of the fluorophore.11-14,33,34 Hence, to obtain a detailed information of photophysics of anthril, fluorescence lifetimes are measured for all the emissions in both the solvents. Based on the availability of the excitation sources, the probe solutions were excited at 370 nm (for n* excitation) and at 283 nm (for * excitation) and the decays were monitored at different wavelengths within the emission bands. Figure 3 depicts the decay profiles of anthril for different emissions in EtOH and MCH upon excitation at 370 nm and 283 nm and the deconvoluted decay parameters are tabulated in Table 1. On exciting the probe solution at its n* band, i.e., 370 nm, the fluorescence decay monitored within the lower energy structured emission band show mono-exponential profile both in EtOH and MCH and the 11 ACS Paragon Plus Environment


Page 12 of 30

corresponding lifetime values are found to be 11.7 ± 0.3 ns and 10.0 ± 0.3 ns respectively in the said solvents. Single lifetime value upon monitoring at different wavelengths within the lower energy structured emission band ranging between 400 nm and 550 nm in the two studied solvents confirm that the emission comes exclusively from a single emitting species. Since excitation at the n* absorption band gives only one emission from the near-trans conformer, the lifetime analyzed from the lower energy emission is ascribed to the near-trans conformer of anthril. Upon excitation at the * absorption band (λex = 283 nm), the fluorescence lifetime measurements of anthril monitoring at both lower and higher energy emission bands (Figure 2B) depict bi-exponential decay profiles and yield two lifetime values in both the solvents. Deconvolution of the bi-exponential decay profiles give lifetime values 6.9 ± 0.3 ns and 11.4 ± 0.3 ns in EtOH and 6.2 ± 0.3 ns and 10.3 ± 0.3 ns in MCH upon monitoring the lower and the higher energy emission bands, respectively (Table 1). Analysis of the fluorescence decays upon excitation at both n* and * absorption bands and the relative values of the pre-exponential factors (a1 and a2) contributing towards the lifetime values (1 and 2) respectively, for both the lower and higher energy emission bands, led us to assign the longer lifetime (11.4 ± 0.3 ns in EtOH and 10.3 ± 0.3 ns in MCH) to the near-trans conformer and the shortlived species (6.9 ± 0.3 ns in EtOH and 6.2 ± 0.3 ns in MCH) to the relaxed skew form of the fluorophore. Thus, the lifetime studies reveal that a single conformer (skew) in the ground state forms two different conformers (relaxed skew and near-trans) in the S1 state for * excitation. Observation of clear isoemissive points in the TRANES spectra (see later) of anthril in both EtOH and MCH upon * excitation corroborate the coexistence of two conformers in the excited singlet state (S1).


Page 13 of 30

Prompt EtOH ex=283 nm, em = 350 nm) EtOH ex=283 nm, em = 440 nm) EtOH (ex=370 nm, em = 440 nm)

1000

MCH ex=283 nm, em = 350 nm) MCH ex=283 nm, em = 430 nm)

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MCH ex=370 nm, em = 430 nm)

100

10 0

20

40

60

Time (ns)

80

100

Figure 3. Fluorescence decay profiles of anthril in EtOH and MCH at various emission wavelengths. Excitation and monitoring wavelengths are indicated in the legend.

Table 1. Time resolved fluorescence data of anthril in EtOH and MCH at room temperature. Solvent EtOH

MCH

ex (nm)

em (nm)

1 ± 0.3 (ns)

2 ± 0.3 (ns)

a1

a2

2

283 283 370 283 283 370

360 440 440 350 430 430

6.90 6.90

11.0 11.8 11.7 10.4 10.3 10.0

0.73 0.08

0.27 0.92

0.87 0.09

0.13 0.91

1.07 1.10 1.03 1.16 1.13 1.10

6.40 6.00

To confirm that the two species having two distinct lifetime values are coming from the two different conformers of anthril in the first excited singlet state, and not from the same species in the different excited electronic states, we have generated room temperature time resolved emission spectra (TRES) and time resolved area normalized emission spectra (TRANES) of anthril in both the solvents upon excitation at * band (see experimental section). Looking at the reasonable disparity in the lifetime values for the lower and the higher energy emission bands, TRES have been collected 13 ACS Paragon Plus Environment


within the spectral range 300-550 nm at various selected time windows, as depicted in Figures 4A and 4C in EtOH and MCH, respectively. The spectral changes in TRES at various time windows may occur because of the two or more emitting species or due to the different solvated cluster of single excited species.18,19 To resolved this, we have opted for a model free extension of TRES, namely, TRANES by normalization of area under the time resolved emission spectra.19-22 Figures 4B and 4D represents the TRANES spectra of anthril in EtOH and MCH respectively at different time windows. Isoemissive points (at 400 nm in EtOH and 389 nm in MCH) are observed in the TRENES spectra, revealing the coexistence of two conformers in the excited state. Therefore, the two fluorescence emission bands are ascribed to originate from the two emitting conformers in the first excited singlet state and the possibility of emission from higher excited singlet state (S2 or S3) is rightaway discarded. The simulated PECs also suggest that upon excitation at S2 or S3 states, anthril molecule smoothly comes down to the two local minima of the S1 state having near-trans and relaxed skew geometries of the fluorophore (see later), and hence, the two fluorescence bands are believed to come from the two torsional isomeric species in the S1 state.

(A)

TRANES in EtOH

Time window (ns) 0.73-1.40 1.46-2.13 2.91-3.53 4.37-5.04 5.82-6.50 7.28-7.95 9.46-10.14 11.65-12.32 13.83-14.50 16.74-17.42 19.66-20.33 23.30-23.97 26.94-27.61 31.30-31.98 35.67-36.34 40.77-41.44

(B)

Time window (ns) 0.73-1.40 1.46-2.13 2.91-3.53 4.37-5.04 5.82-6.50 7.28-7.95 9.46-10.14 11.65-12.32 13.83-14.50 16.74-17.42 19.66-20.33 23.30-23.97 26.94-27.61 31.30-31.98 35.67-36.34 40.77-41.44

Intensity

TRES in EtOH

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

Isoemissive point

300

350

400

450

500

Wavelength (nm)

550

300

350

400

450

500

Wavelength (nm)

550


TRES in MCH

TRANES in MCH

Time window (ns)

(C)

0.67-1.29 1.35-1.96 2.01-2.63 2.69-3.30 3.36-3.98 4.70-5.32 6.05-6.66 7.39-8.01 9.41-10.03 11.42-12.04 13.44-14.56 16.13-16.74 18.82-19.43 22.18-22.79 26.21-26.82 29.56-30.18 36.29-36.90 46.37-47.00

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(D) Time window (ns) 0.67-1.29 1.35-1.96 2.01-2.63 2.69-3.30 3.36-3.98 4.70-5.32 6.05-6.66 7.39-8.01 9.41-10.03 11.42-12.04 13.44-14.56 16.13-16.74 18.82-19.43 22.18-22.79 26.21-26.82 29.56-30.18 36.29-36.90 46.37-47.00

Intensity

Page 15 of 30

Isoemissive point

300

350

400

450

500

Wavelength (nm)

550

300

350

400

450

500

Wavelength (nm)

550

Figure 4. Time resolved emission spectra (A and C) and time resolved area normalized emission spectra (B and D) of anthril in EtOH and MCH (ex = 283 nm). Various time windows for the TRES and TRANES studies are depicted in the legends.

3.4. Emission Studies at 77 K Spectral analysis of 1,2-dicarbonyl compounds at cryogenic temperature (77 K) is a prime experiment to investigate the photophysical behavior in the singlet and triplet states of the fluorophore. Since the rotational motion of a probe is restricted in the frozen matrix, we have performed the emission studies of anthril at 77 K to resolve the fluorescence and phosphorescence emissions precisely. Considering the huge difference in lifetime values (in the range of nanosecond for fluorescence and millisecond for phosphorescence, respectively), the fluorescence and phosphorescence emissions can easily be segregated by taking the emission spectra at various preset time windows.11-14,17 When emissions are collected within time window range 0-0.02 ms, phosphorescence is practically obstructed, and the fluorophore principally gives the fluorescence only. Similarly, within long time window range (0.1-100 ms), the fluorescence should disappear, and actually we obtained exclusively the phosphorescence. However, both fluorescence and phosphorescence bands were observed when we used the combined time window range (0 - 100 ms).



Page 16 of 30

Upon excitation at the n* absorption band (380 nm), the emission spectra of anthril in the EtOH and MCH solid matrices give structured emission band covering the range of 400 nm to 550 nm as depicted in Figure 5A. Since the emission is collected without any preset time window, the figure contains both fluorescence and phosphorescence emissions. The figure reveals that the main structured band (400-500 nm) in EtOH matrix is red shifted compared to that in MCH matrix. The spectral pattern and band position of this emission in both EtOH and MCH glassy matrices show resemblance to the room temperature n* fluorescence in solution phases. Hence, this fluorescence band is ascribed to originate from the near-trans conformer of the fluorophore. Apart from this emission, the spectra contain another structured lower energy emission band in the wavelength range between 460 nm and 600 nm. Emission studies using a much longer time window (0.1-100 ms) in both the matrices, anthril shows structured phosphorescence ranging between 460 - 600 nm because within this long delay time (0.1 ms) fluorescence should vanish (Figures 5B and 5C). Insignificant spectral shift of the phosphorescence band on changing the polarity of the glassy matrices (EtOH and MCH) imply that the trans geometry of anthril in the triplet state is responsible for the phosphorescence band. Very long lifetime (~ 3 ms) of this lower energy band further substantiate the nature of this band to be phosphorescence (described in the upcoming section). Our quantum chemical calculations suggest that the phosphorescence is coming from the triplet state having trans conformation (intercarbonyl dihedral angle 180) (see later). Again, upon collecting the emission spectra within the short time window 0-0.02 ms (phosphorescence is blocked) in both the frozen matrices (EtOH and MCH), anthril shows structured fluorescence covering the range 400-550 nm (Figures 5B and 5C). Thus, there is a considerable overlap between the fluorescence and phosphorescence bands in the cryogenic matrices.


(A)

Fl. Intensity (a.u.)

EtOH MCH

ex = 380 nm

400

450

500

550

Wavelength (nm) Emission Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Emission intensity

Page 17 of 30

(C)

600

MCH

(A)

EtOH

0 - 100 ms 0 - 0.02 ms 0.1 - 100 ms

ex = 380 nm

400

450

500

550

Wavelength (nm)

600

0 - 100 ms 0 - 0.02 ms 0.1 - 100 ms

ex = 380 nm

400

450

500

550

Wavelength (nm)

600

Figure 5. (A) Emission spectra of anthril in EtOH and MCH for the n* excitation at 77 K. Emission spectra of anthril recorded using various time windows in EtOH (B) and in MCH (C) at 77 K upon n* excitation. Excitation wavelengths and various time windows are depicted in the legends.

Alike the room temperature studies, the emission spectra of anthril in the glassy matrices reveals significant modification in its band pattern upon exciting the sample at its * absorption band (ex = 290 nm). The full emission spectra of anthril consists of multiple emissions in the wavelength range 320 – 570 nm upon * excitation in both the frozen matrices as depicted in Figure 6A. A new higher energy structured emission is observed in the wavelength range 310-380 nm in both the glassy matrices, together with the two aforesaid emissions, one fluorescence (neartrans singlet species) and one phosphorescence (trans triplet species), observed upon n* excitation (Figure 5). Consistent with the room temperature emission studies on * excitation and the 17 ACS Paragon Plus Environment


Page 18 of 30

quantum chemical calculations of anthril in different electronic states (see later) divulge that the higher energy structured emission with band maximum at 355 nm in EtOH and 340 nm in MCH is ascribed to the S1 fluorescence coming from the relaxed skew conformer. Moreover, a change in the polarity of the matrices show appreciable shift in the emission maximum of this higher energy band indicating that the relaxed skew conformer has high dipolar character. For the proper assignment of the fluorescence and phosphorescence emissions upon * excitation, we have performed the emission studies of anthril using different time windows (Figures 6B and 6C) in both the rigid matrices (EtOH and MCH), similar to n* excitation. In both EtOH and MCH solid matrices, within 0-0.02 ms time window, anthril gives two distinct structured fluorescence bands in the range of 330-400 nm and 400-550 nm, respectively. Both the fluorescence bands resemble well with the emission studies at room temperature, indicating that the emissions are coming from relaxed skew and the near-trans conformers. However, when we set a longer time window (0.1-100 ms), anthril gives structured emission within the range of 400-600 nm in both the matrices, confirming the emission to be exclusively phosphorescence. An isoemissive point in the TRANES studies for phosphorescence and two distinct phosphorescence lifetimes (to be discussed in the upcoming section) reveal that the 400-600 nm phosphorescence consists of two discrete structured phosphorescence bands; lower energy one in the range 470-600 nm and the higher energy band within 400-470 nm. The triplet state (T1) PEC in the quantum chemical calculations also suggests that the two stable triplet conformers (trans and cis) are responsible for these two phosphorescences (see later). Among these two, the lower energy phosphorescence band (470-600 nm) appears upon the n* excitation as well (see above) and suggests the trans conformer of anthril ( = 180) in the T1 state. The triplet emission in the range 400-470 nm in both EtOH and MCH matrices is only observed upon exciting the system at its * absorption band. Similar to fluorescence studies upon * excitation, the higher energy phosphorescence emission band is 18 ACS Paragon Plus Environment

Page 19 of 30

ascribed to originate from the cis conformer (intercarbonyl dihedral angle 0) in the first triplet state. Reasonable red shift (~ 10 nm) in the higher energy phosphorescence band maximum with increasing the solvent polarity points to an appreciable dipole moment of the cis conformer in the T1

(A)

EtOH MCH ex = 290 nm

350

400

450

500

Wavelength (nm)

MCH

325

350

375

Wavelength (nm)

350

400

400

450

500

550

Wavelength (nm)

600

0 - 100 ms 0 - 0.02 ms 0.1 - 100 ms

ex = 290 nm

400

450

0 - 100 ms 0 - 0.02 ms 0.1 - 100 ms

ex = 290 nm

350

550

(C)

EtOH

(B)

Emission intensity

Emission intensity

300

Emission intensity

state.

Fl. Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

550

Wavelength (nm)

600

Figure 6. (A) Emission spectra of anthril in EtOH and MCH for the * excitation at 77 K. Emission spectra of anthril recorded using various time windows in EtOH (B) and in MCH (C) at 77 K upon * excitation. Excitation wavelengths and various time windows are depicted in the legends.

To confirm that the two phosphorescence bands of anthril are coming from two different species in the triplet state in both the frozen matrices, we have performed the phosphorescence decay 19 ACS Paragon Plus Environment


Page 20 of 30

measurements for single or dual phosphorescence bands upon excitation at the n* and * bands respectively at liquid nitrogen temperature. Upon excitation at 380 nm (n* band), the phosphorescence decay profiles at different monitoring wavelengths within the lower energy phosphorescence band (490-600 nm) are mono-exponential yielding phosphorescence lifetime 3.1 ± 0.3 ms in EtOH and 2.9 ± 0.3 ms in MCH matrices (Figures 7A and 7B). Single as well as identical lifetime values in the glassy matrices of anthril within the 490-600 nm structured phosphorescence band implies that the band originates from a single triplet species, i.e., the trans conformer of the fluorophore. Upon excitation at the * absorption band, the phosphorescence lifetime measurements for low energy phosphorescence band in the frozen matrices also give single phosphorescence lifetime (3.3 ± 0.3 ms in EtOH and 3.2 ± 0.3 ms in MCH) as displayed in Figures 7C and 7D. However, on monitoring the higher energy phosphorescence band (400-470 nm), the decay profile gives a very long phosphorescence lifetime; 5000 ± 100 ms in EtOH and 4200 ± 100 ms in MCH (Figures 7E and 7F for EtOH and MCH matrices). A comparison with the emissions of anthril at 77 K and phosphorescence decay measurements upon n* excitation implies that the shorter lived triplet species correspond to the trans conformer in both the matrices. The very long lived species in the frozen matrices is assigned to the triplet species of anthril having cis geometry.


Page 21 of 30

(B)

EtOH

(D)

10

15

Time (ms)

 = 2.93 ± 0.3 ms 2

R = 0.993

0

20

5

(E)

MCH

 = 3.2 ± 0.3 ms 2

R = 0.996

5

10

Time (ms)

15

20

10

15

Time (ms)

EtOH

20

 = 3.3 ± 0.3 ms 2

R = 0.999

0

5

(F)

ex = 290 nm

10

 = 5000 ± 100 ms 2

R = 0.996

0

3000

6000

9000

Time (ms)

12000

15000

15

Time (ms)

20

MCH ex = 290 nm

em = 425 nm

Counts

ex = 290 nm em = 510 nm

0

ex = 290 nm em = 520 nm

Counts

Counts

Counts

2

R = 0.999

5

EtOH

ex = 380 nm em = 510 nm

 = 3.1 ± 0.3 ms

0

(C)

MCH

ex = 380 nm em = 510 nm

em = 425 nm

Counts

(A)

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


 = 4200 ± 100 ms 2

R = 0.998

0

3000

6000

9000

12000

Time (ms)

15000

Figure 7. Phosphorescence decay profiles (A to F) of anthril in EtOH and MCH matrices. Excitation and monitoring wavelengths are depicted in the legends.

Similar to the room temperature TRES and TRANES studies for the two S1 fluorescence bands, we have also performed the TRES and TRANES studies of anthril in EtOH frozen matrix at 77 K to confirm the coexistence of the two aforesaid phosphorescent species. As a sample presentation, Figure 8 depicts the TRES and TRANES spectra of anthril in EtOH matrix at various preset time windows upon excitation at 290 nm. The TRANES spectra of anthril shows a single isoemissive point appearing at 478 nm; confirming that the 400-600 nm phosphorescence emission consists of two distinct phosphorescence bands, originating from the coexisting emissive species in the T1 state. Quantum chemical calculations in the triplet state corroborate the involvement of two distinct triplet species for the two phosphorescence bands (see below).


(A)

TRES

Time window (ms) 0.05-0.5 0.05-1 0.1-5 0.5-20 2-20 2-30 6-30 6-60 12-60 25-60

Emission Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Emission intensity


(B)

Page 22 of 30

Time window (ms)

TRANES

0.05-0.5 0.05-1 0.1-5 0.5-20 2-20 2-30 6-30 6-60 12-60 25-60 100-450

Isoemissive point

400

450

500

550

Wavelength (nm)

600

400

450

500

550

Wavelength (nm)

600

Figure 8. Time resolved emission spectra (A) and time resolved area normalized emission spectra (B) of anthril in a rigid EtOH matrix at 77 K (ex = 290 nm). Various pre-set time windows for TRES are depicted in the legends.

3.5. Quantum Chemical Calculations The quantum chemical calculations of anthril have been performed using density functional theory with relatively simpler functional (B3LYP) in comparison to the theoretical calculations on 2,2′-pyridil (CAM-B3LYP).14 Due to our computational limitation and the bulkiness of the anthril molecule, we had to opt for this compromization for the assignment of the multiple fluorescence and phosphorescence emissions. Under this situation, we are inclined more towards getting the number of fluorescence/s and phosphorescence/s corresponding to different species in terms of torsional isomers and/or electronic states, rather than the energy of the corresponding emissions. The ground state global energy minimized structure (S0) and dipole moment (4.55 D) of anthril are obtained using density functional theory (DFT) with B3LYP functional and 6-311++G** basis set using the Gaussian 09 software.26-30 The calculations reveal that the dihedral angle between the two carbonyl groups () of anthril is 81.0 at its ground state energy optimized geometry, suggesting the geometry of the fluorophore in the ground state is skew. The position of n* absorption band maximum of anthril relative to that of benzil, -naphthil, -furil, 2,2′-pyridil etc. also suggests the ground state  22 ACS Paragon Plus Environment

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


value of anthril to be close to 2,2′-pyridil ( = 80.5) (see the section of absorption studies). We have constructed the potential energy curves (PECs) of anthril in various electronic states (S1, S2, S3 and T1) at various intercarbonyl dihedral angles (0 to 180) to rationalize the fluoremetric and phosphorimetric observations. Figure 9 represents the variation of potential energies of anthril at different electronic states as a function of intercarbonyl dihedral angle in vacuum phase. The PEC of the ground state of anthril suggests that the fluorophore exists exclusively in a single geometry, i.e., skew form. The potential energy curves in the excited singlet states (S1, S2 and S3) reveal close proximity of energies at all  values. The figure, therefore, predicts that upon exciting the molecule at the higher energy, it undergoes an effortless passage to its S1 state. The molecule in the S1 state can then exhibit barrierless torsional motion to attain either the global minimum for the near-trans geometry ( = 169) or the local minimum having relaxed skew geometry ( = 10). Thus, the PEC of anthril in the S1 state suggests that the two fluorescence bands originate from the two conformers i.e., near-trans and relaxed skew in the S1 state. Dipole moments of these two conformers in the S1 state are calculated to be 0.98 D and 6.1 D respectively from TDDFT calculations. Experimental TRANES studies also establish the coexistence of the two conformers in the S1 state (see above). Based on the experiments supported by the simulated PECs, it is pertinent to mention here that anthril does not exhibit any emission from higher excited state because of the proximity of the S3, S2 and S1 states. Further, the triplet state calculations support that the two phosphorescence emissions come from the two distinct triplet species having geometries close to the singlet emissive species, i.e., trans ( = 180) and cis ( = 0). The ground and excited state quantum chemical calculations of anthril, thus, give a qualitative support to the assignments of the different emissions from the S1 and T1 states. The photophysical behavior of anthril is found to go parallel to that of some other recently studied 1,2-dicarbonyl compounds, namely, -furil and 2,2′-pyridil, but differs markedly from benzil and α-naphthil.11-14 23 ACS Paragon Plus Environment


270

Energy (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

240 S3 S2 S1

210 180

T1

150 15 0

S0

0

20 40 60 80 100 120 140 160 180

Dihedral angle (degree)

Figure 9. Potential energy curves (PECs) of S0, S1, S2, S3 and T1 states of anthril with the variation of intercarbonyl dihedral angles.

4. CONCLUSION The present work unveils the multiple emissions originating from the different excited state conformers of anthril in ethanol and methylcyclohexane at room temperature fluid phase and cryogenic (77 K) glassy matrices. The vivid steady state and time resolved fluorometric studies of anthril at room temperature yield single or dual fluorescence bands depending on whether it is excited at the n* or * absorption band. The lower energy emission band corresponds to the neartrans and the higher energy band to the relaxed skew geometries of anthril in the first excited singlet state. Cryogenic emission studies and phosphorescence lifetime measurements established two phosphorescence emissions, originating from the triplet (T1) state trans and cis geometries. The TRES and TRANES studies for both the fluorescence and phosphorescence bands substantiate the coexistence of two emitting species in the S1 as well as T1 states. Quantum chemical calculations for the PECs in different electronic states provide a qualitative theoretical support for the assignment of the multiple emissions of anthril. The photophysical studies along with the quantum chemical calculations confirm that anthril does not shows any emission from higher excited state (S2). In this 24 ACS Paragon Plus Environment

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


regard, anthril photophysically behaves, more like -furil and 2,2′-pyridil in contrast to benzil and naphthil where S2 emission is observed. Therefore, the present work concludes that the observation of S2 emission is truly system dependent and not a general phenomenon for the series of 1,2dicarbonyl compounds.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Financial

supports

from

the

Department

of

Science

and

Technology

(Project

No.

EMR/2016/001087), Government of India, is gratefully acknowledged. P.K. thanks C.S.I.R., Govt. of India for his Research Fellowship. Authors sincerely thank Dr. A. K. Chandra of Department of Chemistry, North-Eastern Hill University, India for necessary help in the quantum chemical calculations of anthril.



Page 26 of 30

REFERENCES (1) Arnett, J.; McGlynn, S. P. Photorotamerism of Aromatic α-Dicarbonyls. J. Phys. Chem. 1975, 79, 626-629. (2) Leonard, N. J.; Mader, P. M. The Influences of Steric Configuration on the Ultraviolet Absorption of 1,2-Diketones. J. Am. Chem. Soc. 1950, 72, 5388-5397. (3) Morantz, D. J.; Wright, A. J. C. Structures of the Excited States of Benzil and Related Dicarbonyl Molecules, J. Chem. Phys. 1971, 54, 692-697. (4) Miyasaka, H.; Mataga, N. Femtosecond Laser Photolysis Studies on the Conformation Change of Benzil in Solutions. In: Harris, C. B.; Ippen, E. P.; Mourou, G. A.; Zewail A. H. (eds), Ultrafast Phenomena VII. Vol 53. Springer, Berlin, Heidelberg, 1990, p 501. (5) Roy, D. S.; Bhattacharyya, K.; Bera, S. C.; Chowdhury, M. Conformational Relaxation in the Excited Electronic States of Benzil and Naphthyl. Chem. Phys. Lett. 1980, 69, 134-140. (6) Bera, S. C.; Karmakar, B.; Chowdhury, M. Molecular Conformation and Spectra of Mesitil. Chem. Phys. Lett. 1974, 27, 397-400. (7) Singh, A. K.; Palit, D. K. Excited-State Dynamics and Photophysics of 2,2′-Furil. Chem. Phys. Lett. 2002, 357, 173-180. (8) Leonard, N. J.; Blout, E. R. The Ultraviolet Absorption Spectra of Hindered Benzils. J. Am. Chem. Soc. 1950, 72, 484-487. (9) Shimizu, N.; Bartlett, P. D. Photooxidation of Olefins Sensitized by Alpha-Diketones and by Benzophenone. A Practical Epoxidation Method with Biacetyl, J. Am. Chem. Soc. 1976, 98, 41934200.


Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Degen, J.; Hellwig, M.; Henle, T. 1,2-Dicarbonyl Compounds in Commonly Consumed Foods. J. Agric. Food Chem. 2012, 60, 7071-7079. (11) Bhattacharya, B.; Jana, B.; Bose, D.; Chattopadhyay, N. Multiple Emissions of Benzil at Room Temperature and 77 K and their Assignments from ab-Initio Quantum Chemical Calculations. J. Chem. Phys. 2011, 134, 044535(1-7). (12) Jana, B.; Chattopadhyay, N. Multiple Emissions of α-Naphthil: Fluorescence from S2 State. J. Phys. Chem. A 2012, 116, 7836-7841. (13) Kundu, P.; Chattopadhyay, N. Photophysics of -Furil at Room Temperature and 77 K: Spectroscopic and Quantum Chemical Studies. J. Chem. Phys. 2016, 144, 234317(1-10). (14) Kundu, P.; Ghosh, S.; Chattopadhyay, N. Exploration of Photophysics of 2,2-Pyridil at Room Temperature and 77 K: A Combined Spectroscopic and Quantum Chemical Approach. Photochem. Photobiol. Sci. 2017, 16, 159-169. (15) Bhattacharyya, K.; Chowdhury, M. Solvent Shift and Excited State Geometries of Benzyl. J. Photochem. 1986, 33, 61-65. (16) Molnar, J.; Foldeak, S.; Hegyes, P.; Schneider, B.; Holland, I. B. New Plasmid Curing Compounds. Anthril and Phenathril Derivatives. Biochem. Pharmacol. 1979, 28, 261-265. (17) Chattopadhyay, N.; Serpa, C.; Arnaut, L. G.; Formosinho, S. J. Coexistence of Two Triplets for the TICT Probe DMABN in Polar Solvents: An Experimental Evidence. Helv. Chim. Acta. 2002, 85, 19-26. (18) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd Edn. 2006, Springer, New York.



Page 28 of 30

(19) Koti, A. S. R.; Periasamy, N. Application of Time Resolved Area Normalized Emission Spectroscopy to Multicomponent Systems. J. Chem. Phys. 2001, 115, 7094-7099. (20) Koti, A. S. R.; Krishna, M. M. G.: Periasamy, N. Time-Resolved Area-Normalized Emission Spectroscopy (TRANES): A Novel Method for Confirming Emission from Two Excited States. J. Phys. Chem. A 2001, 105, 1767-1771. (21) Ghosh, D.; Chattopadhyay, N. Characterization of the Excimers of Poly(N-vinylcarbazole) using TRANES. J. Lumin. 2011, 131, 2207-2211. (22) Periasamy, N.; Koti, A. S. R. Time Resolved Fluorescence Spectroscopy: TRES and TRANES. Proc Indian Natn Sci Acad. 2003, 69A, 41-48. (23) Becker, H. D.; Sorensen, H.; Hammarberg, E. 9,9′-Anthril (Di-9-anthrylethanedione). Tetrahedron Lett. 1989, 30, 989-992. (24) Kundu, P.; Ghosh, S.; Chattopadhyay, N. Exploration of the Binding Interaction of a Potential Nervous System Stimulant with Calf-thymus DNA and Dissociation of the Drug-DNA Complex by Detergent Sequestration. Phys. Chem. Chem. Phys. 2015, 17, 17699-17709. (25) Ghosh, S.; Kundu, P.; Chattopadhyay, N. DNA Induced Sequestration of a Bioactive Cationic Fluorophore from the Lipid Environment: A Spectroscopic Investigation. J. Photochem. Photobiol. B 2016, 154, 118-125. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (27) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648-5652.


Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785-789. (29) Nazeeruddin, K. M.; Angelis, D. F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gratzel, M. Combined Experimental and DFT-TDDFT Computational Study of Hotoelectrochemical Cell Ruthenium Sensitizers. J. Am. Chem. Soc. 2005, 127, 16835-16847. (30) Krishnan. R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self-consistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wave Functions. J. Chem. Phys. 1980, 72, 650-654. (31) Purkayastha, P.; Chattopadhyay. N. Role of Rotamerisation and Excited State intramolecular Proton Transfer in the Photophysics of 2-(2′-Hydroxyphenyl)Benzoxazole, 2-(2′Hydroxyphenyl)Benzimidazole and 2-(2′-Hydroxyphenyl)Benzothiazole: A Theoretical Study. Phys. Chem. Chem. Phys. 2000, 2, 203-210. (32) Sarkar, A.; Chakravorti, S. Photo-Rotamerism of 2,2′-Pyridil in Different Environments: A Guest for Geometry in Excited States. J. Lumin. 1996, 69, 161-168. (33) Kundu, P.; Ghosh, S.; Karmakar, R.; Maiti, G.; Chattopadhyay, N. Impact of Structural Modification on the Photophysical Response of Benzoquinoline Fluorophores. J. Fluores. 2016, 26, 845-854. (34) Kundu, P.; Banerjee, D.; Maiti, G.; Chattopadhyay, N. Dehydrogenation Induced Inhibition of Intramolecular Charge Transfer in Substituted Pyrazoline Analogue. Phys. Chem. Chem. Phys. 2017, 19, 11937-11946.



Page 30 of 30

TOC graphics


Photophysics of Anthril in Fluids and Glassy Matrixes - ACS Publications

Recommend Documents