Article pubs.acs.org/JPCA
Stereoelectronics of the Hydrogen-Bond-Induced Fluorescence Quenching of 3‑Aminofluorenones with Alcohols Isaac G. Alty and Christopher J. Abelt* Department of Chemistry, College of William and Mary, Williamsburg, Virginia 23185, United States S Supporting Information *
ABSTRACT: Two derivatives of 3-amino-9-fluorenone (1) bearing one (2) and two methyl (3) groups flanking the carbonyl group are prepared. Comparison of their photophysical properties show that all suffer efficient radiationless deactivation in the presence of alcohols. Preferential solvation studies with mono alcohols reveal that a single H-bonding interaction quenches the excited states of 1 and 2, but not that of 3. In contrast, a single molecule of ethylene glycol quenches all three. These results are interpreted in a quenching mechanism similar to one proposed by Inoue and co-workers, but where an out-of-plane H-bond with the carbonyl group gives rise to an emissive species, while an in-plane H-bond results in quenching.
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plane with a π-electron pair. The latter is thought to mostly quench, while the former mostly fluoresces. A doubly Hbonded excited state, with both in-plane and out-of-plane Hbonds, also mostly quenches. Transient absorption experiments have corroborated the existence of two distinct, singly Hbonded excited states of 2-piperidinoanthraquinone.10 The shortening of the fluorescence lifetimes results from an increase in the nonradiative decay rate due to H-bond-induced quenching. In connection with our investigation of the H-bond-induced quenching in 6-propionyl-2-dimethylaminonaphthalene (PRODAN) derivatives,11 we were interested in examining the quenching process in aminofluorenones in greater depth. In particular, we sought to determine the validity of this model through the synthesis and testing of molecules that restrict the formation of one or more in-plane hydrogen bonds to the lone pairs of the carbonyl oxygen in 3-aminofluoren-9-one. We took advantage of the proximity of the aromatic ring system to the carbonyl and used it as a scaffold for the substitution of methyl groups ortho to the carbonyl to sterically block the formation of in-plane hydrogen bonds. As methyl groups are not polar, they should not form intramolecular hydrogen bonds to the carbonyl oxygen. Furthermore, the methyl groups are readily incorporated into a general synthesis of 3-aminofluoren-9-ones, and they should not interfere with the photophysics of the fluorophore except through the blocking of hydrogen bonds. Finally, they are large enough to effectively block the in-plane hydrogen bond, but not bulky enough to also hinder the formation of the out-of-plane H-bond. Derivatives 2 and 3 were synthesized to selectively block one or both in-plane hydrogen bonds to the carbonyl oxygen (Figure 1). This paper reports the synthesis of 3-aminofluoren-9-one derivatives and examines
INTRODUCTION The fluorescence of aminofluorenones has received considerable attention.1−8 These fluorophores show strong emission from an intramolecular charge-transfer (ICT) excited state. Electron density is transferred from the amino group to the carbonyl group. Alcohols efficiently quench the fluorescence of these compounds through the formation of H-bonds with the electron-rich carbonyl oxygen. The high frequencies of the Hbonds allow for the strong coupling of the excited state with the ground state thereby promoting radiationless deactivation. While this coupling speaks to the general nature of the quenching mechanism, specific details of the process are less well established. These details include the number of H-bonds and their stereochemistry with respect to the carbonyl oxygen atom. Inoue and co-workers have studied the molecular mechanism of the fluorescence quenching of aminofluorenones3 and related aminoanthraquinones9,10 by alcohols. They have interpreted the quenching behavior of these compounds in terms of the photophysical processes shown in Scheme 1 for the 3-amino derivative (1). In this mechanism, there are two excited states with a single H-bonding interaction: one that is in-plane with a nonbonding electron pair and one that is out-ofScheme 1. H-Bonding Interactions between the Singlet Excited State of 1 and Alcohols
Received: March 6, 2017 Revised: June 22, 2017 Published: June 23, 2017 © XXXX American Chemical Society
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DOI: 10.1021/acs.jpca.7b02142 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A their fluorescence properties in the presence of hydrogen-bond donating solvents.
On the other hand, this band only increases by 7% in going from THF to EG. The third band shows strong bathochromic and hyperchromic shifts as a result of charge-transfer. The effects of hydroxylic solvents on the shifts in 1 have been interpreted in terms of a ground-state H-bonded complex. The association constant for the complex of 1 with ethanol in benzene was determined to be 0.90 M−1.3 The corresponding association constants for 1−3 with ethanol and isopropanol in toluene and ethylene glycol in tetrahydrofuran are reported in Table 2. The relatively small equilibrium constants ensure most of the fluorenone derivatives do not form ground-state complexes at low alcohol concentrations. The diminished bathochromism in 2 and 3 indicates that there is less charge-transfer character in these compounds compared to 1. The flanking methyl groups not only provide poor dielectric stabilization of the partial negative charge on the neighboring carbonyl oxygen atom, but they also hinder the approach of solvent molecules. The effect is especially pronounced in 3 with its two flanking methyl groups. Fluorescence. Relative quantum yields for 1−3 in toluene were determined using anthracene as a standard (Φ = 0.30): 1, 0.24 ± 0.02; 2, 0.18 ± 0.02; 3, 0.15 ± 0.02. These values compare reasonably to the literature value of 0.16 for 1.5 They show that the extra methyl groups in 2 and 3 do not significantly alter the excited state properties of the aminofluorenone chromophore in an apolar aprotic solvent such as toluene. Note that the quantum yields in cyclohexane are just over an order of magnitude lower because intersystem crossing to the triplet state becomes more efficient. Because aminofluorenone possess an ICT excited state, their emission shows solvatochromism. The relative degree of solvatochromism for 1−3 was determined and the results are displayed in Figure 2. In the plots the emission center-of-mass is used to characterize the fluorescence position and Reichardt’s ET(30) parameter is used to define the solvent polarity.12 This parameter reflects both polarity and specific interactions through H-bonds. The best-fit lines for 1−3 all have negative slope indicative of solvatochromic behavior. The slopes have slightly different magnitudes: 1, −130 ± 11; 2, −121 ± 12; 3, −99 ± 7. As seen with the absorption behavior, the magnitude of the solvatochromism is reduced by adding flanking methyl
Figure 1. Structures of 3-amino-9-fluorenone compounds 1−3.
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EXPERIMENTAL METHODS See the Supporting Information. RESULTS AND DISCUSSION Synthesis. The three aminofluorenone compounds were prepared in several steps shown in Scheme 2. All of the preparations involved a Suzuki coupling that ultimately produced a biphenyl derivative bearing an ortho carboxylic acid group (1a−3a). Intramolecular acylation with polyphosphoric acid gave the fluorenone skeleton. In the case of 2a, cyclization can only give one product, whereas for 3a cyclization occurs ortho to a methyl substituent instead of a nitro group. The last step was the selective reduction of the nitro group to the amine while keeping the carbonyl group intact. The syntheses are reported in the Supporting Information. Absorption. The UV/vis absorption spectra for 1−3 are shown in the Supporting Information (Figures S1−S9). The absorption maxima are presented in Table 1 in order of increasing polarity and H-bonding ability of the solvent (left to right). The maxima for 1 are in agreement with previous reports.1,2,8 All bands shift to longer wavelengths in each row. For the first two bands the three compounds have comparable wavelength maxima in each solvent. However, for the third band the maxima diverge along the row. The magnitude of the bathochromic shift decreases in going from 1 → 3. All bands also show hyperchromic shifts in the solvent series (Figures S1−S9). The most significant increases are seen in the third band in going from toluene to either mono alcohol (∼63%). Scheme 2. Preparation of 1−3
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DOI: 10.1021/acs.jpca.7b02142 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Table 1. Absorption Maxima (nm) of 1−3 in Various Solvents Tol band 1 band 2 band 3 a
a
iPrOH
THF
EtOH
EG
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
345 357 392
349 360 388
350 362 390a
349 362 406
352 364 401
354 366 399
360 372 428
361 374 422
− 373 417
360 371 428
362 374 421
− 374 416
362 372 432
364 375 424
369 420
Shoulder.
Table 2. Equilibrium Constants (M−1) for the Ground-State Complexes of 1−3 with Various Alcoholsa 1 2 3
Tol/EtOH
Tol/iPrOH
THF/EG
0.61 (0.01) 0.23 (0.04) 0.13 (0.05)
0.77 (0.02) 0.33 (0.05) 0.25 (0.02)
0.19 (0.09) 0.20 (0.03) 0.20 (0.04)
significant contribution from preassociation. In Figures 4 and 5, the alcohol concentrations that give rise to 10% ground-state complex formation are 1.4 mol % (11th curve) and 4.4 mol % (13th curve), respectively. Finally, no attempt is made to correlate the ground-state H-bonding equilibrium constants with the kinetic quenching of the excited states because the former are ratios of association and dissociation rates whereas the latter are only association rates. The florescence behavior of 1−3 in the preferential solvation studies was evaluated using the solvent-exchange model of Rosés and Bosch.13 We have applied this model in several previous papers.11,14,15 The mathematical description of the model is provided in the Supporting Information. The model assumes three emitting species with zero, one and two H-bonds to the carbonyl oxygen, respectively. Application of the model allows the relative quantum yield and the fluorescence centerof-mass of the singly-H-bonded species to be elucidated. The results of this analysis for 1−3 in the three binary solvents are presented in Table 3 where Y0, Y1, and Y2 refer to the emitting species with the number of H-bonds given by the subscript. Table 3 shows some of the expected behavior that results from sequential substitution of flanking methyl groups in comparing the behavior in the pure aprotic solvent to the pure protic solvent (Y0 vs Y2). The magnitude of the quenching in the protic solvents decreases going from 1 to 3 as shown by the relative quantum yields (Y2), 0.05, 0.06 and 0.09, averaged over the three binary systems. Nevertheless, these small values indicate that all three suffer significant quenching in the pure protic solvents. Similarly, the magnitude of the quenching for the singly H-bonded fluorophore (Y1) decreases going from 1 to 2 in the toluene/alcohol mixtures (0.43 and 0.56), but with 3 the singly-H-bonded species emits as or more strongly as the non-H-bonded species (1.04). At first glance this result could be interpreted as the methyl groups completely blocking the alcohol from the carbonyl group. However, two other results show that there is a significant interaction between the fluorophore and alcohol. The first is that all of the singly-Hbonded species suffer a decrease (Y0 − Y1) in the fluorescence center-of-mass due to H-bonding (1260, 1110, and 900 cm−1, respectively). The second is that the singly H-bonded species for all three compounds behave similarly in tetrahydrofuran/ ethylene glycol. In particular, they show similar shifts (1140, 1240, and 1100 cm−1, respectively) and relative quantum yields (0.20, 0.18, and 0.25, respectively). These results are consistent with most of the mechanism put forth by Inoue and co-workers.3 There appear to be two anisotropic single H-bonds with the excited state of 1, each with differing quenching characteristics. Multiple H-bonds can form with the excited state of 1 which give rise to more efficient quenching. In contrast with Inoue’s mechanism the behavior with 3 suggests that it is the out-of-plane H-bonded complex that is emissive, and by extension, it is the in-plane H-bonded complex that undergoes radiationless deactivation. The flanking methyl groups in 3 hinder the in-plane H-bonding thereby
a
Numbers in parentheses are standard deviations from determinations at 374 and 423 nm.
Figure 2. Plot of ṽCM vs ET(30) for 1 (◊, - - -), 2 (□, ···) and 3 (○, − − − ). Solvents (in order of ET(30) values) are toluene, ethyl ether, chlorobenzene, tetrahydrofuran, ethyl acetate, methylene chloride, acetone, dimethyl sulfoxide, acetonitrile, ethanol, methanol, and ethylene glycol.
groups. The effect is much stronger for 3 than for 2. Again, this result reflects both the apolar environment of the methyl groups and their blocking of the approach by solvent molecules. Preferential solvation studies were conducted to elucidate the effect of H-bonding. Three binary solvent systems, each with an aprotic and protic combination, were employed: toluene/ isopropanol, toluene/ethanol and tetrahydrofuran/ethylene glycol. In these experiments the aminofluorenone fluorescence is monitored as a function of the mole fraction of the protic component. Representative results in toluene/ethanol for 1 and 3 are shown in Figures 3 and 4. The remaining titrations are in the Supporting Information (Figures S10−S16). As the concentration of the hydroxylic solvent increases, the fluorescence center-of-mass shifts to lower energy. Hydroxylic solvents also give rise to significant quenching. The most noticeable difference in these figures is that 1 is quenched immediately with ethanol, but 3 is not. The effect of the alcohol at low concentrations is dynamic. The extremely small concentrations of alcohol used and the small association constants for the ground-state complex (Table 2) rule out any C
DOI: 10.1021/acs.jpca.7b02142 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
Figure 3. Fluorescence spectra of 24 μM solutions of 1 in toluene/ethanol mixtures. [EtOH] (mol %): 0, 0.09, 0.18, 0.27, 0.36, 0.45, 0.54, 0.63, 0.72, 0.90, 1.4, 1.8, 2.7, 4.4, 8.4, 15, 22, 31, 42, 52, 59, 70, 75, 82, 90, and 100. λex = 366 nm.
Figure 4. Fluorescence spectra of 48 μM solutions of 3 in toluene/ethanol mixtures. [EtOH] (mol %): 0, 0.09, 0.18, 0.27, 0.36, 0.45, 0.54, 0.63, 0.72, 0.90, 1.3, 1.8, 2.7, 4.4, 8.4, 15, 21, 31, 42, 52, 59, 70, 75, 82, 90, and 100. λex = 366 nm.
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CONCLUSIONS
Three aminofluorenone derivatives all show efficient fluorescence quenching in alcohol solvents. Sequential blocking the carbonyl group with methyl groups at the 1- and 8-positions have little effect on the ultimate level of quenching. However, the methyl groups strongly affect the quenching from a single H-bond as revealed through preferential solvation studies. Flanking both sides of the carbonyl guides the H-bonding interaction to occur from the out-of-plane orientation and the excited state is not quenched. As a result, we suggest that the anisotropy proposed by Inoue and co-workers for H-bond induced fluorescence quenching in 3-aminofluorenone be reversed: the in-plane H-bond gives quenching while the outof-plane H-bond is emissive.
Figure 5. Proposed structure for the nonemissive, doubly H-bonded single excited state of 3.
directing the H-bonding out-of-plane. The singly H-bonded excited state in 3 is not quenched with ethanol or isopropanol. In contrast, the doubly H-bonded excited state in 3 is quenched efficiently. This species can be formed not only with moderate concentrations of either mono alcohol but also with a single molecule of ethylene glycol as shown in Figure 5. Note that the in-plane interaction in this case must be nearly colinear with the carbonyl group. D
DOI: 10.1021/acs.jpca.7b02142 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Table 3. Preferential Solvation Parameters for 1−3 in Binary Solventsa Tol/EtOH Y0 1
Y1 Y2
2
Y0 Y1 Y2
3
Y0 Y1 Y2
a
Tol/iPrOH
THF/EG
Qrel
ṽCM (cm−1)
Qrel
ṽCM (cm−1)
Qrel
ṽCM (cm−1)
1.00 (−) 0.42 (0.01) 0.05 (0.01) 1.00 (−) 0.53 (0.07) 0.07 (0.01) 1.00 (−) 1.03 (0.02) 0.10 (0.01)
18920 (20) 17700 (10) 16580 (40) 19020 (50) 17830 (50) 16630 (50) 19210 (20) 18270 (70) 17050 (10)
1.00 (−) 0.44 (0.04) 0.09 (0.02) 1.00 (−) 0.58 (0.03) 0.09 (0.01) 0.97 (0.02) 1.05 (0.05) 0.14 (0.01)
18960 (60) 17670 (110) 16670 (10) 18990 (30) 17960 (60) 16840 (160) 19200 (10) 18350 (80) 17100 (10)
1.00 (−) 0.20 (0.01) 0.02 (0.01) 1.00 (−) 0.18 (0.02) 0.03 (0.01) 1.00 (−) 0.25 (0.02) 0.04 (0.01)
18470 (10) 17330 (10) 16340 (10) 18610 (30) 17370 (30) 16360 (70) 18620 (60) 17520 (60) 17130 (190)
Numbers in parentheses are the standard deviations from two or three experiments.
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fluorenones through Hydrogen Bonds with Alcohols. J. Phys. Chem. A 1998, 102, 8657−8663. (4) Biczók, L.; Bérces, T.; Inoue, H. Effects of Molecular Structure and Hydrogen Bonding on the Radiationless Deactivation of Singlet Excited Fluorenone Derivatives. J. Phys. Chem. A 1999, 103, 3837− 3842. (5) Biczók, L.; Bérces, T.; Yatsuhashi, T.; Tachibana, H.; Inoue, H. The Role of Intersystem Crossing in the Deactivation of the Singlet Excited Aminofluorenones. Phys. Chem. Chem. Phys. 2001, 3, 980−985. (6) Sugita, M.; Shimada, T.; Tachibana, H.; Inoue, H. Factors Controlling the Deactivation Induced by Hydrogen-Bonding Interaction: Steric and Electronic Effects on Dual Anisotropic Relaxation Processes. Phys. Chem. Chem. Phys. 2001, 3, 2012−2017. (7) Morimoito, A.; Yatsuhashi, T.; Shimada, T.; Biczók, L.; Tryk, D. A.; Inoue, H. Radiationless Deactivation of an Intramolecular Charge Transfer Excited State through Hydrogen Bonding: Effect of Molecular Structure and Hard-Soft Anionic Character in the Excited State. J. Phys. Chem. A 2001, 105, 10488−10496. (8) Mondal, J. A.; Samant, V.; Varne, M.; Singh, A. K.; Ghanty, T. K.; Ghosh, H. N.; Palit, D. K. The Role of Hydrogen-Bonding Interactions in the Ultrafast Relaxation Dynamics of the Excited States of 3-and 4Aminofluoren-9-ones. ChemPhysChem 2009, 10, 2995−3012. (9) Yatsuhashi, T.; Inoue, H. Molecular Mechanism of Radiationless Deactivation of Aminoanthraquinones through Intermolecular Hydrogen-Bonding Interaction with Alcohols and Hydroperoxides. J. Phys. Chem. A 1997, 101, 8166−8173. (10) Morimoto, A.; Yatsuhashi, T.; Shimada, T.; Kumazaki, S.; Yoshihara, K.; Inoue, H. Molecular Mechanism of the Intermolecular Hydrogen Bond between 2-Piperidinoanthraquinone and Alcohol in the Excited State: Direct Observation of the Out-of-Plane Mode Interaction with Alcohol by Transient Absorption Studies. J. Phys. Chem. A 2001, 105, 8840−8849. (11) Alty, I. G.; Cheek, D. W.; Chen, T.; Smith, D. B.; Walhout, E. Q.; Abelt, C. J. Intramolecular Hydrogen-Bonding Effects on the Fluorescence of PRODAN Derivatives. J. Phys. Chem. A 2016, 120, 3518−3523. (12) Reichardt, C.; Welton, T. In Solvents and Solvent Effects in Organic Chemistry; Wiley-VCH Verlag GmbH: 2011. (13) Rosés, M.; Ràfols, C.; Ortega, J.; Bosch, E. Solute−Solvent and Solvent−Solvent Interactions in Binary Solvent Mixtures. Part 1. A Comparison of several Preferential Solvation Models for Describing ET (30) Polarity of Bipolar Hydrogen Bond Acceptor-Cosolvent Mixtures. J. Chem. Soc., Perkin Trans. 2 1995, 1607−1615.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b02142. Experimental methods, synthetic details for the preparation of 1−3, description of the preferential solvation analysis, absorption spectra, and fluorescence spectra (PDF)
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AUTHOR INFORMATION
Corresponding Author
*(C.J.A.) E-mail:
[email protected]. ORCID
Christopher J. Abelt: 0000-0002-0123-8390 Author Contributions
The manuscript was written through contributions of both authors who have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society. REFERENCES
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The Journal of Physical Chemistry A (14) Nikitina, Y. Y.; Iqbal, E. S.; Yoon, H. J.; Abelt, C. J. Preferential Solvation in Carbonyl-Twisted PRODAN Derivatives. J. Phys. Chem. A 2013, 117, 9189−9195. (15) Daneri, M.; Abelt, C. J. A Higher-Order Preferential Solvation Model for the Fluorescence of Two PRODAN Derivatives in TolueneAlcohol Mixtures. J. Photochem. Photobiol., A 2015, 310, 106−112.
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DOI: 10.1021/acs.jpca.7b02142 J. Phys. Chem. A XXXX, XXX, XXX−XXX
