Intramolecular Hydrogen-Bonding Effects on the Fluorescence of

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Intramolecular Hydrogen-Bonding Effects on the Fluorescence of PRODAN Derivatives Isaac G Alty, Douglas W. Cheek, Tao Chen, David B. Smith, Emma Q. Walhout, and Christopher John Abelt J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b02398 • Publication Date (Web): 29 Apr 2016 Downloaded from http://pubs.acs.org on May 4, 2016

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The Journal of Physical Chemistry

Intramolecular Hydrogen-Bonding Effects on the Fluorescence of PRODAN Derivatives Isaac G. Alty, Douglas W. Cheek, Tao Chen, David B. Smith, Emma Q. Walhout and Christopher J. Abelt* Department of Chemistry, College of William and Mary, Williamsburg, VA 23185

ABSTRACT

The effects of intramolecular hydrogen-bonding on the fluorescence behavior of three derivatives of 6-propionyl-2-dimethylaminonaphthalene are reported. The H-bonding effects are revealed through comparisons with corresponding reference compounds in which the H-bond donating hydroxyl groups are replaced with methoxy groups. In toluene, intramolecular H-bonding gives rise to a dramatic increase in the fluorescence intensity, but only a slight red-shift in the position. This behavior is attributed to decreased efficiency in intersystem crossing due to an increase in the energy of the n→π* triplet state. The intramolecular H-bond does not induce quenching in acetonitrile.

However, in the presence of a very small concentration of methanol, a dual

intramolecular, intermolecular H-bonding arrangement does lead to partial quenching as revealed by preferential solvation studies.

ACS Paragon Plus Environment

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INTRODUCTION PRODAN (6-propionyl-2-dimethyaminonaphthalene) is well known as a probe of micropolarity.1 It responds to an increase in solvent dipolarity with a red-shift in its fluorescence maximum. This behavior is a result of charge transfer from the amino group to the carbonyl group that gives rise to a larger dipole moment in the excited-state. The ensuing reorganization of polar solvent molecules stabilizes the excited-state and leads to a large Stokes shift. Because the carbonyl group acquires a greater partial negative charge, protic solvents give even larger Stokes shifts due to H-bonding interactions.2-5 As a result, the Stokes shift cannot be exactly correlated with micropolarity.6, 7 In fact, both the Kamlet-Taft solvent polarity/polarizability parameter π* and the acidity parameter α are needed to a give good correlation with the solvent Stokes shifts in a multilinear regression analysis.8 PRODAN derivatives with carbonyl groups that are forced to twist out of the plane of the naphthalene rings show strong quenching in hydroxylic solvents. The magnitude of the quenching can be directly correlated with the H-bond donating ability of the solvent as quantified by Catalán's solvent acidity parameter.9,

10

Preferential solvation studies afforded some insight into the

mechanism of quenching. They suggested that quenching occurs from a doubly H-bonded excitedstate and that a singly H-bonded excited-state was not quenched at all.11, 12 In this paper the fluorescence behavior of the six PRODAN derivatives is studied (Figure 1). Three compounds (1a - 3a) possess hydroxyl groups on the alkyl chains of the ketones that are capable of making five- and six-membered intramolecular H-bonded rings with the carbonyl oxygens. Three corresponding compounds (1b – 3b) have methoxy groups in place of the hydroxyl groups. These reference compounds should mimic the electronic effects of 1a - 3a, but are not capable of intramolecular H-bonding. The impact of H-bonding in 1a - 3a is determined using


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solvatochromism and preferential solvation studies. In the latter case, a two-step exchange solvation model formulated by Rosés and Bosch13-20 is used to elucidate the fluorescence behavior of the mixed intramolecular, intermolecular doubly H-bonded fluorescent excited-state.

Figure 1. Structures of 1-3.

EXPERIMENTAL METHODS See Supporting Information.

RESULTS AND DISCUSSION Compounds 1 – 3 were synthesized using routes A and B in Scheme 1. Both routes culminated in a nucleophilic addition of the 6-dimethylamino-2-naphthyl lithium to a Weinreb amide (route A) or an ester (route B). Esters were used in excess to minimize double addition of the aryl lithium. The Weinreb amide route involved not only preparation of the amide, but also protection of the alcohol group as a methoxymethyl ether, necessitating a deprotection step after nucleophilic addition. Regardless of the route, the final product required chromatography to separate it from side-products, most often 2-(dimethylamino)naphthalene. The syntheses and characterizations of 1 – 3 are documented in the Supporting Information.


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Scheme 1. Preparation of 1-3.

The effects of intramolecular H-bonding on the fluorescence behavior of 1a-3a were probed using two lines of investigation. In the first, the solvatochromism of each fluorophore possessing a hydroxyl group (1a-3a) is compared with those with the corresponding methoxy group (1b-3b). Figure 2 shows an example of such an analysis for 2a vs. 2b. In these plots the emission centerof-mass (ṽCM, see equation S2) is used to define the position of the fluorescence and Reichart’s ET(30) parameter is used to characterize the solvent polarity.21 The corresponding plots for 1 and 3 are in the Supporting Information (Figures S1 and S2). All of the plots are roughly linear. The slopes of the best-fit lines are a measure of the sensitivity of the solvatochromism to solvent polarity. Slopes for all of the plots are collected in Table 1. All of the fluorophores capable of making intramolecular H-bonds show smaller solvatochromism than those that cannot. The smaller sensitivities toward external solvent effects are consistent with the involvement of an additional internal effect like H-bonding.


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23000 22500 22000

wavenumber (cm-1)

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


21500 21000 20500

20000 19500 19000 18500 33

36

39

42

45

48

51

54

ET(30)

Figure 2. Plot of ṽCM vs. ET(30) for 2a (◊,---) and 2b (□,···). Solvents are toluene, chlorobenzene, ethyl ether, methylene chloride, ethyl acetate, acetone, dimethyl sulfoxide, acetonitrile, isopropanol, ethanol and methanol.

Table 1. Slopes of the solvatochromism plots of ṽCM vs. ET(30) for 1 – 3.a

1a 2a 3a

a

slope -178 (8) -162 (6) -148 (8)

1b 2b 3b

slope -213 (8) -201 (8) -169 (7)

Values in parentheses are from the regression analysis of the linear correlation.


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The fluorescence of compounds 1-3 was also investigated through fluorescence titrations using three binary solvent systems. Each solvent system was composed of one aprotic and one protic component: toluene/methanol, acetonitrile/methanol and toluene/isopropanol. Figures 3 and 4 show two examples of these titrations with 3a and 3b in toluene/methanol mixtures.

10 9 toluene 8 7

intensity (A.U.)

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6 5 4

3

methanol

2 1 0 16000

17000

18000

19000

20000

21000

wavenumber

22000

23000

24000

25000

26000

(cm-1)

Figure 3. Fluorescence spectra of 9.7 μM solutions of 3a in toluene/methanol mixtures. [MeOH] (mole %): 0, 0.1, 0.3, 0.4, 0.5, 0.7, 1.3, 1.9, 2.6, 3.2, 6.2, 9.0, 12, 16, 21, 28, 40, 51, 61, 68, 77, 81, 87, 93, 95, 96, 98, 100.


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10 9 8 7

intensity (A.U.)

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6

toluene

5 4 methanol 3 2 1 0 16000

17000

18000

19000

20000

21000

wavenumber

22000

23000

24000

25000

26000

(cm-1)

Figure 4. Fluorescence spectra of 6.7 μM solutions of 3b in toluene/methanol mixtures. [MeOH] (mole %): 0, 0.3, 0.5, 0.8, 1.0, 1.3, 1.9, 2.6, 3.2, 3.8, 5.0, 6.2, 7.3, 10, 12, 14, 16, 21, 28, 40, 51, 61, 68, 77, 81, 87, 93, 95, 96, 98, 100. The fluorescence spectra in each titration are characterized by their emission center-of-masses (ṽCM) and relative quantum yields (Qrel). The quantum yields are normalized by setting the maximum yield in each titration to a value of one (see Supporting Information for a complete description of the data treatment). The relative quantum yields and emission centers-of-masses for 1-3 in the pure components are presented in Tables 2 and 3. Most of the interpretations that follow rely on the behavior of the titration plots and trends in the relative quantum yields, not their absolute values. Nevertheless, because the structures of the six fluorophores are so similar, the maximum yields are thought to have nearly the same absolute value for all compounds. The data


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in the tables offer some support for this interpretation. For example, the relative quantum yields for all compounds in acetonitrile are 1.0 ± 0.01. The quantum yields for 1a-3a in toluene are all 0.84 ± 0.05 (Table 1), irrespective of whether the binary partner is methanol or isopropanol, even though the respective titration plots show dramatically disparate behavior (Figures 3 vs. S16, S3 vs. S5 and S9 vs. S11). In fact, for a given fluorophore, the relative quantum yields in a particular solvent are always independent of the identity of the binary solvent partner (vide infra). The most pronounced structural effect is seen in the toluene/methanol titrations. The fluorescence of 1a-3a is very strong in toluene (Qrel = 0.88 ± 0.01). In contrast, 1b-3b are weakly fluorescent in toluene (Qrel < 0.46). Figures 3, S3 and S9 show that with 1a-3a the intensity of the fluorescence remains relatively unchanged as the fraction of methanol increases. Above a certain methanol fraction the fluorescence is significantly quenched. With 1b-3b, the fluorescence increases greatly as the fraction of methanol increases (Figures 4, S6 and S12). After it reaches maximum intensity, further increases in the proportion of methanol results in quenching. In apolar solvents the weak fluorescence of PRODAN is thought to result from intersystem crossing (ISC) to the triplet state.5 As the polarity of the solvent increases, the energy of the excited singlet state decreases. As a consequence, the difference in energy between the excited singlet state and the triplet state increases, resulting in less efficient ISC.

As the ISC mode of deactivation is

diminished, the fluorescence intensity increases. Maximum intensity is reached in polar solvents like acetonitrile, as with 1-3 (vide supra). With this background in mind, the unusually large fluorescence yields for 1a-3a in toluene must be due to an H-bonding-induced inhibition of ISC and not an H-bonding-lowering of the excited state energy.22 While H-bonding does lead to slightly lower energy excited states according to the ṽCM values, the shifts in the ṽCM values in going from 1a-3a to 1b-3b are too small to account for the high fluorescence yields for 1a-3a in toluene.


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Indeed, for 1a vs. 1b there is no difference in the ṽCM values. For 2a vs. 2b the difference in the ṽCM values is 310 cm-1, but the change for 2a in going from toluene to acetonitrile (polarity corresponding to maximum fluorescence yield) is 1800 cm-1. For 3a vs. 3b the change is slightly greater (430 cm-1), but the change for 3a in going from toluene to acetonitrile is still much larger (1850 cm-1). Intramolecular H-bonds should lower the energy of the carbonyl n-orbital, and therefore increase the energy of the n→π* state.22 The higher energy triplet states will make ISC less favorable. Trends in the relative quantum yield data reveal several specific structural effects on quenching. The quantum yields in methanol decrease in going from 1 → 2 → 3 for both 1a-3a and 1b-3b. The quantum yields for 1a-3a in methanol are 0.38 ± 0.01, 0.22 ± 0.01 and 0.16 ± 0.01, respectively, independent of whether the aprotic component is toluene or acetonitrile. For 1b-3b the corresponding values are 0.30 ± 0.02, 0.22 ± 0.03 and 0.10 ± 0.01, independent of the aprotic partner. This result is consistent with previous work that shows that the degree of quenching in these systems depends on the degree of twist about the carbonyl-naphthalene C-C bond. The gemdimethyl groups in 2 and 3 cause the carbonyl groups to twist out of plane.9 The extra steric factor in 3 compared to 2 (HOCH2 vs. HO) may contribute to the greater quenching in 3, although electronic and rigidity effects may also contribute.


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Table 2. Aprotic and Protic Relative Quantum Yields and Fluorescence Center-of-Masses for 1a3a in several binary solvents.a Tol/MeOH

ACN/MeOH

Tol/iPrOH

Qrel

Qrel

Qrel

Yaprotic 0.89

ṽCM

22720 1.00

(0.02) (100)

1a Yprotic

0.39

19230 0.37

(0.01) (90) Yaprotic 0.87

Yprotic

0.21

Yaprotic 0.89

Yprotic

0.16

19620 0.15

(0.01) (10)

a

20080

22620

(0.04) (10)

19270 0.79

20060

(0.01) (10)

20740 0.83

22590

(0.01) (10)

19640 0.70

(0.01) (50)

22730

(0.01) (10)

20760 0.79

(0.01) (10)

ṽCM

(0.01) (100)

19160 0.95

(0.01) (20)

22590 1.00

(0.01) (10)

3a

(10)

(0.01) (10)

19250 0.22

(0.01) (20)

20720 0.84

(0.01) (10)

22560 0.99

(0.01) (10)

2a

(0)

ṽCM

20200

(0.02) (10)

Values in parenthesis are standard deviations of two or three titrations.


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Table 3. Aprotic and Protic Relative Quantum Yields and Fluorescence Center-of-Masses for 1b3b in several binary solvents.a Tol/MeOH Qrel ṽCM Yaprotic

1b Yprotic Yaprotic

2b Yprotic Yaprotic

3b Yprotic

a

0.23 (0.01) 0.32 (0.01) 0.19 (0.01) 0.25 (0.01) 0.46 (0.01) 0.11 (0.01)

22720 (40) 18600 (10) 22870 (10) 18970 (10) 23020 (10) 19660 (10)

ACN/MeOH Qrel ṽCM 0.99 (0.01) 0.28 (0.01) 1.00 (0.01) 0.20 (0.01) 0.99 (0.01) 0.09 (0.01)

20580 (30) 18600 (10) 20910 (10) 19380 (10) 21130 (140) 19650 (10)

Tol/iPrOH Qrel ṽCM 0.24 (0.02) 0.95 (0.01) 0.16 (0.01) 0.90 (0.01) 0.43 (0.04) 0.86 (0.01)

22560 (120) 19480 (10) 22770 (100) 19810 (10) 22800 (10) 20390 (10)

Values in parentheses are standard deviations of two or three titrations

The effects of intramolecular H-bonding on the quenching of these derivatives is best revealed through the analysis of the preferential solvation titrations in acetonitrile/methanol mixtures. These solvents are both polar, and methanol has a very high H-bond donating ability (high solvent acidity).10 As we have shown, preferential solvation titrations in this binary solvent system can be modeled using a simple solvent exchange model.11, 12 In this description, fluorescence occurs from three species: one where the fluorophore is surrounded by the aprotic solvent (no H-bonds), a second where a single solvent exchange gives a mixed solvation sphere and one H-bond, and a third where a double exchange gives a doubly H-bonded fluorophore. Analysis of the data with this model allows for the extraction of the relative quantum yields and the fluorescence center-of-masses for the mixed solvent system. With carbonyl-twisted PRODAN


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derivatives, analysis of such titrations showed that a single intermolecular H-bond does not give rise to quenching. Quenching is achieved only with two H-bonds.11 The extracted data for the mixed-solvent system shown in Table 4 can be interpreted as a logical extension of this behavior, but only within the context of the carbonyl-twisting requirement. Compounds 1a and 1b lack an adjacent gem-dimethyl group and are not twisted. The quantum yields for the singly H-bonded species are identical indicating no apparent quenching from intramolecular H-bonding.

In

contrast, carbonyl-twisted compounds 2a and 3a both show enhanced quenching in the presence of a single intermolecular H-bond from methanol relative to their methoxy counterparts. For 2a there is a 20% decrease in the quantum yield compared with 2b, and for 3a there is a 35% decrease in the quantum yield compared with 3b. Both 2b and 3b exemplify the same behavior as the carbonyl-twisted PRODAN derivatives where the singly H-bonded fluorophore is not quenched and the relative quantum yield is maximal. The contrasting behavior of 2a and 3a vs. 2b and 3b suggests that some of the fluorophores with mixed solvation spheres have both an intramolecular H-bond with the internal OH group and an intermolecular H-bond with methanol (Figure 5).

Figure 5. Proposed partially emissive intramolecular, intermolecular double H-bonding structure for the singlet ICT state of 3a.


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The modest quenching of the intramolecular, intermolecular doubly H-bonded excited state is consistent with previous results.

Quenching in carbonyl-twisted PRODAN derivatives is

dependent on solvent acidity. For alkanols, solvent acidity is related to the structure of the alcohol. Primary alcohols are better H-bond donors than secondary and tertiary alcohols. For primary alcohols, solvent acidity decreases with chain length. For 2a and 3a, the intramolecular H-bond is formed from a tertiary and primary alcohol, respectively.

The primary alcohol in 3a, being

analogous to neopentyl or isobutyl alcohol, should be less acidic than methanol. As such, the quenching of the intramolecular, intermolecular H-bonded excited state should not be as efficient as with two H-bonds from methanol (in pure methanol). A more subtle effect attributed to intramolecular H-bonding is seen in the quenching behavior in protic solvents. In the case of quenching by methanol, any involvement of the less effective intramolecular H-bond would give rise to less quenching relative to the methoxy derivative. This result is observed in 1 (0.38 for 1a vs. 0.30 for 1b) and 3 (0.15 vs. 0.10) but not 2 (0.22 vs. 0.23). In the case of quenching in pure isopropanol, a much weaker H-bonding solvent, the intramolecular H-bonding would result in greater quenching. In general, there is very little quenching in isopropanol. The average relative quantum yield of the three methoxy compounds is 0.90. There is no difference in the quenching for 1 (0.95 for both 1a and 1b). There is some difference for 2 (0.79 vs. 0.90), and a larger difference for 3 (0.70 vs. 0.86). In the latter case, the difference in behavior as shown in figures S16 and S18 is even more striking.


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Table 4.

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Relative Quantum Yields and Fluorescence Center-of-Masses for the mixed

acetonitrile/methanol solvent species for 1-3.a Qrel

ṽCM

Qrel

ṽCM

0.87 19470 1b 0.89 18920 (0.09) (30) (0.02) (50) 0.82 19550 2a 2b 1.02 19590 (0.01) (30) (0.01) (50) 3a 0.64 19930 3b 0.99 20180 (0.04) (10) (0.02) (10)

1a

a

Values in parentheses are standard deviations of two or three titrations

CONCLUSIONS Intramolecular H-bonding in PRODAN derivatives affects the excited-state behavior in several ways. The most prominent effect is that it leads to greater fluorescence in apolar (and aprotic) solvents by inhibiting ISC. It also leads to slight quenching in the presence of low concentrations of alcohol due to a doubly intramolecular, intermolecular H-bonded excited state. The results corroborate the importance of carbonyl-twisting in quenching by alcohols. They also corroborate the necessity of more than one H-bonding interaction for efficient quenching.

ASSOCIATED CONTENT Supporting Information. Experimental methods, preferential solvation model and analysis, synthetic details for the preparation of 1 – 3, solvatochromism plots, preferential solvation fluorescence and absorption titrations. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail [email protected] (C.J.A.).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund. REFERENCES (1) Weber, G.; Farris, F. J. Synthesis and Spectral Properties of a Hydrophobic Fluorescent Probe: 6-Propionyl-2-(Dimethylamino) Naphthalene. Biochemistry 1979, 18, 3075-3078. (2) Cerezo, F. M.; Rocafort, S. C.; Sierra, P. S.; García-Blanco, F.; Oliva, C. D.; Sierra, J. C. Photophysical Study of the Probes Acrylodan (1-[6-(Dimethylamino)naphthalen-2-yl]prop-2-en1-one), ANS (8-Anilinonaphthalene-1-sulfonate) and Prodan (1-[6-(Dimethylamino) naphthalen-


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2-yl]propan-1-one) in Aqueous Mixtures of various Alcohols. Helv. Chim. Acta 2001, 84, 33063312. (3) Rowe, B. A.; Roach, C. A.; Lin, J.; Asiago, V.; Dmitrenko, O.; Neal, S. L. Spectral Heterogeneity of PRODAN Fluorescence in Isotropic Solvents Revealed by Multivariate Photokinetic Analysis. J. Phys. Chem. A 2008, 112, 13402-13412. (4) Homocianu, M.; Airinei, A.; Dorohoi, D. O. Solvent Effects on the Electronic Absorption and Fluorescence Spectra. Journal of Adv. Res. Phys. 2011, 2. (5) Balter, A.; Nowak, W.; Pawekiewicz, W.; Kowalczyk, A. Some Remarks on the Interpretation of the Spectral Properties of Prodan. Chem. Phys. Lett. 1988, 143, 565-570. (6) Adhikary, R.; Barnes, C. A.; Petrich, J. W. Solvation Dynamics of the Fluorescent Probe PRODAN in Heterogeneous Environments: Contributions from the Locally Excited and ChargeTransferred States. J. Phys. Chem. B 2009, 113, 11999-12004. (7) Catalán, J.; Perez, P.; Laynez, J.; Blanco, F. G. Analysis of the Solvent Effect on the Photophysics Properties of 6-Propionyl-2-(dimethylamino)naphthalene (PRODAN). J. Fluoresc. 1991, 1, 215-223. (8) Moyano, F.; Biasutti, M. A.; Silber, J. J.; Correa, N. M. New Insights on the Behavior of PRODAN in Homogeneous Media and in Large Unilamellar Vesicles. J. Phys. Chem. B 2006, 110, 11838-11846. (9) Green, A. M.; Naughton, H. R.; Nealy, Z. B.; Pike, R. D.; Abelt, C. J. Carbonyl-Twisted 6Acyl-2-Dialkylaminonaphthalenes as Solvent Acidity Sensors. J. Org. Chem. 2013, 78, 17841789.


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(10) Catalán, J.; Díaz, C. A Generalized Solvent Acidity Scale: The Solvatochromism of o-tertButylstilbazolium Betaine Dye and its Homomorph o,o′-Di-tert-butylstilbazolium Betaine Dye. Liebigs Annalen 1997, 1997, 1941-1949. (11) Nikitina, Y. Y.; Iqbal, E. S.; Yoon, H. J.; Abelt, C. J. Preferential Solvation in CarbonylTwisted PRODAN Derivatives. J. Phys. Chem. A 2013, 117, 9189-9195. (12) Daneri, M.; Abelt, C. J. A Higher-Order Preferential Solvation Model for the Fluorescence of Two PRODAN Derivatives in Toluene-Alcohol Mixtures. J. Photochem. Photobiol. A. 2015, 310, 106–112. (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. (14) Bosch, E.; Rived, F.; Rosés, M. Solute–solvent and Solvent–solvent Interactions in Binary Solvent Mixtures. Part 4. Preferential Solvation of Solvatochromic Indicators in Mixtures of 2Methylpropan-2-ol with Hexane, Benzene, Propan-2-ol, Ethanol and Methanol. J. Chem. Soc., Perkin Trans. 2 1996, 2177-2184. (15) Ortega, J.; Ràfols, C.; Bosch, E.; Rosés, M. Solute–solvent and Solvent–solvent Interactions in Binary Solvent Mixtures. Part 3. The ET(30) Polarity of Binary Mixtures of Hydroxylic Solvents. J. Chem. Soc., Perkin Trans. 2 1996, 1497-1503. (16) Bosch, E.; Roses, M.; Herodes, K.; Koppel, I.; Leito, I.; Koppel, I.; Taal, V. Solute‐solvent and Solvent‐solvent Interactions in Binary Solvent Mixtures. 2. Effect of Temperature on the ET


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(30) Polarity Parameter of Dipolar Hydrogen Bond Acceptor‐hydrogen Bond Donor Mixtures. J. Phys. Org. Chem. 1996, 9, 403-410. (17) Ràfols, C.; Rosés, M.; Bosch, E. Solute–solvent and Solvent–solvent Interactions in Binary Solvent Mixtures. Part 5. Preferential Solvation of Solvatochromic Indicators in Mixtures of Propan-2-ol with Hexane, Benzene, Ethanol and Methanol. J. Chem. Soc., Perkin Trans. 2 1997, 243-248. (18) Rosés, M.; Buhvestov, U.; Ràfols, C.; Rived, F.; Bosch, E. Solute–solvent and Solvent– solvent Interactions in Binary Solvent Mixtures. Part 6. A Quantitative Measurement of the Enhancement of the Water Structure in 2-Methylpropan-2-Ol–water and Propan-2-Ol–water Mixtures by Solvatochromic Indicators. J. Chem. Soc., Perkin Trans. 2 1997, 1341-1348. (19) Buhvestov, U.; Rived, F.; Ràfols, C.; Bosch, E.; Rosés, M. Solute–solvent and Solvent– solvent Interactions in Binary Solvent Mixtures. Part 7. Comparison of the Enhancement of the Water Structure in Alcohol–water Mixtures Measured by Solvatochromic Indicators. J. Phys. Org. Chem. 1998, 11, 185-192. (20) Herodes, K.; Leito, I.; Koppel, I.; Rosés, M. Solute–solvent and Solvent–solvent Interactions in Binary Solvent Mixtures. Part 8. The ET(30) Polarity of Binary Mixtures of Formamides with Hydroxylic Solvents. J. Phys. Org. Chem. 1999, 12, 109-115. (21) Reichardt, C.; Welton, T. In Solvents and solvent effects in organic chemistry; Wiley-VCH Verlag GmbH: 2011; Weinheim. (22) Botrel, A.; Corre, F.; Le Beuze, A. Theoretical Solvent Effects on Molecular Structure and Spectroscopic Properties through the Virtual-Charge Model. Application to the Inverted


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Solvatochromy of Benzophenone n→ π* and π → π * Absorption Bands. Chem. Phys. 1983, 74, 383-394.

Table of Contents Graphic and Synopsis


19


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

O R

O

C H2

Page 20 of 26

O

C

R N

CH3

O

C

C

R

H3C CH3

CH3 1a, R = H 1b, R = CH3

N

CH3

O

H2 C

O

C H3C CH3

CH3 2a, R = H 2b, R = CH3


N

CH3

CH3 3a, R = H 3b, R = CH3

Page 21 of 26

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


O

A) HO

X

O

R

O

1) iPrMgCl 2) CH3ONHCH3-HCl

O

O

3) MOM-Cl, (iPr)2NH

X

N O

1)

O O

O

O

N

HO

Li

X

N

X 1a, X= CH2 2a, X= C(CH3)2 3a, X= CH2C(CH3)2

2) HCl/EtOH

O

O

B) HO

X

O

R

N

O

O

1) NaI 2) CH3I

H3C

X = CH2, C(CH3)2, CH2C(CH3)2 R = CH3, CH3CH2

O

X

O

R

H3C N

Li


O

X

1b, X= CH2 2b, X= C(CH3)2 3b, X= CH2C(CH3)2

N


23000 22500 22000

wavenumber (cm-1)

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 22 of 26

21500 21000 20500

20000 19500 19000 18500 33

36

39

42

45

ET(30)


48

51

54

Page 23 of 26

10 9 toluene 8 7

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


6 5 4 3

methanol

2 1 0 16000

17000

18000

19000

20000

21000

wavenumber

22000

(cm-1)


23000

24000

25000

26000


10 9 8 7

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

Page 24 of 26

6 toluene 5 4 methanol 3 2 1 0 16000

17000

18000

19000

20000

21000

wavenumber

22000

(cm-1)


23000

24000

25000

26000

Page 25 of 26

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


H 3C

O

1* H O

N

O

C H 3C

H

C

CH2

CH3



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

1*

emissive

O

H

C

O CH2

C

N

H3C

CH3 H3C

O

1* H O

H

C

O CH2

C

N

H3C partially quenched

CH3


Page 26 of 26

Intramolecular Hydrogen-Bonding Effects on the Fluorescence of

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