7-(Dimethylamino)coumarin-3-carbaldehyde and Its

Article pubs.acs.org/JPCA

7‑(Dimethylamino)coumarin-3-carbaldehyde and Its Phenylsemicarbazone: TICT Excited State Modulation, Fluorescent H‑Aggregates, and Preferential Solvation Marek Cigáň,* Jana Donovalová, Vojtech Szöcs, Jan Gašpar, Klaudia Jakusová, and Anton Gáplovský Institute of Chemistry, Faculty of Natural Sciences, Comenius University, Mlynská dolina CH-2, SK-842 15 Bratislava, Slovak Republic S Supporting Information *

ABSTRACT: The photophysical properties of 7-(dimethylamino)coumarin-3-carbaldehyde 3 and its phenylsemicarbazone 4 were investigated in solvents of various polarity and in differing solvent mixtures. The different fluorescent quantum yield (ΦF) behavior of 3 and 4 in highly polar solvents is discussed in terms of Twisted Intramolecular Charge-Tranfer (TICT) state formation and the specific solute−solvent interactions. Because of the weak intermolecular hydrogen bonding ability of both the radiative ICT and nonradiative TICT excited state of 3 and the linear steep decrease in ΦF from a medium to high polarity region, coumarin 3 could be a useful polarity probe for microenvironments containing hydrogen bonding groups. Compared to 3, coumarin 4 exhibits the highest ΦF values in highly polar solvents with strong hydrogen bond acceptor ability. The high quantum yield of fluorescence in DMSO, DMF, and alcohols qualifies coumarin 4 as a laser dye in the given medium, with kF higher than knr. Contrary to previous reports that many H-aggregates are nonfluorescent in nature, coumarin 3 forms highly fluorescent H-aggregates in MeOH and EtOH. On the basis of the restrictions of the Kasha-exciton theory model, we assume that the formation of fluorescent H-dimer aggregates of 3 is driven by π+−π− interactions. To the best of our knowledge, this is the first report on aggregation of coumarin dye in alcoholic solutions. In addition, restrictions in the fitting procedure relating to determination of the solvation number, n, using the Covington−Newman model of preferential solvation and also the solvent nonideality parameter, h′, are discussed in this article. fluoride, acetate, benzoate, dihydrogenphosphate, pyrophosphate, citrate, ATP, and ADP; (2) various metal ions including Hg2+, Cu2+, Zn2+, Mg2+, and Pb2+; (3) biologically important compounds such as thiols, amines, amino acids, enzymes, DNA, RNA, H2O2, O2, and hydroxyl radicals; and (4) chemical warfare agents.30−44 Moreover, this class of compounds has received considerable attention due to their ability to lase in the blue−green region.9,27 In designing efficiently fluorophores such as laser dyes, it is absolutely essential to control the formation of the TICT state(s) acting as fluorescence quencher(s).9 In our recent study, substituted coumarins (2-oxo-2Hchromenes) with substituents of various electron donating ability in position 7, such as H, CH3, OCH3, and N(CH3)2, and substituted in position 3 by CHO or −CHNNHCONHPh, were investigated in chloroform and methanol solutions and in the three polymer matrixes PMMA, PVC, and PS.27 The fluorescent quantum yield of derivatives 3 and 4 (Scheme 1), with a dimethylamino group in position 7 in polymer matrixes,

1. INTRODUCTION 7-Aminocoumarins (7-amino-2-oxo-2H-chromenes) are arguably the most important and applicable subset of coumarins, and therefore, they have been the focus of intense study1−29 and widespread application as fluorescent probes. The fluorescence of 7-aminocoumarins is strongly dependent on the polarity, hydrogen bonding ability, and pH and microviscosity or rotational hindrance in their local environment.2−29 The photophysical properties of these dyes depend on the nature and pattern of substitution of the nitrogen atom of the amino group and appropriate substituents at the 3-, 4-, and 6positions of the coumarin moiety. In highly polar solvents, the polar intramolecular charge transfer excited state of 7aminocoumarins with strong push−pull character of a πconjugated system undergoes a nonradiative rotational (twisting) decay mechanism that leads to full charge separation in the excited state.4,9,15,21,28 In general, as a result of this twisted intramolecular charge-tranfer (TICT) state formation,14 the fluorescence of 7-aminocoumarins is red-shifted and its intensity (quantum yield) decreased with increasing polarity of the medium.22 7-Dialkylaminocoumarin derivatives have also served as good chemosensors of the following: (1) anions including cyanide, © 2013 American Chemical Society

Received: March 15, 2013 Revised: May 13, 2013 Published: May 22, 2013 4870

dx.doi.org/10.1021/jp402627a | J. Phys. Chem. A 2013, 117, 4870−4883

The Journal of Physical Chemistry A

Article

determined values of ΦF for 3 and 4 in chloroform (Φ3‑CHCl3 = 0.44; Φ4‑CHCl3 = 0.49)27 as

Scheme 1. Molecular Structure of the Studied Molecules

⎞⎛ F ⎞⎛ η 2 ⎞ ⎛A ΦF(X) = ΦF(CHCl3)⎜ CHCl3 ⎟⎜ X ⎟⎜⎜ X 2 ⎟⎟ ⎝ AX ⎠⎝ FCHCl3 ⎠⎝ ηCHCl3 ⎠

(1)

where the subscript X denotes corresponding solvent or binary mixture, A is the value of absorbance at the excitation wavelength, F the plane under the emission curve, and η the refractive index of the solvent (or solvent mixture). Corresponding absorbances never exceeded 0.1. The refractive indexes of MeOH/CHCl3 and MeOH/EA binary solvent mixtures were taken from refs 50 and 51, respectively. Solvents used were of UV-spectroscopy and HPLC grade (Merck, Darmstadt, Germany). These solvents comprised benzene, dioxane (Diox), chloroform (CHCl3), ethyl acetate (EA), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), N,Ndimethylformamide (DMF), acetone, ethanol (EtOH), acetonitrile (MeCN), and methanol (MeOH). Except for DMSO, all were dried and purified by standard methods before use. Following Lippert and Mataga, the solvent polarity function for different solvents was estimated as52,53

approaches 1. The high quantum yield qualifies them as laser dyes, which have kF higher than knr in the given medium. The 7-diethyl analogue of coumarin 3 has been utilized as a doubly activated Michael acceptor type of chemodosimeter for cyanide in water40 and has shown a selective and sensitive response to homocysteine or cysteine over other various amino acids41 and a more highly selective fluorescence turn-on response toward proline than other amino acids; even exhibiting micromolar sensitivity.42 Moreover, the replacement of the formyl group with a phenylsemicarbazide group led to a significant increase in fluorescent quantum yield of the 7diethyl analogue of 3 in polar solvents (due to hindering formation of the TICT state) and subsequent synthesis of an effective laser dye in DMSO.9 However, the photophysical characteristics of these derivatives were investigated only in four solvents.9 The urea and/or thiourea moiety plays an important role in various chemosensors; for example, as an anion binding site.43−45 The acidity of hydrogen ions on the urea moiety of 4 can be modified via para-substitution on the phenyl ring in the phenylsemicarbazide substituent and thus affect anion binding. Moreover, tuning of fluorescent properties via parasubstitution could have a positive influence on the lasing action of these dyes. Our comprehensive study of spectral properties and anion binding of para-phenyl substituted derivatives of coumarin 4 with various electron-withdrawing or electrondonating substituents in the para-position on the phenyl ring is now in progress. Herein, we report results of a more comprehensive investigation of spectral properties of these two 7-dimethylaminocoumarin derivatives 3 and 4 in solution, which is necessary for their practical application as chemosensors or laser dyes and for better understanding of the photophysical properties of their derivatives. This study led to revelation of some unexpected spectral behavior of 3 in alcoholic solutions. Thus, it completes our previous conclusions relating to fluorescence of 3 in MeOH and it highlights some constraints in the use of the Covington−Newman model of preferential solvation.46−48

Δf =

ε−1 n2 − 1 − 2 2ε + 1 2n + 1

(2)

where Δf is the solvent polarity factor (orientation polarizability), ε is the dielectric constant of the solvent, and n is its refractive index. The decay rate constants for the excited dyes 3 and 4 were calculated as54 kF ‐ calc = 1/t0

(3)

k nr ‐ calc = (1 − ΦF)/t F ‐ calc

(4)

where τF‑calc = τ0ΦF is the fluorescence lifetime, and τ0 signifies the radiative lifetime (the natural lifetime) of the excited singlet state. The τ0 values for 3 and 4 in CHCl3 and MeOH were previously obtained from absorption spectra.27 The experimental data xL1 , describing preferential solvation of 3 and 4 by the corresponding solvent, were treated according to the accepted solvation relationship:55 x1L = x1K12/[1 + (K12 − 1)x1]

(5)

xL1 ,

where the parameter K12 depends, beside also on the solvent−solvent interaction energy, K1/n, on solvent nonideality, h′, and on the coordination number n (see Supporting Information section S2.3: Preferential solvation (PS)−Theoretical background). The above, highly nonlinear, equation describes an implicit relationship of xL1 on x1. Thus, direct fit to the function is impossible. Nevertheless, the inverse relationship of x1 on xL1 is in fact explicit (K12 depends on xL1 ). The least-squares fitting56,57 of inverse ordered experimental data, {x1L, x1} (for known coordination number n), offers a satisfactory starting point to obtain the parameters K1/n and h′ for the final fit of (direct) data, {x1, xL1 }, in a standard nonlinear least-squares approach (Nelder-Mead minimization method56,57). 2.3. Preferential Solvation (PS). The following quantitative parameters were used to achieve more detailed insight into the observed PS of 3 and 4 by MeOH in MeOH/CHCl3 and MeOH/EA binary solvent mixtures: δs1, the index of PS; K12, the PS constant; K1/n, the index of solute−solvent interaction; n, the solvation number (Covington−Newman model); h′, the

2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Synthesis. The synthesis of compounds under study (Scheme 1) has been published in ref 27 and that of 7dimethylaminocoumarin (DAC) in ref 49. 2.2. Spectroscopic Measurements. Electronic absorption spectra were obtained on a HP 8452A (Hewlett-Packard, USA) diode array spectrophotometer, and fluorescence measurements were performed on an FSP 920 (Edinburgh Instruments, U.K.) spectrofluorimeter. The fluorescence of solutions of 3 and 4 was measured in 1 cm fluorescence cuvette in the rightangle arrangement, using the temperature-controlled cuvette holder. In a few cases, the front-face arrangement was used to eliminate the self-absorption of fluorescence due to high solute concentration. The quantum yields of fluorescence (ΦF) of 3 and 4 solutions were calculated relative to previously 4871



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Table 1. Basic Spectral Properties of Coumarins 3 and 4 (T = 25°C)a compd

a

3

4

solvent

Δf

λA (nm)

λF (nm)

νA−νF (cm−1)

benzene diox CHCl3 EA THF DMSO DMF acetone EtOH MeCN MeOH

0.0021 0.021 0.148 0.201 0.210 0.264 0.276 0.284 0.290 0.305 0.309

432 430 441 434 435 447 441 437 443 437 436

461 464 471 474 474 494 493 484 491 486 494

1456 1704 1399 1944 1891 2128 2392 2222 2207 2307 2517

0.26 0.30 0.44 0.25 0.30 0.02 0.02 0.03 0.02 0.02 0.01

ΦF

λA (nm)

λF (nm)

νA−νF (cm−1)

± ± ± ± ± ± ± ± ± ± ±

427 427 433 425 427 439 432 429 432 430 431

485 488 491 494 494 513 507 500 503 503 504

2779 2927 2705 3190 3156 3286 3405 3310 3267 3375 3280

0.03 0.03 0.04 0.03 0.03 0.002 0.002 0.003 0.002 0.002 0.001

ΦF 0.52 0.49 0.49 0.48 0.47 0.65 0.62 0.50 0.63 0.50 0.64

± ± ± ± ± ± ± ± ± ± ±

0.05 0.05 0.05 0.05 0.05 0.07 0.06 0.05 0.06 0.05 0.06

Solvent polarity factor (Δf), absorption (λA) and fluorescence (λF) maxima, Stokes shift (νA−νF), and the fluorescent quantum yields (ΦF).

Figure 1. Absorption and emission maxima of coumarin 3 (A) and 4 (B) in solvents with various polarity (λEX = λA; T = 25 °C; c ≈ 2−3 × 10−6 mol dm−3).

solvent nonideality parameter; ΔWm(g,e), the average free orientation interaction energy for a single molecule in the solvation shell; and Nm, the solvation number (Mazurenko’s model).46−48,58−63 These parameters describe changes in the solvation shell of 3 and 4 due to the increasing amount of a more polar solvent in a binary mixture. The determination and physical interpretation of these parameters are described in greater detail in the Supporting Information section S2.3: Preferential solvation (PS)−Theoretical background.

in CHCl3 as a solvent of low polarity is unexpected, halogenated solvents often exert unpredictable effects on solvatochromism of molecules with high ICT character.64 These effects cannot be explained solely by their dielectric constant values, and we assume that they are connected with decreased stabilization of the ground states of 3 and 4 by CHCl3 in comparison with other solvents of similar polarity, such as EA and THF. The slight increase in λA and λF for DMSO and DMF in respect to their dielectric constants is most likely due their enhanced ability to stabilize the ICT excited states of coumarins compared to other polar aprotic solvents. DMSO and DMF are highly polar solvents and according to some classifications they are more polar than MeOH.65,66 Furthermore, they are strong hydrogen bond acceptor solvents with a high value of the Kamlet−Taft parameter β.54 This same effect on λA and λF in DMSO and DMF was also previously reported for other coumarins.12,28 A slight increase in λA and λF was also observed in alcohols (MeOH and EtOH), and this can be explained by additional stabilization of the S1 excited state by specific interactions (intermolecular hydrogen bonding) between the studied 7-aminocoumarins and corresponding solvent molecules. The stronger dependence of λF on solvent polarity and its bathochromic shift (Figure 1; Table 1) with increasing solvent polarity is in accordance with expected stabilization of the intramolecular charge transfer (ICT) character of the excited

3. RESULTS AND DISCUSSION 3.1. Spectral Properties. Basic photophysical properties of the studied coumarins 3 and 4, such as the absorption (λA) and fluorescence (λF) maxima, the Stokes shift (νA−νF) and the fluorescent quantum yield (ΦF), in solvents with various polarity are presented in Table 1. Figures 1 and S1, Supporting Information, plot λA, λF, and νA−νF of 3 and 4 against the solvent polarity factor (Δf). Absorption maxima of both 3 and 4 are shifted to longer wavelengths with increasing polarity of the solvent, compounds 3 and 4 thus exhibit positive solvatochromism. The bathochromic shifts of λA upon increasing the solvent polarity implies that the electronic transition corresponding to the main absorption band of both studied compounds is π−π* transition. Compared to λF, the value of λA and consequently also the value of νA−νF slightly deviate from linearity in CHCl3. Although the deviation of λA and νA−νF toward lower energies 4872



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Figure 2. Values of fluorescent quantum yield (ΦF) of 3 (A) and 4 (B) plotted against solvent polarity factor (Δf); λEX = λA (CHCl3).

Scheme 2. Charge Redistribution in the Excited Singlet States of Molecules 3 and 4 Dependent on the Solvent Polarity

rotation of the carbonyl group in position 3 due to charge delocalization between two carbonyl groups in molecule 3 (Scheme 2, structure II). This effect has already been observed for 3-phenylcoumarin68 and also in our previous study on molecules 3 and 4 in polymer matrixes.27 Increasing polarity of the solvent leads to increasing charge-transfer character of the excited S1 state of 3 and thus enhances the possibility of charge delocalization between two carbonyl groups in molecule 3, which subsequently prevents the rotation of the carbonyl in position 3. Another interesting feature in the plot of ΦF vs Δf (Figure 2A) is a breaking point at Δf ≈ 0.15. After the slow linear increase in the ΦF value in the lower to medium polarity region (Δf < 0.15), the quantum yield of fluorescence of 3 decreases more rapidly in highly polar solvents (Δf > 0.15). Rapid decrease in ΦF in highly polar solvents contradicts our previously published results,27 and this will be discussed in the next section (Self-Aggregation). To gain a better understanding of the de-excitation processes involved, decay constants for the radiative (kF) and nonradiative (knr) deexcitation were calculated for 3 in two solvents of different polarity and different ΦF behavior: CHCl3 and MeOH (Table 2). The calculated values of kF exhibit no systematic deviations with the change in solvent polarity, and they agree with the kF literature values for other 3- or 4-acceptor substituted 7-

singlet state of 3 and 4 in polar solvents, typical for 7(dialkylamino)coumarins.17,21 In comparison to the parent 7(dimethylamino)coumarin-3-carbaldehyde 3, the λF of phenylsemicarbazone 4 is bathochromically shifted (Table 1). We assume that the red shift of λF in the case of 4 is associated with the rapid additional charge delocalization over the −CH NNHCONHPh moiety in the excited state.27 The linear dependence of λF over the entire Δf range suggests that the emission occurs from the locally excited state (LE) of 3 and 4. However, the emission spectrum of 4 exhibits two bands at approximately 505 and 520 nm, which changes the ratio of their intensities on moving from nonpolar to more polar solvents (Figure S2, Supporting Information). In highly polar solvents, the second emission band with higher ICT character (more batochromically shifted band) almost disappears, and the fluorescence from only one excited state dominates. The existence of two fluorescence bands and their polaritydependent behavior indicates equilibrium between two emissive ICT excited states. Moreover, the fluorescent decay of 4 in CHCl3 exhibited better fit by the biexponential mathematical function (τ1 ≈ 2 ns; τ2 ≈ 3.5 ns), thus supporting this conclusion. As expected, the reduced electron-donating ability of the dimethylamino group in 3 and 4 results in a decrease in both λA and λF, compared to their diethylamino analogues.9 Figure 2 plots the quantum yield of fluorescence (ΦF) of compounds 3 and 4 against the solvent polarity factor (Δf). Both compounds exhibit relatively high ΦF in some of the solvents (Table 1). As seen in Figure 2, the behavior of ΦF is completely different for 3 and 4. Compared to other 3- or 4acceptor substituted 7-(dimethyl- or diethylamino)coumarins,12,15,28,67 an unexpected increase in ΦF for compound 3 was observed with increasing solvent polarity and it follows linear dependence on Δf until Δf reaches a value of approximately 0.15. We assume that the initial increase in ΦF with increasing Δf value is connected with the decreasing

Table 2. Calculated Radiative and Nonradiative Decay Rate Constants for the Excited S1 State of 3a solvent

Δf

kF (109 s−1)

knr (109 s−1)

CHCl3 MeOH

0.148 0.309

0.15 0.12

0.12 3.70

a

Solvent polarity factor (Δf) and the radiative (kF) and the nonradiative (knr) decay rate constants for the S1 excited state.

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Figure 3. (A) Fluorescence spectra of 3 in MeOH at various temperatures (λEX = λA). (B) Linear form of Arrhenius plot (eq 6) expressing temperature dependence of ΦF of 3 in MeOH.

(dimethyl- or diethylamino)coumarins.12,15,67 However, comparison of knr values in CHCl3 and MeOH clearly indicates that solvation of the excited state of 3 by highly polar solvent molecules leads to rapid nonradiative de-excitation of this electronic state. Therefore, the significant increase of knr in highly polar solvents is responsible for the small ΦF values in these solvents. We assume that the additional increase in the ICT character in the S1 state and its stabilization in highly polar solvents is responsible for the increase in knr. This is due to the high value of the dielectric constant and/or hydrogen bonding formation. Thus, solvation in a high polarity region most likely leads to population of the nonradiative TICT14 excited state of 3, which results in increase in the nonradiative decay rate constant knr of the excited state. Different knr and thus different ΦF values for coumarin 3 in solvents with different polarity can be explained by different charge redistribution in the excited singlet state of 3 in particular types of solvents (Scheme 2). The excited state of molecule 3 in different solvents is as follows: (1) in solvents with low to medium polarity (Δf < 0.15), it is of ICT character where a lone electron pair at the nitrogen atom of the dimethylamino group is involved in conjugation with the coumarin skeleton and exists somewhere between resonance structures I and II depicted below, and (2) in highly polar solvents (Δf > 0.15) is in the form of the TICT excited state (structure III) after complete electron transfer from the nitrogen atom to the coumarin residue, with a higher probability of transitions S1(ICT) → TICT → S0 as the additional pathway for nonradiative decay (S0 = ground state of molecule 3). Full charge separation and formation of the TICT excited state in highly polar solvents (DMF and DMSO) was previously observed in the diethylamino analogue of coumarin 3.9 There is controversy in the literature on the exact role that hydrogen bonding plays in the formation of the TICT states of 7-aminocoumarins in protic solvents.8,13,16−18 Recently, Barik et al. published a detailed study with evidence for TICT formation of coumarin C1 only in high polar protic solvents, and no evidence was supplied for TICT in highly polar aprotic solvents.18 For coumarin 3, we observed a rapid decrease in ΦF in both MeCN and MeOH. Moreover, the ΦF values in these solvents were almost equal (Table 1). These results suggest that hydrogen bonding to the solvent is not a necessary condition

for TICT stabilization of 7-aminocoumarins in highly polar solvents. We assume that such stabilization depends on the specific charge redistribution in a particular coumarin, and thus, its effect varies from coumarin to coumarin. Compared to 3, the ΦF of molecule 4 is almost constant (ΦF(4) ≈ 0.5) over the whole polarity range (Figure 2B). The exceptions are ΦF values in highly polar solvents such as DMSO and DMF and in the alcohols MeOH and EtOH. In these highly polar solvents, the ΦF of 4 exhibits surprisingly higher values than in other solvents. The high quantum yield of fluorescence in DMSO, DMF, and alcohols qualifies coumarin 4 as a laser dye in the given medium (kF higher than knr). The observed increase inΦF in these solvents could be caused by a reduction of isomerization of imine CN double bond and/or a reduction of twisting of the single bonds in the −CH NNHCONHPh side chain (an increase in the electron density distribution of the C−C single bonds) due to effective charge transfer in the excited state of 4 in highly polar solvent. However, this increase was not observed in MeCN solvent, which has similar polarity. Therefore, we assume that specific interactions between 4 and corresponding solvent molecules with strong hydrogen bond acceptor ability are responsible for the unexpected increase in ΦF for 4 in these solvents. The 2fold higher values of the Kamlet−Taft parameter β for DMSO, DMF, MeOH, and EtOH in comparison to MeCN support this assumption. Although these specific interactions do not affect the charge redistribution in the excited state of 4 (λA value), they most likely influence the rigidity of the molecule in this electronic state. On the basis of suppression of CN isomerization in forming a complex with a guest cation, a few 7-diethylaminocoumarin-derived imines have been designed as novel fluorescent chemosensors using CN isomerization as a signal unit.33 No isomerization of the imine CN double bond was observed for coumarin 4, as reflected in the lack of observed changes in absorption and emission spectra of 4 following irradiation at λA. However, because of the similar energies of both isomers and a possible fast thermal back-reaction, the isomerization of the imine CN double bond could still be one of the nonradiative deactivation channels of the excited state of coumarin 4. Analogous to the diethylamino analogue of 4,9 the weaker electron-withdrawing character of the newly formed substituent (after condensation of the phenylsemicarbazide and the 4874



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Figure 4. (A) Fluorescence spectra of 4 in CHCl3 at various temperatures (λEX = λA). (B) Linear form of Arrhenius plot (eq 6) expressing temperature dependence of ΦF of 3 in MeOH.

indicates the presence of a new activation-controlled deexcitation channel. As stated above, we associate this new activation-controlled de-excitation channel with the population of the nonradiative TICT excited state of 3. Although the increase in ΔEa with increasing Δf is not typical for most TICT molecules,69 it appears to be a characteristic feature of 3- or 4acceptor substituted 7-(dialkylamino) coumarins, with TICT character of the excited state in highly polar solvents.15,21,67 The observed behavior of 3 supports the conclusion previously found by Pal et al.15,21,67 that the TICT→ ground-state conversion, rather than the ICT → TICT conversion, is the activation controlled rate-determining step for the effective nonradiative process in 3- or 4-acceptor substituted TICT 7(dialkylamino) coumarins in highly polar solvents. Together with the stabilization of the TICT state of 3, the potential energy surface of this state also becomes steeper with increasing polarity of the solvent (with respect to the solvent polarization axis). Consequently, the TICT and the ground state potential energy surfaces cross gradually at higher energy with increasing solvent polarity15 and thus an increase in Δf leads to unusual increase in ΔEa. An interesting feature of the de-excitation process of 3 is revealed in the approximately same value of ΔEa (∼1 kJ·mol−1) for 3 in 1:1 CHCl3/MeOH mixture (volume ratio) as that in pure MeOH. This indicates the PS of 3 by MeOH, which is discussed in greater detail in the section on Preferential Solvation (Quantitative Parameters). Compared to the excited state deactivation of 3 in CHCl3, the activation barrier ΔEa in CHCl3 for the nonradiative channel in molecule 4 is approximately 4-fold higher and is almost constant in both CHCl3 and MeOH. 3.2. Self-Aggregation. As stated above, rapid decrease in the ΦF for 3 in highly polar solvents contradicts our previously published results,27 where we determined an approximately 10fold higher value for ΦF in MeOH (ΦF = 0.40). However, two different excitation wavelengths were used in these experiments. Figure 5 shows the fluorescence spectra of 3 in MeOH for both excitation wavelengths, and on the basis of the large difference between the extinction coefficient for 3 at these two excitation wavelengths (Figure S3, Supporting Information), this confirms that the two different excitation wavelength certainly lead to two markedly different values of ΦF for coumarin 3 (ΦF = 0.03 for excitation at 430 nm and ΦF = 0.40 for excitation at 357 nm, respectively).

carbonyl group in position 3 of coumarin 3) results in an increase in the TICT excited state energy compared to the LE ICT state energy (in a decrease in the S1(ICT) → TICT state transition probability) and thus leads to maintenance of the high ΦF values in highly polar solvents. However, compared to 4, a slight decrease in ΦF in DMSO and DMF was observed for its diethylamino analogue. Unfortunately, no other polar solvents were employed in this study.9 In comparison to 4, the replacement of the dimethylamino group with a diethylamino group in position 7 of the coumarin ring of 3 had almost no effect on ΦF, and the ΦF values of both 3 and its diethylamino analogue are very similar. To obtain deeper insight into the de-excitation process of the excited state of 3 and 4, the temperature effect on ΦF was investigated in solvents with different polarity. The temperature dependence of ΦF in the given solvent, considering kF to be temperature-independent, can be expressed using the modified Arrhenius equation (eq 6):12,15 ⎛1⎞ ⎛ ΔE ⎞ k 1 = 1 + nr = 1 + ⎜ ⎟k nr0 exp⎜ − a ⎟ ⎝ RT ⎠ ΦF kF ⎝ kF ⎠

(6)

where k0nr is the pre-exponential factor and ΔEa is the activation barrier for the nonradiative channel. According to this equation, linear ln{(1/ΦF) − 1)} vs. 1/T plots were obtained for both 3 and 4. Typical plots in MeOH and CHCl3 are shown in Figures 3 and 4. The ΔEa values for different solvents, estimated from the slopes of ln{(1/ΦF) − 1)} vs. 1/T plots, are listed in Table 3. As seen in Table 3, the ΔEa value for 3 markedly increases with an increase in Δf (on going from CHCl3 to MeOH), and this Table 3. Activation Energies for the Nonradiative Deactivation Process in the Excited S1 State of Coumarins 3 and 4a compd

3

4 −1

solvent

Δf

ΔEa (kJ·mol )

ΔEa (kJ·mol−1)

CHCl3 EA MeOH

0.148 0.201 0.309

0.337 ± 0.022 0.381 ± 0.033 0.907 ± 0.021

1.426 ± 0.065 b 1.401 ± 0.031

a The solvent polarity factor (Δf) and activation energies (ΔEa) for the nonradiative deactivation process in the excited S1 state, obtained from analysis of the modified Arrhenius equation (eq 6). bNot measured.

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Figure 6. Dependence of fluorescence excitation spectra of 3 in MeOH on increasing concentration of 3 (the front face arrangement; T = 25 °C).

Figure 5. Fluorescence excitation and emission spectra of coumarin 3 in MeOH obtained at different emission and excitation wavelengths, respectively (T = 25 °C; c = 6 × 10−6 mol dm−3).

Three types of processes may be responsible for this result: (1) concentration-independent equilibrium between the two different excited forms of 3, (2) concentration-dependent ground-state equilibrium between the given fluorophore 3 and its aggregates, or (3) chemical reaction between 3 and the solvent molecules. The formation of poloacetals and acetals, respectively, is a possible reaction pathway between the carbonyl group in position 3 of the coumarin ring and MeOH. However, neither UV−vis spectra nor 1H NMR spectra exhibited the presence of poloacetal or acetal. The 1H NMR spectrum of 3 in CDCl3 after 24 h in MeOH did not indicate the presence of the new signal from methoxy protons (there was no difference between the NMR spectra before dissolution in MeOH and after 24 h in MeOH, followed by evaporation of the solvent), and similarly, no change was also observed in the UV−vis spectrum of 3 in MeOH within a few hours. On the basis of the prediction that photophysical properties of the poloacetal or acetal, respectively, formed from aldehyde 3, are close to photophysical properties of 7-dimethylaminocoumarin (DAC), we decided to compare the fluorescence characteristics of the solution of 3 in MeOH (using the excitation wavelength of 357 nm) with those of DAC in MeOH. Although the values of λF for both molecules are similar, the ΦF for 3 is approximately 17-times higher compared to DAC (ΦF‑DAC ≈ 0.06 × ΦF‑3). Moreover, different temperature dependence of fluorescence intensity of 3 and DAC at 357 nm excitation wavelength was also observed (Figure S4, Supporting Information). These facts definitely exclude the responsibility of acetal (poloacetal) from 3 for the high ΦF of 3 in MeOH at 357 nm excitation wavelength. Although the absorption spectra of 3 in MeOH did not show any significant change with increasing concentration of 3 at first sight (Figure S3, Supporting Information), apparent dependence on the concentration and on the type of solvent was observed in the excitation spectra of 3 (Figures 6 and 7). The appearance of a new hypsochromically shifted excitation band of 3 in MeOH and EtOH (Figure 7) and the apparent concentration dependence of this band (Figure 6) indicate the formation of highly fluorescent H-aggregates. Contrary to previous reports of many H-aggregates to be nonfluorescent in nature,70 the H-aggregates of 3 in MeOH and EtOH are clearly characterized here by hypsochromically shifted fluorescence emission of high efficiency (Figure 8).

Figure 7. Normalized fluorescence excitation spectra of 3 in various solvents (T = 25 °C; c = 1 × 10−5 mol dm−3; λEM = 490 nm).

Figure 8. Fluorescence emission spectra of 3 in various solvents (T = 25 °C; c = 1 × 10−5 mol dm−3; λEX = 357 nm).

In general, the formation of H-aggregates can be explained in terms of the Kasha-exciton theory model, describing transition dipole interactions between two or more chromophores arranged parallel to each other (H-aggregates).71,72 Interaction 4876



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Scheme 3. Formation of H-Dimer Aggregates of 3 Driven by π+−π− Interactions

of two transition dipoles in the H-dimer yields energetic splitting of the excited states in two components (two exciton states). In the case of parallel alignment of dye molecules in Hdimer (parallel transition dipoles of molecules), only the transition to the higher energy exciton state is allowed because the resulting transition moment for the lower energy exciton state equals zero. This leads to a hypsochromic shift of the absorption maximum with respect to the monomer. Rapid internal conversion from the upper into the lower energy exciton state quenches the fluorescence as a result of the zero transition probability for a radiative process from the lower energy exciton state to the ground state. In accordance with that, the H-aggregates should exhibit no fluorescence, and only few exceptions to this rule have been reported.70,73 On the basis of the exciton theory model, dipole−dipole interactions between monomers of 3 are able to offer only nonfluorescent H-dimers with the opposite dipole orientation of monomers and thus with the zero overall transition moment of the resultant chromophore. Therefore, we assume that the formation of fluorescent H-dimer aggregates of 3 is driven by π+−π− interactions (Scheme 3). The observed high fluorescence intensity of coumarin 3 H-aggregates could be a consequence of suppression of the nonradiative decay channels due to rigidity in the molecular chain in the π−π-stacked aggregate. However, it is difficult to depict the concrete structure of the dimer at this stage. More detailed analysis of the absorption spectra of 3 showed that spectra in MeOH and EtOH differ little from the spectra in other solvents. The spectra of 3 in MeOH and EtOH are broader, and the indication of a new absorption band with maximum at approximately 375 nm appears in these solvents (Figure 9). The ratio of intensity of the two absorption bands at 375 and 435 nm (A 375 /A 435 ) slightly increases with increasing concentration of 3. Together with the expected stronger temperature dependence of the fluorescence intensity of the new short-wavelength band in excitation spectra of 3 in MeOH, compared to that of long-wavelength band (corresponding to the main absorption band) (Figure S4, Supporting Information), this confirms the existence of ground-state equilibrium, i.e., self-aggregation of 3 in MeOH. Moreover, static (aggregation) and dynamic quenching of fluorescence in solvent can be distinguished by their differing dependence on temperature.74 Higher temperatures result in faster diffusion (larger amounts of collisional quenching) and also in the dissociation of weakly bound complexes (smaller amounts of static quenching). The observed steeper decrease in monomer fluorescence intensity (IF) of 3 in MeOH with increased concentration of the dye (c3) at lower temperature (Figure S5A, Supporting Information) has thus proven the formation of

Figure 9. Normalized absorption spectra of coumarin 3 in various solvents (T = 25 °C; c = 1 × 10−4 mol dm−3).

fluorescent coumarin 3 H-aggregates in this solvent. Compared to temperature behavior of fluorescence for 3 in MeOH, the slope of the plot IF vs c3 in CHCl3 is higher at higher temperature (Figure S5B, Supporting Information). As shown in Figure 7, aggregates of 3 are formed only in MeOH and EtOH. No observation of aggregate formation in other highly polar solvents of both hydrogen bond acceptor and hydrogen bond donor ability (DMSO, DMF, MeCN, and H2O) or in solvents with medium polarity (CHCl3 and EA) indicates the specific equilibrium between solute−solvent, solute−solute, and solvent−solvent interactions of 3 in MeOH and EtOH. To the best of our knowledge, this is the first report on aggregation of coumarin dye in alcoholic solutions. Recently, Pal et al.26 discovered the presence of Hdimers and higher H-aggregates of coumarin-481 in aqueous solution. It should be noted that coumarin 4 does not exhibit any tendency for fluorescent aggregate formation in MeOH and EtOH at the same concentration range (10−6−10−3 mol·dm−3). Increasing concentration of 4 does not influence the shape of excitation or emission spectra of 4. Moreover, the observed steeper decrease in fluorescence intensity (IF) of 4 in both MeOH and CHCl3 with increased concentration of the dye (c4) at higher temperature (Figure S6, Supporting Information) indicates the collisional quenching of coumarin 4 fluorescence. 3.3. Preferential Solvation (Quantitative Parameters). The Covington−Newman parameters describing the PS of molecules 3 and 4 in MeOH/CHCl3 and MeOH/EA binary solvent mixtures are summarized in Tables 4−7. K1/n >1 in all three cases confirms the PS of both 3 and 4 by MeOH as a 4877



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reasonable range), as shown in both Table 4 and Figure 10. The fitting curves for various n in Figure 10 melt into one coarser curve, and these can not be differentiated by the naked eye. Although the model theoretical plots for three different K 1/n values at constant h′ and various n are clearly distinguishable curves (Figure S9, Supporting Information), the variance of both K1/n and h′ parameters in the fitting procedure completely reduces the effect of parameter n (see coefficients R2 in Tables 4 and S1, Supporting Information). This characteristic feature was observed in all investigated cases. On the basis of these findings, it can be concluded that the fitting procedure using Covington−Newman model of PS is not suitable for the determination of the solvation number (i.e., the number of molecules in the first solvation shell). Because of this complication, we decided to determine the solvation number n using the theoretical model for PS proposed by Mazurenko and co-workers and to compare these two applied models; as explained in our Supporting Information section S3.3: Preferential Solvation (Quantitative Parameters). To compare the solvation of 3 in its radiative ICT excited state by protic and aprotic solvents, the PS of 3 was also investigated in an MeCN/CHCl3 mixture (Table S1 and Figures S10 and S11, Supporting Information). The higher value of K1/n in the MeCN/CHCl3 mixture indicates a rather surprising stronger solute−solvent interaction between 3 and MeCN, compared to the 3-MeOH interaction. We assume that this observation is connected with (1) an only weak intermolecular hydrogen bonding interaction between 3 and MeOH due to the soft anionic character of the excited molecule 3 (a substantial delocalization of the negative charge) and the hard cationic character of MeOH (HSAB concept),11 and (2) no clustering tendency of the MeCN molecules (although the MeCN−MeCN long-ranging electrostatic interactions are strong),75 compared to MeOH molecules. Likewise, a markedly higher value of h′ in the MeCN/CHCl3 mixture (Table S1, Supporting Information) is most likely connected with strong intermolecular hydrogen bonding between MeOH molecules compared to MeCN molecules. Because of only weak intermolecular hydrogen bonding ability of both radiative ICT and nonradiative TICT states, and the steep linear dependence of ΦF on solvent polarity from medium to highly polar solvents, coumarin 3 could be a useful polarity probe for microenvironments containing hydrogen bonding groups. As shown by Samanta et al., with PRODAN [6-propionyl-2(dimethylaminonaphthalene)], probing the polarity of various chemical and biological systems such as protein binding sites and phospholipid membranes, contains carbonyl and tertiary amine hydrogen bond acceptor groups, so care should be taken when interpreting data from solvatochromic shifts of PRODAN in microenvironments containing hydrogen bond donor groups.76 An interesting point in the investigation of the PS of molecule 3 in MeOH/CHCl3 mixture using the Covington− Newman model is the change in the sign of the nonideality factor h′ (Tables 4 and 5) for two input emission data, fluorescence maxima λF and quantum yield of fluorescence ΦF (Figures 10 and 11), despite the same value of K1/n. This behavior is also evident in the dependence of parameter K12 for molecule 3 on the bulk mole fraction of MeOH in the MeOH/ CHCl3 mixture (Figure S12A, Supporting Information). Parameter h′ describes the solvent−solvent interaction. h′ < 0 means that the solvent components of the mixture repel each other, and a positive value of h′ (h′ > 0) indicates attraction

Table 4. Covington−Newman Parameters Describing PS of Molecule 3 in MeOH/CHCl3 Binary Solvent Mixture: Results of the Fit of Experimental λF Valuesa K1/n

h′ (h/RT)

n

R2

2.803 2.800 2.798 2.796 2.795

−0.15 −0.12 −0.09 −0.08 −0.07

4 5 6 7 8

0.9981 0.9980 0.9980 0.9980 0.9980

a Index of solute−solvent interaction (K1/n), solvation number (n), parameter representing solvent nonideality (h′), and the coefficient of determination (R2).

cosolvent. Because of the unexpected shape of λA = f(x1) curves for 3 and 4 in the MeOH/CHCl3 mixture (Figures S7 and S8, Supporting Information), the Covington−Newman parameters in this binary mixture were determined only from analysis of emission characteristics. Details of the resultant PS of 3 and 4 by MeOH are discussed in the Supporting Information section S3.3: Preferential Solvation (Quantitative Parameters). Figure 10 shows the calculated values of the mole fraction of MeOH in the solvation shell of molecule 3 (based on

Figure 10. Preferential solvation of coumarin 3 in MeOH/CHCl3 binary mixture using the values of fluorescence maxima (λF) as reference data (λEX = λA; x1, bulk mole fraction of MeOH; xL1 , mole fraction of MeOH in the solvation shell of 3). Data are fitted for different solvation numbers n = 4−8. However, very small difference in values of K1/n and h′ in fits with various n were observed, Table 4. Consequently, the fitting curves for different solvation number are overlapping.

experimental values of the fluorescence maxima λF) as a function of the bulk mole fraction of MeOH in the MeOH/ CHCl3 mixture. As shown in this figure, the mole fraction of MeOH in the solvation shell xL1 is always greater than the bulk mole fraction of MeOH (x1). The shape of the xL1 = f(x1) curve thus clearly indicates the PS of 3 by MeOH. The experimental data (xL1 = f(x1)) are well represented by K1/n = 2.8, h′ = −0.09. The positive value of K1/n indicates stronger 3-MeOH interaction in comparison with 3-CHCl3 interaction energy, and the negative value of h′ = −0.09 indicates that the solvent components strongly repel each other. A similar h′ value (h′ = −0.09) was previously reported by Banerjee et al. for the acetone−ethanol mixture.48 Although the fitting curve xL1 = f(x1) perfectly reproduces the experimentally determined values of xL1 , it is practically insensitive to parameter n (in the 4878



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Table 5. Covington−Newman Parameters Describing PS of Molecule 3 in MeOH/CHCl3 Binary Solvent Mixture: Results of the Fit of Experimental ΦF Valuesa K1/n

h′ (h/RT)

n

R2

2.58 2.58

0.10 0.083

6 7

0.9912 0.9911

a

Index of solute−solvent interaction (K1/n), solvation number (n), parameter representing solvent nonideality (h′), and the coefficient of determination (R2).

Figure 12. Preferential solvation of dye 4 in MeOH/CHCl3 binary mixture using different fluorescence parameters: maxima of fluorescence (λF) and quantum yield of fluorescence (ΦF); x1, bulk mole fraction of MeOH; xL1 , mole fraction of MeOH in the solvation shell of 4.

Approximately equal values of h′ (∼ −0.1) for both 3 and 4 in the MeOH/CHCl3 binary mixture (Tables 4 and 6) further Table 6. Covington−Newman Parameters Describing PS of Molecule 4 in MeOH/CHCl3 and MeOH/EA Binary Solvent Mixtures: Average Values from the Nonlinear Fit of Experimental λA, λF, and ΦF Valuesa

Figure 11. Preferential solvation of coumarin 3 in MeOH/CHCl3 binary mixture using the values of fluorescent quantum yield (ΦF) as reference data (λEX = λA (CHCl3); x1, bulk mole fraction of MeOH; xL1 , mole fraction of MeOH in the solvation shell of 3).

K1/n 9.50

between the solvent components. On the basis of our previous conclusions relating to fluorescent characteristics in different polarity regions, we assume that the rapid change in h′ from −0.09 to 0.09 is connected with the existence of a nonemissive TICT state, which markedly affects the fluorescent quantum yield but, however, has almost no effect on the position of the fluorescence maximum. The increasing amount of MeOH in the MeOH/CHCl3 binary mixture thus significantly influences the equilibrium between the emissive ICT state and the nonemissive dark TICT state. Figure 12 depicts the xL1 = f(x1) curve for molecule 4 in the MeOH/CHCl3 mixture based on experimental values of the emission characteristics of both the fluorescence maxima and the quantum yield of fluorescence. The higher value of K1/n in comparison with molecule 3 in the same binary mixture indicates that the interaction between 4 and MeOH is approximately 3-times stronger than the interaction between 3 and MeOH. As previously mentioned, we assume specific interactions between 4 and MeOH molecules involving hydrogen bonding between the urea part of coumarin 4 and solvent molecules. Although the xL1 data from the two sources (λF and ΦF) are a little different, their nonlinear fit using the Covington−Newman model gives almost the same values of h′. This result is also apparent from the plot of K12 vs x1 (Figure S13B, Supporting Information). The differences in xL1 are most likely induced by the different effect of 4−MeOH hydrogen bonding on the charge redistribution in the excited state of 4 (λA value), compared to the influence on the rigidity of the molecule in this electronic state. Contrary to the K12 = f(x1) curve for molecule 3, parameter K12 for molecule 4 decreases in both cases (λF and ΦF sources) with increasing bulk mole fraction of MeOH.

3.00

h/RT MeOH/CHCl3 Mixture −0.11 MeOH/EA Mixture −0.02

n 5 5

a

Index of solute−solvent interaction (K1/n), solvation number (n), and the parameter representing solvent nonideality (h′).

support our previous conclusion that the existence of the dark TICT state does not allow correct determination of h′ from the experimentally determined values of the quantum yield of fluorescence. Both the above-mentioned experiments also confirm the fact that methanol and chloroform in their binary mixture repel each other quite strongly. The PS of molecule 4 by MeOH in the MeOH/EA mixture is shown in Figure 13. In this case, both absorption (λA) and emission (λF; ΦF) characteristics were used to calculate the values of xL1 . The obtained xL1 data in the present case are well represented by nonlinear fit (x1L = f(x1) curve) with the following Covington−Newman parameters: K1/n = 3.0, h′ = −0.02 (Table 6). Substitution of CHCl3 by EA in the methanolic binary mixture leads to the 3-fold decrease in K1/n and increased h′. The value of parameter h′ fluctuates around zero, and contrary to the relatively strong repulsion between MeOH and CHCl3, this indicates only a slight repulsion between MeOH and EA. In comparison with the MeOH/CHCl3 mixture, the decrease in the Covington parameter K1/n for molecule 4 in the MeOH/EA mixture was observed as a consequence of the decrease in repulsion between cosolvents in this binary mixture. Previous thermodynamic analysis of the MeOH/EA mixture by Oswal et al. showed the dominance of interstitial accommodation of small MeOH molecules into the free volumes of EA and formation of ester−alkanol complex over (1) the reduction of hydrogen bonds between the 4879



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4. CONCLUSIONS In this article, the spectral properties of 7-(dimethylamino)coumarin-3-carbaldehyde 3 and its phenylsemicarbazone 4 were investigated in solvents of various polarity and also in several solvent mixtures. Coumarins 3 and 4 exhibit different fluorescent behavior in highly polar solvents. The solvation of 3 in the high polarity region leads to a rapid decrease in ΦF due to the population of the nonradiative TICT excited state of 3, which results in the increased nonradiative decay rate constant knr of the excited state. The temperature dependence of ΦF of 3 in solvents of various polarity showed that although the increase in the activation barrier for the nonradiative channel (ΔEa) with increasing Δf is not typical for most of the TICT molecules, it appears to be a characteristic feature of 3- or 4-acceptor substituted 7-(dialkylamino)coumarins with TICT character of the excited state in the high polarity region. Compared to 3, the ΦF of 4 exhibits a higher value in highly polar solvents with strong hydrogen bond acceptor ability. Although the hydrogen bond interactions between 4 and molecules of these solvents do not affect the charge redistribution in the excited state of 4 (λA value), they suppress the nonradiative de-excitation of this electronic state. The high quantum yield of fluorescence in DMSO, DMF, and alcohols qualifies coumarin 4 as a laser dye in the given medium by having kF higher than knr. Contrary to previous reports that many H-aggregates are nonfluorescent in nature, coumarin 3 forms highly fluorescent H-aggregates in MeOH and EtOH. On the basis fo restrictions of the Kasha-exciton theory model, we assume that the formation of fluorescent H-dimer aggregates of 3 is driven by π+−π− interactions. The high fluorescence efficiency of coumarin 3 H-aggregates could thus be a consequence of suppression of the nonradiative decay channels due to the increased rigidity of the molecular chain in the π−π-stacked aggregate. The quantitative analysis of preferential solvation of 3 and 4 in MeOH/CHCl3, MeOH/EA, and MeCN/CHCl3 binary solvent mixtures showed that the fitting procedure using the Covington−Newman model of PS is unsuitable for determination of the solvation number (that is, the number of molecules in the first solvation shell) and that combination with Mazurenko’s model of PS was required. We also found that the existence of the dark TICT state does not allow correct determination of the Covington−Newman parameter h′, representing solvent nonideality, from the experimentally determined ΦF values. The higher value of the index of solute−solvent interaction (K1/n), in comparison with molecule 3 in the same binary mixture, indicates that the interaction between 4 and MeOH is approximately 3-times stronger than the interaction between 3 and MeOH. We attribute this effect to hydrogen bonding between the urea moiety of coumarin 4 and molecules of MeOH. The comparison of the PS of 3 in MeCN/CHCl3 and MeOH/CHCl3 mixtures rather surprisingly indicates stronger solute−solvent interaction between 3 and MeCN compared to 3−MeOH interaction. We assume that this observation is connected with (1) only weak intermolecular hydrogen bonding interaction between 3 and MeOH due to the soft anionic character of the excited molecule 3 providing a substantial delocalization of the negative charge, and the hard cationic character of MeOH, and (2) the lack of clustering

Figure 13. Preferential solvation of dye 4 in MeOH/EA binary mixture using different fluorescence parameters: maxima of fluorescence (λF), quantum yield of fluorescence (ΦF), and absorption maxima (λA); x1, bulk mole fraction of MeOH; xL1 , mole fraction of MeOH in the solvation shell of 4.

molecules of MeOH, (2) the reduction of dipole−dipole interactions between EA molecules, and (3) the presence of unfavorable interactions between parts of the cosolvent molecules.77 This dominance leads to the negative values of excess molar volume VE of the MeOH/EA binary mixture. As previously mentioned, we decided to determine the solvation number n (Nm in Mazurenko’s model) using the theoretical model proposed by Mazurenko and co-workers. The calculated Mazurenko’s physicochemical parameters describing the PS of the studied molecules in both binary mixtures are shown in Table 7. Table 7. Physico-Chemical Parameters Describing Solvation of 3 and 4 in MeOH/CHCl3 and MeOH/EA Mixtures Calculated Using Mazurenko’s Modela compd

γA

3 4

0.43 ± 0.05 1.07 ± 0.06

4

0.65 ± 0.05

γF

−ΔWm(g)

MeOH/CHCl3 Mixture 0.87 ± 0.06 87.6 1.46 ± 0.20 228.1 MeOH/EA Mixture 1.02 ± 0.15 132.4

−ΔWm(e)

Nm

177.2 297.4

6−7 5

207.7

5

Mazurenko’s absorption and fluorescence parameters (γA; γF), average free orientational interaction energy for a single molecule in the solvation shell in the ground and excited states (ΔWm(g); ΔWm(e)), and the solvation number (Nm).

a

Calculated values of ΔWm(g) and ΔWm(e) show that the interactions of 3 and 4 in their ground and excited states with solvent molecules are very different. The ratio ΔWm(e)/ΔWm(g) for molecules 3 and 4 in the MeOH/CHCl3 mixture equal ∼2.0 and ∼1.4, and for molecule 4 in the MeOH/EA mixture it equals ∼1.6. This difference reflects the fact that the packing of solvent molecules around the solutes in their ground and excited states is very different. The obtained order of ΔWm (Table 7), that is ΔWm(4 in MeOH/CHCl3) > ΔWm(4 in MeOH/EA) > ΔWm(3 in MeOH/CHCl3), for both ground and excited states agrees well with the corresponding order in the Covington−Newman index of solute−solvent interaction K1/n. The determined Nm value for 3 is slightly higher than that for 4, where there are 6−7 solvent molecules for 3 compared to 5 solvent molecules for 4 (Table 7). 4880



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(11) 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. (12) Nad, S.; Pal, H. Unusual Photophysical Properties of Coumarin151. J. Phys. Chem. A 2001, 105, 1097−1106. (13) Królicki, R.; Jarzęba, W.; Mostafavi, M.; Lampre, I. Preferential Solvation of Coumarin 153: The Role of Hydrogen Bonding. J. Phys. Chem. A 2002, 106, 1708−1713. (14) Grabowski, Z. R.; Rotkiewicz, K. Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures. Chem. Rev. 2003, 103, 3899−4031. (15) Nad, S.; Kumbhakar, M.; Pal, H. Photophysical Properties of Coumarin-152 and Coumarin-481 Dyes: Unusual Behaviour in Nonpolar and in Higher Polarity Solvents. J. Phys. Chem. A 2003, 107, 4808−4816. (16) Moog, R. S.; Kim, D. D.; Oberle, J. J.; Ostrowski, S. G. Solvent Effects on Electronic Transitions of Highly Dipolar Dyes: A Comparison of Three Approaches. J. Phys. Chem. A 2004, 108, 9294−9301. (17) Dahiya, P.; Kumbhakar, M.; Mukherjee, T.; Pal., H. Effect of Protic Solvents on Twisted Intramolecular Charge Transfer State Formation in Coumarin-152 and Coumarin-481 Dyes. Chem. Phys. Lett. 2005, 414, 148−154. (18) Barik, A.; Kumbhakar, M.; Nath, S.; Pal, H. Evidence for the TICT Mediated Nonradiative Deexcitation Process for the Excited Coumarin-1 Dye in High Polarity Protic Solvents. Chem. Phys. 2005, 315, 277−285. (19) Satpati, A. K.; Senthilkumar, S.; Kumbhakar, M.; Nath, S.; Maity, D. K.; Pal, H. Investigations of the Solvent Polarity Effect on the Photophysical Properties of Coumarin-7 Dye. Photochem. Photobiol. 2005, 81, 270−278. (20) Raikar, U. S.; Renuka, C. G.; Nadaf, Y. F.; Mulimani, B. G.; Karguppikar, A. M.; Soudagar, M. K. Solvent Effects on the Absorption and Fluorescence Spectra of Coumarins 6 and 7 Molecules: Determination of Ground State and Excited State Dipole Moment. Spectrochim. Acta, Part A 2006, 65, 673−677. (21) Satpati, A. S.; Kumbhakar, M.; Nath, S.; Pal, H. Photophysical Properties of Coumarin-7 Dye: Role of Twisted Intramolecular Charge Transfer State in High Polarity Protic Solvents. Photochem. Photobiol. 2009, 85, 119−129. (22) Wagner, B. D. The Use of Coumarins as EnvironmentallySensitive Fluorescent Probes of Heterogeneous Inclusion Systems. Molecules 2009, 14, 210−237. (23) Hrdlovič, P.; Donovalová, J.; Stankovičová, H.; Gaplovský, A. Influence of Polarity of Solvents on Spectral Properties of Bichromophoric Coumarins. Molecules 2010, 15, 8915−8932. (24) Danko, M.; Szabo, E.; Hrdlovič, P. Synthesis and Spectral Characterization of Fluorescence Dyes Based on Coumarin Fluorophore and Hindered Amine Stabilizer in Solution and Polymer Matrices. Dyes Pigments 2011, 90, 129−138. (25) Barooah, N.; Mohanty, J.; Pal, H.; Bhasikuttan, A. C. NonCovalent Interactions of Coumarin Dyes with Cucurbit[7]uril Macrocycle: Modulation of ICT to TICT State Conversion. Org. Biomol. Chem. 2012, 10, 5055−5062. (26) Verma, P.; Pal, H. Intriguing H-Aggregate and H-Dimer Formation of Coumarin-481 Dye in Aqueous Solution As Evidenced from Photophysical Studies. J. Phys. Chem. A 2012, 116, 4473−4484. (27) Donovalová, J.; Cigáň, M.; Stankovičová, H.; Gašpar, J.; Danko, M.; Gaplovský, A.; Hrdlovič, P. Spectral Properties of Substituted Coumarins in Solution and Polymer Matrices. Molecules 2012, 17, 3259−3276. (28) Cigáň, M.; Filo, J.; Stankovičová, H.; Gaplovský, A.; Putala, M. Spectral Properties of Binaphthalene−Coumarins Interconnected Through Hydrazone Linkage. Spectrochim. Acta, Part A 2012, 89, 276−283.

tendency of MeCN molecules compared to MeOH molecules. Because of the weak intermolecular hydrogen bonding ability of both the radiative ICT and the nonradiative TICT excited state of 3 and the steep linear dependence of ΦF on solvent polarity from medium to highly polar solvents, coumarin 3 could be a useful polarity probe for microenvironments containing hydrogen bonding groups.

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ASSOCIATED CONTENT

S Supporting Information *

Additional tables and figures, theoretical background of PS, and details of the resultant PS of 3 and 4 by MeOH. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(M.C.) Tel: +421-2-60296306. Fax: +421-2-60296337. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication is the result of the project implementation “The Competence Center for Intelligent Technologies to Enable Electronization and Informatization of Systems and Services,” ITMS 26240220072, supported by the Research & Development Operational Programme funded by the ERDF.

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REFERENCES

(1) Woods, L. L.; Shamma, S. M. Synthesis of Substituted Coumarins with Fluorescent Properties. J. Phys. Chem. 1970, 74, 4234−4240. (2) Jones, G., II; Jackson, W. R.; Kanoktanaporn, S.; Halper, A. M. Solvent Effects on Photophysical Parameters for Coumarin Laser Dyes. Opt. Commun. 1980, 33, 315−320. (3) Jones, G., II; Jackson, W. R.; Choi, C.; Bergmark, W. R. Solvent Effects on Emission Yield and Lifetime for Coumarin Laser Dyes. Requirements for a Rotatory Decay Mechanism. J. Phys. Chem. 1985, 89, 294−300. (4) Nag, A.; Bhattacharyya, K. Role of Twisted Intramolecular Charge Transfer in the Fluorescence Sensitivity of Biological Probes: Diethylaminocoumarin Laser Dyes. Chem. Phys. Lett. 1990, 169, 12− 16. (5) Patalakha, N. S.; Yufit, D. S.; Kirpichënok, M. A.; Gordeeva, N. A.; Struchkov, Y. T.; Grandberg, I. I. Luminiscence-Spectral and AcidBase Characteristics of 3-Aryl-7-Diethylaminocoumarins. Khim. Geterotsikl. Soedin. 1991, 1, 40−46. (6) Abdel-Mottaleb, M. S. A.; Antonious, M. S.; Abo Ali, M. M.; Ismail, L. F. M.; El Sayed, B. A.; Aherief, A. M. K. Photophysics and Dynamics of Coumarin Laser Dyes and Their Analytical Implications. Proc. Indian Acad. Sci. 1992, 104, 185−196. (7) Yip, R. W.; Wen, Y.-X.; Szabo, A. G. Decay Associated Fluorescence Spectra of Coumarin 1 and Coumarin 102: Evidence for a Two-State Solvation Kinetics in Organic Solvents. J. Phys. Chem. 1993, 97, 10458−10462. (8) López Arbeloa, T.; López Arbeloa, F.; Tapia, M. J.; López Arbeloa, I. Hydrogen-Bonding Effect on the Photophysical Properties of 7-Aminocoumarin Derivatives. J. Phys. Chem. 1993, 97, 4704−4707. (9) Bangar, R. B.; Varandajaran, T.S. Substituent and Solvent Effects on the Twisted Intramolecular Charge Transfer of Three New 7(Diethylamino)coumarin-3-aldehyde Derivatives. J. Phys. Chem. 1994, 98, 8903−8905. (10) Raju, B. B.; Costa, S. M. B. Photophysical Properties of 7Diethylaminocoumarin Dyes in Dioxane−Water Mixtures: Hydrogen Bonding, Dielectric Enrichment and Polarity Effects. Phys. Chem. Chem. Phys. 1999, 1, 3539−3547. 4881



Article

(29) Chatterjee, A.; Maity, B.; Seth, D. The Photophysics of 7-(N,N′Diethylamino)coumarin-3-carboxylic Acid in Water/AOT/Isooctane Reverse Micelles: An Excitation Wavelength Dependent Study. Phys. Chem. Chem. Phys. 2013, 15, 1894−1906. (30) Demchenko, A. P. Advanced Fluorescence Reporters in Chemistry and Biology I, Fundamentals and Molecular Design; Springer Series on Fluorescence; Springer: Heidelberg, Germany, 2010; Vol. 8. (31) Li, H.; Cai, L.; Chen, Z. In Coumarin-Derived Fluorescent Chemosensors. Advances in Chemical Sensors; Wang, W., Ed.; InTech: New York, 2012. Available from http://www.intechopen.com/books/ advances-in-chemical-sensors/coumarin-derived-fluorescentchemosensors. (32) Prasanna de Silva, A.; Gunaratante, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; Mc Coy, C. P.; Rademacher, J. T.; Rice, T. E. Signaling Recognition Events with Fluorescent Sensors and Switches. Chem. Rev. 1997, 97, 1515−1566. (33) Wu, J.; Liu, W.; Ge, J.; Zhang, H.; Wang, P. New Sensing Mechanism for Design of Fluorescent Chemosensors Emerging in Recent Years. Chem. Soc. Rev. 2011, 40, 3483−3495. (34) Kim, G.-J.; Kim, H.-J. Doubly Activated Coumarin As a Colorimetric and Fluorescent Chemodosimeter for Cyanide. Tetrahedron Lett. 2010, 51, 185−187. (35) Park, S.; Kim, H.-J. Highly Selective Chemodosimeter for Cyanide Based on a Doubly Activated Michael Acceptor Type of Coumarin Thiazole Fluorophore. Sens. Actuators, B 2012, 161, 317− 321. (36) Lin, Q.; Bao, C.; Fan, G.; Cheng, S.; Liu, H.; Liu, Z.; Zhu, L. 7Amino Coumarin Based Fluorescent Phototriggers Coupled with Nano/Bio-Conjugated Bonds: Synthesis, Labeling and Photorelease. J. Mater. Chem. 2012, 22, 6680−6688. (37) Lv, X.; Liu, J.; Liu, Y.; Zhao, Y.; Sun, Y.-Q.; Wang, P.; Guo, W. Ratiometric Fluorescence Detection of Cyanide Based on a Hybrid Coumarin−Hemicyanine Dye: the Large Emission Shift and the High Selectivity. Chem. Commun. 2011, 47, 12843−12845. (38) Kwon, H.; Lee, K.; Kim, H.-J. Coumarin−Malonitrile Conjugate As a Fluorescence Turn-On Probe for Biothiols and Its Cellular Expression. Chem. Commun. 2011, 47, 1773−1775. (39) Yang, Z.; Liu, Z.; Chen, Y.; Wang, X.; He, W.; Lu, Y. A New Ratiometric and Colorimetric Chemosensor for Cyanide Anion Based on Coumarin−Hemicyanine Hybrid. Org. Biomol. Chem. 2012, 10, 5073−5076. (40) Kim, G.-J.; Kim, H.-J. Coumarinyl Aldehyde As a Michael Acceptor Type of Colorimetric and Fluorescent Probe for Cyanide in Water. Tetrahedron Lett. 2010, 51, 2914−2916. (41) Kim, T.-K.; Lee, D.-N.; Kim, H.-J. Highly Selective Fluorescent Sensor for Homocysteine and Cysteine. Tetrahedron Lett. 2008, 49, 4879−4881. (42) Kim, G.-J.; Kim, H.-J. Highly Selective and Sensitive Fluorescence Turn-On Probe for Proline. Tetrahedron Lett. 2010, 51, 4670−4672. (43) Shao, J. A Novel Colorimetric and Fluorescence Anion Sensor with a Urea Group As Binding Site and a Coumarin Group As Signal Unit. Dyes Pigments 2010, 87, 272−276. (44) Ghosh, K.; Adhikari, S. Colorimetric and Fluorescence Sensing of Anions Using Thiourea Based Coumarin Receptors. Tetrahedron Lett. 2006, 47, 8165−8169. (45) Boiocchi, M.; Del Boca, L.; Gómez, D. E.; Fabbrizzi, L.; Licchelli, M.; Monzani, E. Nature of Urea−Fluoride Interaction: Incipient and Definitive Proton Transfer. J. Am. Chem. Soc. 2004, 126, 16507−16514. (46) Covington, A. K.; Newman, K. E. Approaches to the Problems of Solvation in Pure Solvents and Preferential Solvation in Mixed Solvents. Pure Appl. Chem. 1979, 51, 2041−2058. (47) Covington, A. K.; Newman, K. E. Application of Kirkwood− Buff Theory to Free Energies of Transfer of Electrolytes from One Solvent to Another. J. Chem. Soc., Faraday Trans. I 1988, 84, 1393− 1404.

(48) Banerjee, D.; Laha, A. S.; Bagchi, S. Preferential Solvation in Mixed Binary Solvent. J. Chem. Soc., Faraday Trans. 1995, 91, 631− 636. (49) Flašík, R.; Stankovičová, H.; Gáplovský, A.; Donovalová, J. Synthesis and Study of Novel Derivatives Potentially Utilizable As Memory Media. Molecules 2009, 14, 4838−4848. (50) PHYWE Series of Publications. Boiling Point Diagram of a Binary Mixture, LEC 03.04; PHYWE Systeme GmbH & Co.: Göttingen, Germany. Available from http://www.phywe.fr/index.php/fuseaction/ download/lrn_file/versuchsanleitungen/P3030401/e/LEC03_04_LV. pdf. (51) Mancini, P. M.; del C. Pérez, A.; Vottero, L. R. Nonspecific Solute-Solvent Interactions in Binary Solvent Mixtures Containing an Aprotic Hydrogen-Bond Acceptor and a Hydrogen-Bond Donor: Dipolarity/Polarizability and Refractive Index. J. Sol. Chem. 2001, 30, 695−707. (52) Lippert, E. Dipolmoment und Elektronenstruktur von Angeregten Molekulen. Z. Naturforsch., A: Phys. Sci. 1955, 10, 541−545. (53) Mataga, N.; Kaifu, Y.; Koizumi, M. Solvent Effects upon Fluorescence Spectra and the Dipole Moments of Excited Molecules. Bull. Chem. Soc. Jpn. 1956, 29, 465−470. (54) Valeur, B. Molecular Fluorescence: Principles and Aplications; Wiley-VCH: Berlin, Germany, 2001. (55) Banerjee, D.; Bagchi, S. Solution Photophysics of Ketocyanine Dyes in Neat and Mixed Binary Solvents. J. Photochem. Photobiol., A 1996, 101, 57−62. (56) Wolfram, S. The Mathematica Book, 5th ed.; Addison-Wesley: Redwood City, CA, 2003. (57) Press, W. H.; Teukolsky, S.; Vetterling, W.; Flannery, B. Numerical Recipes: The Art of Scientific Computing; Cambridge University Press: New York, 2007. (58) Ben-Naim, A. Preferential Solvation in Two-Component Systems. J. Phys. Chem. 1989, 93, 3809−3813. (59) Sasirekha, V.; Vanelle, P.; Terme, T.; Ramakrishnan, V. Solvatochromism and Preferential Solvation of 1,4-Dihydroxy-2,3dimethyl-9,10-anthraquinone by UV−Vis Absorption and LaserInduced Fluorescence Measurements. Spectrochim. Acta, Part A 2008, 71, 766−772. (60) Chatterjee, P.; Bagchi, S. Preferential Solvation of a Dipolar Solute in Mixed Binary Solvent: A Study by UV−Visible Spectroscopy. J. Phys. Chem. 1991, 95, 3311−3314. (61) Mazurenko, Y. T. Universalnie Bzaimodeistvii v Trex Componentov Jidkostiax. Opt. Spektrosk. 1972, 33, 1060−1064. (62) Józefowicz, M.; Heldt, J. R. Preferential Solvation of Fluorenone and 4-Hydroxyfluorenone in Binary Solvent Mixtures. Chem. Phys. 2003, 294, 105−116. (63) Józefowicz, M. The Influence of Hydrogen Bonds and Preferential Solvation on Spectroscopic Properties of Methyl pDimethylaminobenzoate and Its Ortho Derivative in Binary Solvent Mixture. Chem. Phys. 2011, 383, 19−26. (64) Ooyama, Y.; Asada, R.; Inoue, S.; Komaguchi, K.; Imae, I.; Harima, Y. Solvatochromism of Novel Donor−π−Acceptor Type Pyridinium Dyes in Halogenated and Non-Halogenated Solvents. New J. Chem. 2009, 33, 2311−2316. (65) Kawski, A. On the Estimation of Excited-State Dipole Moments from Solvatochromic Shifts of Absorption and Fluorescence Spectra. Z. Naturforsch. 2002, 57a, 255−262. (66) Rauf, M. A.; Soliman, A. A.; Khattab, M. Solvent Effect on the Spectral Properties of Neutral Red. Chem. Cent. J. 2008, 2, 19−26. (67) Senthilkumar, S.; Nath, S.; Pal, H. Photophysical Properties of Coumarin-30 Dye in Aprotic and Protic Solvents of Varying Polarities. Photochem. Photobiol. 2004, 80, 104−111. (68) Kaholek, M.; Hrdlovič, P. Spectral Properties of Coumarin Derivatives Substituted at Position 3. Effect of Polymer Matrix. J. Photochem. Photobiol., A 1997, 108, 283−288. (69) Rettig, W. Photoinduced Charge Separation via Twisted Intramolecular Charge Transfer States; Topics in Current Chemistry, Electron Transfer-I; Mattay, J., Ed.; Springer-Verlag: Berlin, Germany, 1994; Vol. 169, pp 253−299. 4882



Article

(70) Rösch, U.; Yao, S.; Wortmann, R.; Würthner, F. Fluorescent HAggregates of Merocyanine Dyes. Angew. Chem., Int. Ed. 2006, 45, 7026−7030. (71) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. The Exciton Model in Molecular Spectroscopy. Pure Appl. Chem. 1965, 11, 371−392. (72) Davydov, A. S. Theory of Molecular Excitons; McGraw-Hill: New York, 1962. (73) Kumar, V.; Baker, G. A.; Pandey, S. Ionic Liquid-Controlled Jversus H-Aggregation of Cyanine Dyes. Chem. Commun. 2011, 47, 4730−4732. (74) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer Science + Business Media: New York, 2006; Chapter 8, p 280. (75) Kovacs, H.; Kowalewski, J.; Laaksonen, A. Molecular Dynamics Simulation of Liquid Mixtures of Acetonitrile and Chloroform. J. Phys. Chem. 1990, 94, 7378−7385. (76) Samanta, A.; Fessenden, R. W. Excited State Dipole Moment of PRODAN As Determined from Transient Dielectric Loss Measurements. J. Phys. Chem. A 2000, 104, 8972−8975. (77) Oswall, S. L.; Putta, S. S. R. Excess Molar Volumes of Binary Mixtures of Alkanols with Ethyl Acetate from 298.15 to 323.15 K. Thermochim. Acta 2001, 373, 141−152.

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