J. Phys. Chem. 1993,97, 4704-4707
4704
Hydrogen-Bonding Effect on the Photophysical Properties of 7-Aminocoumarin Derivatives Teresa Mpez Arbeloa, Fernando Mpez Arbeloa, M. Jose Tapia, and Iaigo Mpez Arbeloa' Departamento de Quimica-F'isica, Universidad del Pais Vasco-EHU, Apartado 644, 48080 Bilbao, Spain Received: December 29, 1992
The photophysical properties of 7-amimocoumarin derivatives with different alkylation degrees are studied in water and in different monoalcohols. Specific hydrogen bonds between the solute and protic solvents are considered to explain spectral shifts and internal conversion process. The mechanism for the internal conversion process via a twisted intramolecular charge transfer state and the umbrella-like motion model is discussed.
Introduction Fluorescence dyes have been extensively used as laser dyes, solar energy concentrators, and fluorescence Certain dye families show an important charge redistribution upon excitation, and their photophysical properties depend on the molecular structure and on environmental factors. Coumarin derivatives with an amino group in the 7-position are an example of this type of dye. Investigationsare still necessary to explain not only the effect of the molecular structure and the nature of the solvent but also how specific solute/solvent interactions (particularly hydrogen bonds) can affect these properties. The nonradiative deactivation process competes with the fluorescence emission of these dyes, controlling their laser properties. In the past decade, an increasing number of researchers have attributed the nonradiative deactivation of 7-aminocoumarin derivatives to the formation of the so-called twisted intramolecular charge transfer (TICT) state from the SI single excited ~ t a t e . ~ The - ' ~ population probability of this TICT state depends on the electron donor-acceptor capacities of the involved partners and on the solvent polarity which would stabilize the highly polar structure. Furthermore, solvent viscosity could also affect the nonradiative deactivation since the process involves a rotational motion of the bond linking the partners. The concept of the TICT state was first proposed by Grabowski et a1.15 to explain the dual fluorescenceof N,N-dimethylaminobenzonitrile derivatives in polar solvents. Recently, an alternativeand, to a certain extent, complementary mechanism has been considered to explain the internal conversion of rhodamines, another amino dye family.l63l7This mechanism identifies the internal conversion process to a structural change of the amino group from a planar N+-aromatic configuration (with sp2 hybridation for the nitrogen atom) to a pyramidal N-aromatic one (with sp3 hybridation for the nitrogen atom), the so-called open-closed umbrella-like motion (ULM) mechanism. Other authors haveattributed the photophysicalproperties of some aminoaromatic molecules to the hybridation change of the amino group.'* The present work investigates the photophysical properties of 7-aminocoumarins in solution. The effect of the molecular structure of the dye is studied using 7-aminocoumarins with different alkylation of the amino group: non-, mono- and diethylaminocoumarin (Cl20, C2, and C 1, respectively, Figure 1). Although the effect of the solvent polarity has been largely investigated,8J2J3hydrogen bonds between coumarins and protic solvents can also affect the photophysics of the dyes.'9JO To know how the latter interactions affect not only the absorption and emission spectra but also the nonradiative process, water and a series of linear monoalcohols are chosen as solvent. TICT and ULM models are considered to explain the nonradiative deactivation in which solvent polarity and viscosity and hydrogenbond solute/solvent interactions are taken into account. 0022-3654/93/2097-4704%04.00/0
R3v CH3
Figure
1.
C2:
Et H Me
C1:
Et Et H
Molecular structure of the 7-aminocoumarin derivatives.'
Experimental Section 7-AminocoumarinsC120, C2, and C1 were supplied by Kodak (laser grade) and used without further purification. Water was double-distilled' and absolute methanol (MeOH, Merck for spectroscopy), ethanol (EtOH, Merck pro-analysis), and l-propanol (PrOH, Merck pro-analysis) were dried by distillation.*' The other alcoholic solvents, 1-butanol (BuOH, BDH, 99.5%), 1-hexanol(HxOH, Merckfor synthesis), and 1-decanol(DcOH, Merck for synthesis), were used without any purification. The dye concentration was always less than 1 t 5 M to avoid dye aggregation.22 Since the atmospheric oxygen quenches to some extent the S1excited state of these dyes? samples were desgassed by bubbling argon during 10 min. Absorption and emission spectra were recorded on a Shimadzu (Model UV-240) spectrophotometer and a Shimadzu (Model RF-5000) spectrofluorometer, respectively, immediately after sample preparation. The fluorescence quantum yield was evaluated using the quinine bisulfate (Aldrich Chemie)/ 1.O N H2S04 aqueous solution (4' = 0.55 at 20 'CZ3) as reference. Radiative decay curves were registered by the time-correlating single photon counting technique using an Edinburgh Instrument (Model qF-900). The excitation source was a flash lamp with a full-wide at the middle-height pulse of 1.6 ns. The goodness of the fit of the deconvoluted decay curves was controlled by the reduced chi-square statistical parameter. All of the decay curves were analyzed as a single exponential (x2< 1.2).
Results and Discussion The absorption and emission spectral maxima of the studied 7-aminocoumarin derivatives in water and in different linear monoalcohols are listed in Table I. The large Stokesshift indicatesan increase in the dipole moment upon excitation. This behavior has been attributed to a charge redistribution of the excited state with respect to the ground state.1q11.2"26Thus, the statistical weight of the resonance structures a and b of Figure 2 changes from predominantly structure a in the ground state to predominantly structure b in the first excited state. The latter structure can be described as a planar intramolecular charge transfer (ICT) state, explaining the greater solvent polarity effect observed on the emission maximum wavelength than on that of absorption (Table I). The maximum of absorption and emission spectra depends on the molecular structure of the dye. When the alkylation degree 0 1993 American Chemical Society
Photophysical Properties of 7-Aminocoumarin Derivatives
The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 4705
TABLE I: Wavelength of the Absorption and Emission Maximum, &,and L,,, (nm), of 7-Aminocoumarin Derivatives in Different Solvents Xab
H20 MeOH EtOH
PrOH BuOH HxOH DcOH
DcOH
480
HxOH BuOH M H &OH MIOH H 2 0 I
I
I
I
I
I
1
I
'
1
I
I
'
I A
Xeln
c120
c2
c1
c120
c2
c1
342.0 351.5 353.5 354.0 354.5 354.5 355.0
361.5 365.5 365.0 365.0 366.0 366.0 365.5
381.5 374.5 373.0 373.5 373.5 373.5 371.5
439.5 429.0 427.5 427.0 427.0 427.0 425.0
456.0 438.0 434.0 432.5 431.5 431.0 430.0
472.0 455.0 447.0 444.5 443.0 441.0 435.0
460
440
> 0
1
a b (ICT) Figure 2. Resonance structures of the 7-aminocoumarin derivatives. at the amino group is increased (C120< C2 < Cl),a spectral shift to lower energies is observed (Table I). This is due to the inductive effect +I of the ethyl substituent, which stabilizes the structure resonance b of Figure 2, increasing its statistical ~eight.I*~$' The effect of the solvent on the observed spectral shifts depends on the molecular structure of the dye, as Jones et al. have reported.25 Furthermore, these shifts are different for the absorption spectrum from those for the emission one (Table I). Thus, the wavelength of the absorption maximum of the nonethylated C120 increases by changing water for any alcohol, whereas the maximum of diethylated C1 decreases (Figure 3). The emission maximum shifts to higher energies in alcoholic solutions than in water for all coumarins studied, though it is greater for the diethylated derivative than for the nonethylated one (Figure 3). Lippert representation for the general solvent effect on the spectral shifts predicts a red shift in the absorption and emission spectra and a increase of the Stokes shift by increasing solvent p ~ l a r i t y . For ~ ~ coumarin ~~~ dyes, two relationships have been reported:8J2~'3J9~25~29 one obtained in nonpolar and polar aprotic solvents and a second in alcoholic solutions. This difference has been attributed to the formation of hydrogen bonds between the solute and the alcoholic molecules. The results of Table I1 show a Lippert relationship for the monoalcoholsolvents,but a different behavior is obtained in aqueous solution. Specific solute/solvent interactions and, more concretely, H-bondings would be different in water than in alcohols, and these interactions would depend on the ethylation degree at the amino group of the dye. Different hydrogen bonds have been proposed for 7-aminocoumarin derivatives in protic solvent^^^^^^ (Figure 4). Interaction A would be favored in alcoholic solutions for the coumarin with diethylaminogroup, C 1, by hydrophobic forces, whereas the water molecules would be avoided at the diethylamino fragment of the dye. This fact could explain the very poor solubility of this coumarin derivative in water; besides, similar arguments have been used to explain the much higher tendency to aggregate in water than in ethanol of other aminoaromatic dyes, such as rhodamines.30JI In theunsubstituted aminocoumarin C120,water molecules are not avoided at the amino group and a higher participation of interaction A can be expected in aqueous than in alcoholic solutions due to the greater acidity of the former solvent. For the same reason, interaction B is more probable in water and interaction C, which is possible only in C120 and C2 dyes, is more probable in alcohols, increasing with the alcohol size. Due to the different electron distribution of the electronic states, interaction A is more probable in the ground state (predominantly structure a), whereas interactions B and C are
5
l
1 3401 19
I
22
I
26
I
29
,& J 33
Af.102 Figure 3. Wavelength of the absorption (open symbols) and emission (solid symbols) maximum of C120 (circles), C 2 (squares), and C 1 (triangles), in different solvents, vs Lippert parameter.
more important in the first excited state (predominantly structure b) (Figure 4). A long-range electrostatic interaction between the positive charge of the iminium group and the electron lone pair of the solvent OH group would also be possible for aminoaromatic compounds.16J7 In the case of coumarins 120 and 2, this interaction would suppose the stabilization of the resonance structure b, favoring interaction B. These differences between the specific interactions of the 7-aminocoumarin derivatives in water and in alcohols and in the two electronic states could explain the spectral shifts observed (Table I). The shift to higher energies of the absorption band of C120 in water with respect to those in alcohols would be attributed to an increase of interaction A in the former solvent, whereas for C1 this interaction is more important in alcohols and therefore an opposite effect is observed. The behavior of C2 is an intermediate case. The larger shift to lower energies of the emission spectra of C1 with respect to C2,and the latter with respect to C120,by increasing the solvent polarity could be due to an augmentation of interaction B upon the ethylation at the amino group since the resonance structure b (Figure 4) is more probable. Moreover, interaction C in C120 and C2 could also contribute to the shorter emission spectral shift of these derivatives with respect to that for C1. From the obtained fluorescence lifetimes (TO) and the fluorescence quantum yield (#'), the rate constant of the radiative (kf) and nonradiative (k,,) deactivation can be separately determined by kf = #"/TO and k,, = (1/~") - kf.The results are given in Table 11. The k,, values are temperature-independent except for C1,which shows an Arrhenius behavior characterized by an activation energy (Ea),also given in Table 11. The kf/n2value (where n is the refraction index of the solvent) of the 7-aminmumarin derivatives tends to decrease when the size of the alcoholic solvent increases, and it is lower in water
4706 The Journal of Physical Chemistry, Vol. 97, No. 18, 1993
Ldpez Arbeloa et al.
TABLE II: Fluorescence Quantum Yield, r$O, Fluorescence Lifetimes, 7’ (ns), and Nonradiative Rate Constant, k., (d) of 7-AminocoumaMs in Different Solvents at 20 OC’ To
Hz0 MeOH EtOH
PrOH BuOH
HxOH DcOH I?
c120
c2
c1
c120
c2
c1
c120
0.94 0.95 0.90 0.86 0.82 0.76 0.72
0.95 0.96 0.93 0.8 1 0.85 0.74 0.56
0.07 0.46 0.75 0.77 0.78 0.74 0.59
4.88 4.40 4.10 4.00 3.90 3.84 3.80
4.78 4.32 3.96 3.82 3.75 3.71 3.60
0.40 2.07 3.38 3.65 3.71 3.72 3.50
0.12 0.1 1 0.24 0.35 0.46 0.63 0.74
knr X c2 0.12 0.09 0.15 0.50 0.40 0.70 1.22
(Ea) c1 23.2 (69.0) 2.61 (5.35) 0.75 (2.84) 0.66 0.59 0.69 1.17 (5.00)
The activation energy of the nonradiative deactivation, Ea (kJ/mol), of C1 is shown in parentheses.
R.
R ‘0
R
7’
H’
‘0
i Interaction A
Interaction C
Interaction B
j
Interaction C
a
b Fipre4. Specific interactions for 7-aminmumarins i n p r o t i c s ~ l v e n t s . ~ ~ ~ ~ ~
than in methanol. Similar variations are observed for the molar absorptivity. Since kf/n2 values should be independent of the these results corroborate that the specific 7-aminmumarin/protic solvent interactions aredifferent in water from those in alcohol. The intersystem crossing probability of 7-aminocoumarins is very mall;^,)^ thus, the nonradiative deactivation of these dyes is mainly due to internal conversion process. Table I1 shows that the rate constant of this process increases upon the alkylation of the amino group. The influence of the solvent on the k,, value depends on the molecular structure of the dye. Hence, for C120 and C2 the internal conversion rate constant is higher in alcohols (except methanol) than in water but for C1 shows the opposite behavior. These experimental results suggest that the specific solute/solvent interactions have an important effect on the nonradiative deactivation process of these dyes, as has been previously proposed by Masilamani et al.33 Different authors9J have identified the nonradiative deactivation of 7-aminocoumarinswith the formation of a twisted intramolecular charge transfer (TICT) state from the ICT state of the dye. The TICT state formation in 7-aminmumarin derivativesis characterized by an electron transfer from the amino group to the coumarin ring followed by a twisting rotation between both partners (Figure 5 ) . Therefore, the formation of this state depends on the electron donor-acceptor capacities of the moieties and on the solvent polarity. Since a rotational motion is involved in the TICT state, this process would also depend on the rigidity of the molecular system and on the solvent viscosity, as Jones et al. have previously proposed.9 In spite of the numerous publications, the formation of the TICT state in coumarins has not received any experimental confirmation. The emission from this state, to our knowledge,has not been observed yet. Some authors have reported amplified spontaneous emission (ASE) in two distinct bands in protic solvents for Cl33 and C120.i2 One band was first attributed to the TICT state of the dye, but this fact was denied later.34 On the other hand, no evidence was found for the formation of the TICT state by the isolated jet-cooled coumarin 152 using continuous supersonic jet t e c h n i q ~ e . ~ ~ The TICT model could explain to some extent the results of k,, obtained in this work (Table 11). Thus, the electron donor capacity of the amino group increases upon alkylation (due to the inductive effect +I), leading to a higher probability of TICT state formation in the diethylaminocoumarin. Since the TICT 1-14925
m
U M (a)
Figure 5. Mechanisms of the internal conversion process proposed for 7-aminocoumarin derivatives.
state is characterized by an increase of the charge separation, it should be stabilized by the solvent polarity, explaining the higher k,, value in water than in alcohols for C1. However, the TICT model, consideringonly the general solvent effect, cannot explain the decrease of nonradiative rate constant for C120 and C2 upon solvent polarity, as well as the increase ink,, of C1 in long-chain alcohols (Table 11). A similar tendency is observed in other amino dyes such as rhodamines, where the TICT state is thought not to be formed for no- and monoethyl derivative^.)^,^^ Since the k,, value does not correlate with either solvent polarity or solvent viscosity, other medium effects should be taken into account, such as the above-mentionedspecificsolute/ solvent interactions. Thus, Masilamani et proposed that the stabilization of the TICT state by the solvent is due mainly to specific solute/solvent interactions rather than solvent polarity. As interaction A is not important in the SIexcited state and interaction B is common in all of the coumarins, these hydrogen bonds would not be responsible for the different solvent effect on the k,, values observed for C120, C2, and C1. Interaction C (Figure 4), which is only possible in C120 and C2 dyes, could explain the increase of the k,, value with the size of the alcohol in these derivatives. Thus, this interaction stabilizes the positive charge at the nitrogen atom and, therefore, increases the k,, value, as is observed (Table 11). The nonradiative deactivation via a TICT state could not adequately explain the k,, values of C1 in long-chain alcohols. As the alcoholic size rises, both the polarity and acidity of the solvent decrease while theviscosity increases. All of these factors are expected to diminish the k,, values via the TICT state: for instance, a low-polar and high-viscoussolvent would lead to a less stable TICT state and would reduce the twisting motion of the amine-coumarin bond, respectively; in low-acid solvent, interaction B would be less probable, inducing a less stable TICT state. However, theopposite behavior is experimentallyobserved (Table 11). An alternative mechanism for the radiationless deactivation of aminoaromatic dyes could be the ULM model, which identifies
Photophysical Properties of 7-Aminmumarin Derivatives the internal conversion of rhodamines as a structural change of the amino group from a planar structure to a pyramidal one.16J7,38,39 Applying this model to 7-aminmumarins, it involves a disruption of the amine+-coumarin-double bond to an aminecoumarin single bond and a redistribution of the ?r-system(Figure 5 ) . The nonradiative deactivation leads to a structure which is similar to that of the ground state. So, the k,,valuecouldindicate the tendency of the dye to reach the resonance structure a of the ground state from the resonance structure b of the S1 excited state (Figures 2 and 5 ) . Consequently, 7-aminocoumarin derivatives with a high electron density in the amine+=coumarinbond in theexcitedstate (a high statistical weight ofthe resonance structure b) would have a high probability for internal conversion. The increase of the k,, values upon alkylation at the amino group can be also explained by the ULM model. The inductive effect +I of the ethyl group increases the statistical weight of the resonance structure b (Figure 2) and, consequently, the double amine+=coumarin- bond character. Since polar solvents stabilize the resonance structure b, the amine+-coumarindouble-bond character increases. Therefore, the ULM model predicts an augmentation of the k,, value with solvent polarity.16J7-38,39 Thus, this solvent effect, as in the TICT model, can explain the decrease of the probability of the nonradiative process for C1 in water with respect to that in shortchain alcohols (Table 11). However, the solvent polarity and/or viscosity cannot explain either the behavior of C120 and C2 in the solvents used in this workor that of C1 in long-chain alcohols. So, specific solute/solvent interactions have to be taken into account also in this model. The ULM model assumes that all interactions which restrict the electron flow of the mystem would decrease the nonradiative deactivation p r ~ b a b i l i t y . ’ ~On , ~ one ~ , ~hand, ~ the three coumarin derivatives show similar tendencies with the size of the longchain alcoholic solvents; the increase in the k,, value could be attributed to the weakness of interaction B due to the decrease of the solvent acidity. On the other hand, the anomalous behavior of the nonradiative process for C120 and C2 with solvent polarity in water and short-chain alcohols could be due to the decrease of interaction C. This interaction rises the amine+-coumarindouble-bond character and, consequently, the k,, value. The solvation qf the iminium group (above-mentioned electrostatic interaction) could also affect the different behavior of C1 with respect to C120 and C2. This interaction depends on alkylation at the amino group of the dye due to hydrophobic forces. Thus, this interaction would stabilize structure bin alcohols with respect to that in water for C1, while the opposite behavior would be expected for C120 and C2. The temperature dependence of the k,, value could be considered as a corroboration of the ULM model. Thus, systems showingan important activation energy are precisely those whose k,, value is higher (Table 11).
Conclusion The photophysical properties of 7-aminocoumarin derivatives in water are very different from those in alcoholic solutions due to solute/solvent hydrogen bonds. These interactions depend on the alkylation degree of the amino group and affect not only the absorption and emission spectral shifts but also the nonradiative deactivation process. The influence of these specific
The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 4707 interactions on the nonradiative process is better explained by the ULM model than the TICT one, although both could be complementary.
Acknowledgment. We thank Gobierno Vasco for financial support and for awarding grants to T.L.A. and M.J.T. References and Notes (1) (2) (3) 1976. (4)
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