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Rhodamine 3B+ ClO4− electronic transitions: reaction field and vibrational structure
Chemical Physics 273 (2001) 39±49
www.elsevier.com/locate/chemphys
Rhodamine 3B ClO4 electronic transitions: reaction ®eld and vibrational structure Jose A.B. Ferreira, Sõlvia M.B. Costa * Centro de Quõmica Estrutural, Complexo I, Instituto Superior T ecnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Received 6 July 2001; in ®nal form 27 August 2001
Abstract The electronic absorption and emission in the visible region of the spectrum of a xanthene dye, Rhodamine 3B perchlorate, have been studied as a function of solvent and temperature. The spectral shifts correlate well with the function f n2 n2 1= 2n2 1 (Ooshika, Bayliss and McRae theory) and are essentially due to dispersion and induced electronic polarization. A further energy lowering occurs due to orientation polarization giving an estimation of le lg 1:7 D. The broadenings in absorption reveal the contribution from orbital expansion whilst in emission provide evidence for the opposite eect. The contribution of an electrostriction eect is inferred from the good correlation obtained between a vibronic function containing the elongation of molecular coordinates and the temperature. The double con®guration coordinate model describes well the eect of temperature on both the S1 $ S0 transitions Stokes shift and broadening at the vibronic 0 $ 0 bands. Ó 2001 Published by Elsevier Science B.V.
1. Introduction After electronic excitation of a molecule in the condensed phase, the solvation of the new state gives rise to varying electric multipole interactions [1]. Repulsions, dispersions and reorientations contribute to the energy release [2,3]. The inertial component of the response occurs in a few tenths of femtoseconds and is slightly solvent dependent. Slower contributions are associated with the collective reorientation and diusion processes and thus vary several orders of magnitude depending on the viscosity. The Debye [4] and Onsager [5] models of dielectrics account for the energy vari-
*
Corresponding author. Tel.: +351-21-8419271; fax: +35121-8464455/7. E-mail address: [email protected] (S.M.B. Costa).
ation by means of the electric dipole in a cavity approximation [6]. Along with the perturbational treatment of the solute±solvent interaction [1], these approaches make the solvatochromism of excited electronic states understandable by the theory developed by Ooshika, Bayliss and McRae (OBM). In a dielectric medium of given optical refractive index (n) and static dielectric constant (e), it leads to a straightforward interpretation of spectral shifts in electronic absorption and emission transitions (Eq. (1)) [1] n2 1 e 1 n2 1 v ma;e ma;e aa;e ba;e ; 2n2 1 e 2 n2 2 1 where the coecient aa;e is related to the induced polarization and dispersion forces and the ba;e one to the orientation polarization. If speci®c interactions are absent and the contribution from the
0301-0104/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 3 0 1 - 0 1 0 4 ( 0 1 ) 0 0 4 7 8 - 5
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J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
solvent Stark eect is small, Eq. (1) gives the red shifts relatively to the corresponding values in vacuum (v). The contributions of the electronic and orientational polarization, respectively, are accounted for by the last two terms. The spectral shifts and broadening of the absorption and emission bands induced by a change in temperature are related to ¯uctuations in the thermal bath in which the solute is included [7±9]. These can be estimated by using the double con®guration coordinate model [10,11], which explains the alterations in spectral band shape through the existence of more than one coordinate to the relaxation after the electronic excitation. In the absence of low frequency intramolecular relaxation processes, the thermal broadening of the vibronic bands re¯ects the width of the distribution of solvent±solute con®gurations, while the Stokes shift reveals mainly the evolution of the most probable con®guration. It is expected that both fast and slow components of the solvation process contribute to both broadening and Stokes shift. Thus, the spectral behavior is associated with the mechanical and dielectric contributions to the energy relaxation [11,12]. The free energy variations are taken into consideration by the double con®guration coordinate model [10,11] through the correlation of the half-bandwidths of absorption and emission spectra ra;e with the equilibrium Stokes shifts Seq (Eq. (2)): r2a r2e 2kB TSeq :
2
Thermally induced spectral variations can thus elucidate the kinetic behavior of excited states and solute±solvent interaction mechanisms. Both can be devised with the understanding of the relevant structural features. In xanthene dyes dierent molecular forms can occur [13]: lactone, zwitterion, acid and ester. In low polar media, ion-pair association can occur [14±16]. The thermally induced spectral alterations can re¯ect the amounts of the various rhodamine molecular forms. However, in the absence of equilibrium, the changes observed with increasing temperature in cationic 9carboxyphenyl±rhodamines (e.g. with Rhodamine 6G) cannot only be attributed to the liquid density decrease [13]. The variations can be globally in-
terpreted considering the vibronic spectral weights, broadening and shifts. The latter have been reported for the cases of Rhodamine 101, Rhodamine 6G and Rhodamine B [17±19]. In this paper the electronic absorption and emission spectra of the ester salt Rhodamine 3B perchlorate are investigated in order to address speci®cally the dependence of absorption and emission wavelength and bandwidths on solvent and temperature. Correlations with Stokes shifts and molecular changes re¯ected in the vibronic patterns are discussed.
2. Experimental 2.1. Materials Rhodamine 3B perchlorate (Radiant Dyes Chemie, laser grade) and hexyl±Rhodamine B chloride (Molecular Probes Inc.) were used as received. Water was distilled twice over quartz. Alcohols were spectrophotometric grade ethanol (Merck), glycerol (Aldrich). 1-Hexanol was spectroscopic grade (Merck) and methanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 1,4-butanediol, 1,5-pentanediol and 2-methyl-2-propanol (Merck), 1-decanol (Fluka), 1,3-propanediol (BDH), tetraethyleneglycol (Fluka) and cyclohexanol (Eastman Kodak) are proanalysis grade. Acetonitrile and formamide (Merck UVASOLâ ), dichloromethane (Merck) and glycerol triacetate (Fluka) were used as received. 1,2-ethanediol (Fluka), propylene carbonate (Merck) and 1-chloro-naphthalene (Janssen) were chromatographed using a silica gel column. In all cases, the purity was checked by the absorption and ¯uorescence spectra in the UV/VIS range. 2.2. Sample preparation The samples (5 ml) were prepared from ethanol stock solution aliquots. Ethanol was evaporated using a stream of dry nitrogen. Rhodamine 3B perchlorate concentration was kept at 6:8 10 7 M (to avoid dye aggregation and self-absorption/ reemission eects) by addition of each solvent. The
J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
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samples were kept for 24 h in the dark before the measurements. 2.3. Apparatus Absorption spectra were recorded with a Jasco V-560 UV/VIS spectrophotometer with blank correction and emission spectra were recorded with a Perkin±Elmer LS-50B spectro¯uorimeter, already described [15,20]. In both absorption and emission measurements, 10 mm quartz rectangular cells were used and the temperature was controlled within 1 K using a circulating water stream on the apparatus cell holders. Excitation was selected at 516 nm keeping the optical density at this wavelength below 0.02. Correction of emission spectra was made over the experimental wavelength range using the curve provided by the manufacturer.
3. Results Rhodamine 3B perchlorate (Scheme 1), cationic and ion-pair molecular forms, have nearly indistinguishable visible absorption and emission spectra which present three shoulders of vibronic origin (similarly to other 9-carboxyphenyl rhodamines). The electronic transition behaves as a characteristic p±p and the shoulders relate to the vibronic transitions 2 0, 1 0 and 0 0 by decreasing order of energy [21,22]. The S1 ! S0 emission band is a quasi-specular image of the S1 S0 absorption with corresponding vibronic
Fig. 1. Rhodamine 3B perchlorate visible absorption (A) and emission If spectral intensities (normalized) in 1-decanol and 1,2-ethanediol at T 296 K.
assignment. The visible absorption band spreads from 450 until 620 nm. The spectral overlap between absorption and emission is high and covers 90 nm because the emission of the ®rst singlet begins at 530 nm and reaches 760 nm. Rhodamine 3B perchlorate typical absorption and emission spectra in 1-decanol and 1,2-ethanediol are shown in Fig. 1. The spectral maxima in 1,2-ethanediol are red shifted relatively to 1-decanol. Since the refractive indexes of both liquids are similar, the lowering of energy is attributed to better stabilization due to the static dielectric constant of 1,2ethanediol (see Table 1). The discussion of these relative contributions is presented in the following sections considering both the dipole moment changes as well as the vibrational modes involved upon interaction with the radiation ®eld. 3.1. Solvent dependence
Scheme 1. Molecular structure of Rhodamine 3B perchlorate illustrating one of the possible resonance structures.
The total shift of the spectral maxima is very small and does not exceed 11 nm in all the accessible polarity range covered by the solvents listed in Table 1. In fact, Rhodamine 3B perchlorate is not soluble in hydrocarbons (e.g. 2,2,4-trimethylpentane), aromatics (e.g. toluene), or chlorinated solvents without signi®cant permanent dipole moment, (e.g. carbon tetrachloride). The amplitude of shifts observed in absorption spectra corresponds
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J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
Table 1 Rhodamine 3B perchlorate wavelengths of visible absorption and ¯uorescence emission maxima in various solvents along with refractive indexes and static dielectric constants (T 296 K) Solvent
ka 0:5 (nm)
ke 0:5 (nm)
nD
e
Methanol Ethanol 1-Propanol 1-Butanol 1-Hexanol (H) 1-Decanol (D) 2-Propanol 2-Methyl-2-propanol 2-Butanol Cyclohexanol Water 1,2-Ethanediol (EG) 1,3-Propanediol 1,4-Butanediol 1,5-Pentanodiol Tetraethyleneglycol Glycerol (G) Glycerol triacetate (T) Acetonitrile Formamide Propylene carbonate Dichloromethane 1-Chloronaphthalene
555.4 556.0 557.0 558.0 558.6 559.2 556.0 557.0
577.5 579.2 578.3 579.3 582.1 580.5 579.2 579.4
1.328 1.361 1.386 1.399 1.418 1.437 1.378 1.388
33.6 24.3 20.1 17.4 13.3 8.10 18.3 12.4
556.8 561.0 558.6 561.8
578.8 582.5 583.9 584.0
1.398 1.464 1.333 1.432
15.8 15.0 80.4 37.0
561.2 562.0 561.0 563.2
584.6 582.7 583.0 586.7
1.440 1.446 1.449 1.460
35.0 32.0 25.0 40.2
563.4 560.0
586.6 583.9
1.476 1.430
43.2 7.11
555.6 560.5 559.0
582.6 586.5 585.0
1.344 1.447 1.422
37.5 109 63.1
557.0 565.6
577.7 589.5
1.424 1.633
9.08 4.72
to 325 cm 1 (3.9 kJ mol 1 ) and in emission to 350 cm 1 (4.2 kJ mol 1 ). Since the experimental uncertainty is about 10%, these variations are virtually equal. In Table 1, the experimental peak wavelengths are shown, along with the solvents' refractive indexes and static dielectric permittivities. 3.1.1. Polarization contributions The wave numbers of absorption and emission maxima correlate with the Onsager function f n2 . In Fig. 2A are shown the experimental values and those obtained subtracting from the former the third term of Eq. (1). The correlation improves meaning that the orientational polarization is contained in the experimental data. From the intercepts (Table 2), it is obtained the value of 7.5 kJ mol 1 (1.8 kcal mol 1 ) for the Stokes shift in vacuum. In this way, it was possible to calculate the wave numbers of the absorption and emission maxima with a bilinear regression using Eq. (1) (Fig. 2B). The contributions of each type of polarization were evaluated through the regression parameters that also allow the estimation of dipole moment variation (see Section 3.1.2). The data obtained (Table 2) show that the electronic contribution of the polarization is the dominant one in the interaction with the dielectric environment. Also, the comparison with the hexyl Rhodamine B data shows that the benzoate group does not participate in the p-conjugation, by contrast with
Fig. 2. Absorption and emission spectral maxima of Rhodamine 3B perchlorate obtained in various solvents (cf. Table 1). (A) Experimental values ( ) and calculated by subtraction of the orientational contribution ( ) (Eq. (1), Table 2). (B) Values calculated using Eq. (1) (cf. Table 2) represented as function of the experimental values. The error bars represent uncertainty of instrumental resolution of (0.5 nm).
J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49 Table 2 Correlation between Rhodamine 3B perchlorate absorption and emission wave numbers at the spectral maxima and the Onsager (a) and Debye (b) dielectric functions OBM (Eq. (1))
kv (nm)
aa;e (cm 1 )
ba;e (cm 1 )
Rh 3B (ClO4 ) S1 S0 a 523a (4)b S1 ! S0 e 541 (7)
5209 (527) 5185 (909)
310 (84) 440 (145)
Hexyl Rh B (Cl ) S1 S0 a 524a (13)b 541 (10) S1 ! S0 e
4238 (1510) 4164 (1070)
558 (414) 795 (293)
Regression parameters referring to data on the same transitions in hexyl Rhodamine B chloride [23] are included for comparison. a The wave numbers foreseen for the transitions in vacuum (v) are converted to nanometers. b The standard errors of the parameters estimated are shown in parentheses.
amine alkyl substituents, as will be highlighted in the following sections. 3.1.2. Dipole moment variation From the values of the slopes (b in Eq. (1)), it was possible to estimate the dipole moment variation between the two electronic states (Eq. (3)): ba
be
2 l hca3 0
l1 2 :
3
Here indexes 0 and 1 refer to ground state and ®rst excited singlet respectively. There are a variety of possibilities to estimate a, the more intuitive being the one which assumes that this parameter is related with the molecular dimensions. Rettig and Klock [24] use 40% of the bigger molecular axis for the radius of the Onsager cavity (a), which agrees with the calculated van der Waals radius. We have the Onsager length compatible used a 6:0 A, with the measured dipole strengths and similar to the hydrodynamic radius inferred from ¯uorescence anisotropy data [15]. The value thus obtained for the variation of dipole moment was Dl 1:7 D, close to the experimental one determined for the parent moleculeÐRhodamine B (Dl 1:8 0:2 D, [25]). The dipole moment variation found for other rhodamines (e.g. Rhodamine 110 with Dl 2:81 D [26,27]; Rhodamine 101 with Dl 1:7 D [28] and Rhodamine 6G with
43
the Dl value reported between 4.6 and 5.7 D [29]) allow to envisage a clear dependence on the substitution pattern. In fact, a bilinear correlation as function of both the number of methylene and methyl groups linked to the xanthene skeleton can be drawn. Each methylene and methyl contribute, respectively, with a 0.14 D decrease and a 1.16 D increase in the dipolar moment as compared with the value obtained without substitution. 3.2. Thermochromism The temperature-induced eects of the visible absorption and emission of Rhodamine 3B perchlorate were studied in a selection of solvents. These are abbreviated with capital letters in Table 1. Weakly polar non-protic glycerol triacetate and the two longer chains 1-alkanols with similar dielectric properties were used. Besides, higher polarity solvents, such as 1,2-ethanediol and 1,2,3propanetriol (glycerol) were also chosen. Since they present similar refractive indexes and their static dielectric constants vary considerably, it was expected that polarity eects could be monitored. In the absorption spectra of Rhodamine 3B perchlorate the temperature increase produces a decreasing contribution of the 0 0 vibronic which is accompanied by a progressive red shift and broadening. In the emission, the latter eect dominates the alterations. 3.2.1. Relation between the broadening and Stokes shifts The absorption (and emission) spectra can be decomposed in sums of three Gaussian functions in the frequency domain [22]. This procedure is outlined for the absorption spectra in Appendix A. The vibronic transitions 0 0 and 0 ! 0 are shifted dierently in the various solvents and their contributions to the total spectrum vary with the temperature. By contrast, the associated 1 0 and 1 ! 0 and 2 0 and 2 ! 0 vibronics are affected in less extent. Thus, the quantitative analysis of thermochromic eects was done using the data referring to the 0 $ 0 transitions only. Just as the values of the maximum energy, both the vibronic half-width broadenings show a clear dependence with the temperature increase. The
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J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
Fig. 3. Thermally induced broadening of the 0±0 vibronic absorption (A) and emission (B) of Rhodamine 3B perchlorate in various solvents represented as function of the temperature. Glycerol (G), 1-decanol (D), 1-hexanol (H), glycerol triacetate (T) and 1,2ethanediol (EG).
results obtained for the 0±0 vibronic, respectively for the case of the absorption and emission, are shown in Fig. 3. It is observed that the nature of the medium aects the broadenings of the absorption spectrum, but leaves (within experimental accuracy) the broadening of the emission spectrum invariant. The thermochromic behavior is interpreted using the relationship between the equilibrium Stokes shifts Seq and the broadenings ra;e (Eq. (2)). In these expressions ra;e are the standard deviations of absorption (a) or emission (e) main Gaussian peaks and S is the Stokes shift considering only the 0±0 vibronic band peaks obtained from absorption and emission spectra using the corresponding compatible decomposition in three
Gaussian functions. A global comparison was achieved with the separate calculation of the ®rst and second members, respectively recast in 1=2 energy units as Fr NA hc r2a r2e and FS 1=2 NA hc 2kB TS . The calculations were made considering the experimental dependence with the temperature. The behavior anticipated from Eq. (2) is observed in lower viscosity media. In fact, the shifts as well as the broadenings show linear dependence with temperature (except for the emission shift in glycerol, object of study in a separate publication [30]). In Fig. 4 are represented the functions Fr and FS . The behavior foreseen is obeyed in what concerns the correlation with the temperature dominated by a slope dependent on the magnitude of the Stokes shift S. The con-
Fig. 4. Broadening (A) and Stokes shifts (B) of the 0±0 vibronic of the spectra of Rhodamine 3B perchlorate in: 1-decanol (D), 1hexanol (H), 1,2-ethanediol (EG), glycerol triacetate (T) and glycerol (G). The values are expressed in terms of the functions Fr and FS (see Eq. (2) and text).
J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
vergent high temperature behavior is evident, while the opposite is observed for the lower temperature data. 3.3. Vibrational structure The 0 $ 0 vibronic half-width can be compared with the variations in the polarization dielectric functions. By contrast with the emission, the absorption half-width does not correlate with the orientational polarization. Other contributions present lead to a less systematic variation. The 0 0 vibronic cannot be described by an interaction of simply dielectric origin, the dominant mechanism being probably related to a mechanical interaction. To corroborate the hypothesis we show in Fig. 5 the representation of the values of the 0 0 vibronic half-widths as function of the relaxational component of the longitudinal modulus of compression Kr =Pa [31]. It is observed that the 0 0 broadening grows with the compression modulus stabilizing at higher values. The broadening of the 0 ! 0 emission generates, however, a good correlation with the electronic polarization. The information contained in the vibronic progressions can be used to assess whether an expansion in the orbital volume is re¯ected in the
45
broadening of the absorption spectra. The ratio of 0 the upper vibronic an relatively to the 0 bands a0 in aromatic rings can be described by Eq. (4) [32]: an mn x2n : a0 m0 n!
4
Here mn is the peak frequency associated with the quantum number n and x is a function of the reduced mass l and vibrational frequency m. R is the normal displacement associated with the nuclear vibration. For absorption, x is given by Eq. (5): 2 1=2 2p lm x R: 5 h Based on the present model, the evaluation of the reduced mass and of the normal displacements associated with the present transitions is limited. However, the vibrational progression can be evaluated and is based on an 800 cm 1 mode m0 with 300 cm 1 increments m. The dependence of x with solvent and temperature variations of the dominant spectral vibronic weights are shown in Fig. 6A. It is noteworthy that the lower x values which are likely to re¯ect the dominant contributions from R, are found in glycerol which is the solvent that presents the saturation eect at the higher compression modulus. The solvent variation of x temperature dependence (Fig. 6B) correlates with the total polarization function f e of the solvents, changing sign at higher dielectric constants. 4. Discussion
Fig. 5. Broadening of the 0±0 vibronic absorption of Rhodamine 3B perchlorate in various media, at T 295 K, identi®ed by their compression module Kr by increasing order of magnitude: 1-decanol, 1-hexanol, 1,2-ethanediol, glycerol triacetate and glycerol.
Alterations on the molecular polarizabilities are revealed from solvent dependence on the vibronic contributions, which in turn are related with the refractive indexes [17±19]. The herein estimated vibronic 0 0 (800 cm 1 ), 1 0 (1100 cm 1 ) and 2 0 (1400 cm 1 ) Franck±Condon absorption transition frequencies mn are in agreement with infrared and Raman spectroscopic data of rhodamine molecules. The xanthene ring breathing modes are assigned in this
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J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
A
B
Fig. 6. (A) Solvent and temperature dependence of the vibrational progression factor x (Eq. (5)). (B) dx=dT correlated with the Onsager reaction ®eld function of total polarization, f e. Lines are regressions of experimental data and bars correspond to standard errors.
frequency range [33,34]. The participation of the amine group modes is compatible, since similar values are observed both in aryl-amine compounds jet spectroscopy [35] as well as in the resonances found in the dielectric relaxation spectroscopy of alkyl amines [36]. A viscoelastic model allows the identi®cation of the non-polar solvation and vibrational relaxation and predicts that the electronic spectral bandwidth will have an increasing value that stabilizes at high viscosities [37,38]. The liquid response induced by the breathing mode of benzene, located around 1000 cm 1 , is compatible with the change in cavity size associated with the orbital expansion [38]. An electrostriction eect results as a consequence of a density increase at the solvation shell [39]. If the polarizing molecule is a cation, the solvent molecules can become nearer than in the case of an anion due to the electrostatic repulsion of the electronic clouds. Thus, if a positive charge is developed at a given atom of a polyatomic molecule upon the electronic transition, it is expected that the solvating molecules will produce a local density increase as a result of the decreased distance towards the new charge center. In water [40,41] the electrostriction pressure P equals the elastic modulus [41,42]. Our results show that large bandwidths occur at high elastic module, making possible the presence of an electrostriction eect associated with mechanical contributions felt in
the process of con®guration averaging. Semiempirical calculations assign to the S1 S0 transition an increase of positive charge on the carbon atom at the 9 position of the xanthene ring at the expense of negative charge increase at the amino groups [26,27]. Thus, an expansion of the molecular cavity due to the enhanced electron density can be assumed. The weight of the 1 0 transition increases with decreasing refractive index as reported for Rhodamine 3B chromophore earlier [22]. However, an opposite behavior can be envisaged from the pattern of 0 0 and 1 0 vibronic contributions to the absorption spectrum. Accordingly, the low dielectric constants associated with high electric ®eld strengths allow that the similar Rhodamine B ¯uorophore will show a pronounced increase in the 1 ! 0 spectral contribution in its electroluminescence spectrum [25]. This fact may be related with the geometrical anisotropy involved in the charge displacement occurring upon the relevant electronic transitions [26,27]. The correlation between the temperature variation of the vibronic function x and the total polarization can thus be understood in terms of the electrostriction eect on the vibrational coordinate involved in the photon absorption. It contains the elongation on the molecular coordinates and re¯ects the change in solute cavity size imposed by the electronic variations. The increase in size is easier in solvents having low modulus of
J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
compression and is more dicult otherwise. A similar conclusion can be inferred from the slope of the temperature dependence at varying polarizations in agreement with the temperature variations associated with the refractive index and thermal expansion (Lorentz±Lorenz equation) and with the dielectric constant (Arrhenius behavior) [43]. The slope is high, positive at low polarizations (and compression moduli), and low at high polarization (and high moduli). The broadening pattern of the 0 0 vibronic transition can be explained if the absorption is accompanied by an expansion of the orbital volume while in the emissive process the inverse process occurs. In absorption, the expansion can be limited by the compression modulus while after emission the response to the decompression reveals a dominant orientational contribution. Recently the activated non-radiative decay of the ®rst excited singlet state of Rhodamine 3B was described as a process controlled by both the frequency dependent hydrodynamic response and the solvent polarity [44]. Since the eect of the viscoelastic response on the reaction coordinate depends on the correlation time of the density ¯uctuations, it is expected that the electrostriction eect, suggested herein, while inducing density variations in the solvation shell can contribute to explain the ¯uorescence quantum yield variations produced under the in¯uence of an external electric ®eld reported earlier [25]. The study of the broadening of the vibronic absorption and emission spectral bands Gaussian functions provides some dynamic information, complementary to that obtained from the analysis of the corresponding energies. The broadening detected in a Gaussian band contains the spread of the distribution by the accessible con®gurations. It is possible to conclude that while the broadening varies linearly with the temperature and becomes quite independent of the solvent medium suggesting fast contributions, the Stokes shifts reveal mainly slow reorientation dynamics. At higher temperatures, the double con®guration coordinate model explains the results obtained from the absorption and emission spectra of Rhodamine 3B perchlorate in a variety of solvents. However, it does not explain the behavior at lower tempera-
47
tures since equilibrated Stokes shifts are considered. In fact, in these cases the Stokes shifts do not relate with the situation of full equilibrium [30]. Kinoshita and Nishi [47] observed similar behavior concerning the ¯uorescence emission of Rhodamine 6G in ethanol also at low temperature and gave evidence for the presence of fast contributions well accounted for by the double con®guration coordinate model. The fast relaxation process was associated with phonon propagation. In this line, Stein and Fayer [48] have also shown that the Rhodamine B spectral shifts in 1-propanol and in glycerol, at low temperature, re¯ect long time orientational components of the solvation. In fact, it was previously observed that the emission spectrum shifts to the red at the expense of components of orientational nature that determine the solvatochromism of the ¯uorescence from ®rst singlet [30]. The wavelength values obtained for transitions in vacuum compare well with those obtained by semi empirical calculations [26,27]. The spectroscopic behavior observed is similar to dierent rhodamines, cyanines and other heterocyclic compounds [13,17±19,46]. Particularly, the independence of the molecular form (cation and ion pair) was also observed in the solvatochromism of (a; x)-amino-polyenes [46]. The interionic distance supports the argument of the solvent separated ion pair as the more stable form of the associated species [26,27]. The correlations with the dielectric functions are also similar to those observed for cyanines [17±19], and for b-carotene [45] where the faster contributions should dominate the non-polar electronic state in polar solvents. Dierent relaxation models account for the energy relaxation. The mechanical model for the solvent response, based on the viscoelastic response of the liquid, can be used along with the dielectric model, the instantaneous normal mode approach and mode coupling theory [37,38,49±51]. In fact, the liquid response induced by an oscillating external electric ®eld is linked to a particular molecular relaxation mechanism, but can also account for the eective friction understood in terms of dipole coupling with the liquid modes. The present work shows identical features for the broadenings, which are less sensitive to the
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J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
Acknowledgements This work was supported by Project PRAXIS/ 2/2.1/443/94. Rhodamine 3B perchlorate is a kind gift of Prof. L.F. Vieira Ferreira. J.A.B.F. is grateful to PRAXIS XXI by the ®nancial support through Ph.D. grant BD/3616/94. Appendix A
Fig. 7. Three Gaussian function decomposition of absorption spectra of Rhodamine 3B perchlorate in 1-hexanol at T 285 K (upper spectrum) and T 325 K (lower spectrum). Inset: Semi-logarithmic representation of the same data.
dielectric properties while the reorientation processes shown by the Stokes shifts foresee dynamics associated with the longer time scales, re¯ecting the evolution of the most probable solute±solvent con®guration.
5. Conclusion The spectral pattern of Rhodamine 3B perchlorate was discussed in terms of the in¯uence of dielectric and mechanical contributions. These were monitored through the study of solvent shifts, and vibronic half-width broadenings (associated with the aromatic ring breathing modes) as the dominant aspects to the chromophore interactions. The ¯uorophore solvation process was interpreted through fast and slowly attained con®gurational distributions. OBM theory and the double con®guration coordinate model provide a consistent description in terms of electronic and orientation polarization contributions to the reaction ®eld. The anisotropy associated with charge displacement upon photon absorption can be related to the pattern present in the vibronic structure and the thermochromic variations interpreted invoking the contribution of electrostriction. Accordingly, the increase in the contribution of the 1 0 vibronic to the absorption is re¯ected in the spectrum thermal response through the variation of the normal coordinate displacements.
The spectra decomposition was made in terms of Eq. (A.1): ! 3 X Ai k 1 li 2 1 p exp S k : A:1 2r2i i1 ri 2p The fractions ai (Eq. (A.2)), are the relative contributions of each vibronic band to the spectral distribution: Ai ai P3
i1
Ai
:
A:2
Using each experimental spectrum, the nine global parameters are obtained by a least squares procedure. The number of experimental points in each data ®le is higher than 400 separated by no more than 0.5 nm. A typical result is shown in Fig. 7 for the absorption spectra at two temperatures in 1hexanol. References [1] N. Mataga, T. Kubota, Molecular Interactions and Electronic Spectra, Marcel Dekker, New York, 1970. [2] R.M. Stratt, M. Maroncelli, J. Phys. Chem. 100 (1996) 12981. [3] M. Maroncelli, J. Mol. Liq. 57 (1993) 1. [4] P. Debye, Polar Molecules, Dover, New York, 1929. [5] L. Onsager, J. Am. Chem. Soc. 58 (1936) 1486. [6] C.J.F. B ottcher, Theory of Electric Polarization, Elsevier, Amsterdam, 1973. [7] J. Ma, D.V. Bout, M. Berg, J. Chem. Phys. 103 (1995) 9146. [8] Y.T. Mazurenko, N.G. Bakshiev, Opt. Spectrosc. 28 (1970) 490. [9] R.A. Marcus, J. Chem. Phys. 38 (1963) 1858. [10] R.F. Loring, J. Phys. Chem. 94 (1990) 513. [11] R.A. Marcus, J. Chem. Phys. 43 (1965) 1261. [12] G. van der Zwan, J.T. Hynes, J. Phys. Chem. 89 (1985) 4181.
J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49 [13] D.A. Hinckley, P.G. Seybold, D.P. Borris, Spectrochim. Act. A 42 (1986) 747. [14] K. Kemnitz, K. Yoshihara, J. Phys. Chem. 95 (1991) 6095. [15] J.A.B. Ferreira, S.M.B. Costa, Chem. Phys. Lett. 307 (1999) 139. [16] F. K ohn, J. Hofkens, F.C. De Schryver, Chem. Phys. Lett. 321 (2000) 372. [17] I. Renge, Chem. Phys. 167 (1992) 173. [18] I. Renge, J. Photochem. Photobiol. A: Chem. 69 (1992) 135. [19] I. Renge, J. Phys. Chem. A 104 (2000) 7452. [20] G.M.M. Medeiros, M.F. Leit~ao, S.M.B. Costa, J. Photochem. Photobiol. A: Chem. 72 (1993) 255. [21] J.L. Clark, P.F. Miller, G. Rumbles, J. Phys. Chem. A 102 (1998) 4428. [22] J.A.B. Ferreira, S.M.B. Costa, J. Mol. Struct. 565±566 (2001) 35. [23] J.A.B. Ferreira, Ph.D. Thesis, Technical University of Lisbon, Lisbon, 2000. [24] W. Rettig, A. Klock, Can. J. Chem. 63 (1985) 1649. [25] R.F Code, Z.D. Popovic, J.H. Sharp, Chem. Phys. 83 (1984) 181. [26] V.Y. Artyukhov, Sov. Phys. J. 29 (1986) 534. [27] V.Y. Artyukhov, Sov. Phys. J. 30 (1987) 850. [28] S. Schmidt, R. Reich, Ber. Bunsenges. Phys. Chem. 76 (1972) 599. [29] N.V. Dubinin, L.M. Blinov, S.V. Yablonskii, Opt. Spectrosc. 44 (1978) 473. [30] J.A.B. Ferreira, S.M.B. Costa, Chem. Phys. 269 (2001) 313.
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[31] K.F. Herzfeld, T.A. Litovitz, Absorption and Dispersion of Ultrasonic Waves, Academic Press, New York, London, 1959. [32] E.F. McCoy, I.G. Ross, Aust. J. Chem. 15 (1962) 573. [33] R. K onig, A. Lau, H.-J. Weigmann, Chem. Phys. Lett. 69 (1980) 87. [34] S. Matsubara, H. Takahashi, Chem. Phys. Lett. 108 (1984) 475. [35] D. Phillips, J. Photochem. Photobiol. A: Chem. 105 (1997) 307. [36] J. Gilchrist, Chem. Phys. 65 (1982) 1. [37] M. Berg, J. Phys. Chem. A 102 (1998) 17. [38] M.A. Berg, H.W. Hubble, Chem. Phys. 233 (1998) 257. [39] J.-K. Hyun, T. Ichiye, J. Chem. Phys. 109 (1998) 1074. [40] D. Aherne, V. Tran, B.J. Schwartz, J. Phys. Chem. B 104 (2000) 5382. [41] I. Danielewicz-Ferchmin, A.R. Ferchmin, J. Chem. Phys. 109 (1998) 2394. [42] D. Bertolini, A. Tani, Phys. Rev. E 51 (1995) 1091. [43] R.B. Altman, I. Renge, J. Chem. Phys. 97 (1992) 5316. [44] J.A.B. Ferreira, S.M.B. Costa, L.F.V. Ferreira, J. Phys. Chem. A 104 (2000) 11909. [45] P.O. Andersson, T. Gillbro, L. Ferguson, R.J. Cogdell, Photochem. Photobiol. 54 (1991) 353. [46] T. Harder, J. Bendig, Chem. Phys. Lett. 228 (1994) 621. [47] S. Kinoshita, N. Nishi, J. Chem. Phys. 89 (1988) 6612. [48] A.D. Stein, M.D. Fayer, J. Chem. Phys. 97 (1992) 2948. [49] I. Martini, G.V. Hartland, J. Phys. Chem. 100 (1996) 19764. [50] R.M. Stratt, Acc. Chem. Res. 28 (1995) 201. [51] W. G otze, L. Sj ogren, Rep. Prog. Phys. 55 (1992) 241.
www.elsevier.com/locate/chemphys
Rhodamine 3B ClO4 electronic transitions: reaction ®eld and vibrational structure Jose A.B. Ferreira, Sõlvia M.B. Costa * Centro de Quõmica Estrutural, Complexo I, Instituto Superior T ecnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Received 6 July 2001; in ®nal form 27 August 2001
Abstract The electronic absorption and emission in the visible region of the spectrum of a xanthene dye, Rhodamine 3B perchlorate, have been studied as a function of solvent and temperature. The spectral shifts correlate well with the function f n2 n2 1= 2n2 1 (Ooshika, Bayliss and McRae theory) and are essentially due to dispersion and induced electronic polarization. A further energy lowering occurs due to orientation polarization giving an estimation of le lg 1:7 D. The broadenings in absorption reveal the contribution from orbital expansion whilst in emission provide evidence for the opposite eect. The contribution of an electrostriction eect is inferred from the good correlation obtained between a vibronic function containing the elongation of molecular coordinates and the temperature. The double con®guration coordinate model describes well the eect of temperature on both the S1 $ S0 transitions Stokes shift and broadening at the vibronic 0 $ 0 bands. Ó 2001 Published by Elsevier Science B.V.
1. Introduction After electronic excitation of a molecule in the condensed phase, the solvation of the new state gives rise to varying electric multipole interactions [1]. Repulsions, dispersions and reorientations contribute to the energy release [2,3]. The inertial component of the response occurs in a few tenths of femtoseconds and is slightly solvent dependent. Slower contributions are associated with the collective reorientation and diusion processes and thus vary several orders of magnitude depending on the viscosity. The Debye [4] and Onsager [5] models of dielectrics account for the energy vari-
*
Corresponding author. Tel.: +351-21-8419271; fax: +35121-8464455/7. E-mail address: [email protected] (S.M.B. Costa).
ation by means of the electric dipole in a cavity approximation [6]. Along with the perturbational treatment of the solute±solvent interaction [1], these approaches make the solvatochromism of excited electronic states understandable by the theory developed by Ooshika, Bayliss and McRae (OBM). In a dielectric medium of given optical refractive index (n) and static dielectric constant (e), it leads to a straightforward interpretation of spectral shifts in electronic absorption and emission transitions (Eq. (1)) [1] n2 1 e 1 n2 1 v ma;e ma;e aa;e ba;e ; 2n2 1 e 2 n2 2 1 where the coecient aa;e is related to the induced polarization and dispersion forces and the ba;e one to the orientation polarization. If speci®c interactions are absent and the contribution from the
0301-0104/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 3 0 1 - 0 1 0 4 ( 0 1 ) 0 0 4 7 8 - 5
40
J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
solvent Stark eect is small, Eq. (1) gives the red shifts relatively to the corresponding values in vacuum (v). The contributions of the electronic and orientational polarization, respectively, are accounted for by the last two terms. The spectral shifts and broadening of the absorption and emission bands induced by a change in temperature are related to ¯uctuations in the thermal bath in which the solute is included [7±9]. These can be estimated by using the double con®guration coordinate model [10,11], which explains the alterations in spectral band shape through the existence of more than one coordinate to the relaxation after the electronic excitation. In the absence of low frequency intramolecular relaxation processes, the thermal broadening of the vibronic bands re¯ects the width of the distribution of solvent±solute con®gurations, while the Stokes shift reveals mainly the evolution of the most probable con®guration. It is expected that both fast and slow components of the solvation process contribute to both broadening and Stokes shift. Thus, the spectral behavior is associated with the mechanical and dielectric contributions to the energy relaxation [11,12]. The free energy variations are taken into consideration by the double con®guration coordinate model [10,11] through the correlation of the half-bandwidths of absorption and emission spectra ra;e with the equilibrium Stokes shifts Seq (Eq. (2)): r2a r2e 2kB TSeq :
2
Thermally induced spectral variations can thus elucidate the kinetic behavior of excited states and solute±solvent interaction mechanisms. Both can be devised with the understanding of the relevant structural features. In xanthene dyes dierent molecular forms can occur [13]: lactone, zwitterion, acid and ester. In low polar media, ion-pair association can occur [14±16]. The thermally induced spectral alterations can re¯ect the amounts of the various rhodamine molecular forms. However, in the absence of equilibrium, the changes observed with increasing temperature in cationic 9carboxyphenyl±rhodamines (e.g. with Rhodamine 6G) cannot only be attributed to the liquid density decrease [13]. The variations can be globally in-
terpreted considering the vibronic spectral weights, broadening and shifts. The latter have been reported for the cases of Rhodamine 101, Rhodamine 6G and Rhodamine B [17±19]. In this paper the electronic absorption and emission spectra of the ester salt Rhodamine 3B perchlorate are investigated in order to address speci®cally the dependence of absorption and emission wavelength and bandwidths on solvent and temperature. Correlations with Stokes shifts and molecular changes re¯ected in the vibronic patterns are discussed.
2. Experimental 2.1. Materials Rhodamine 3B perchlorate (Radiant Dyes Chemie, laser grade) and hexyl±Rhodamine B chloride (Molecular Probes Inc.) were used as received. Water was distilled twice over quartz. Alcohols were spectrophotometric grade ethanol (Merck), glycerol (Aldrich). 1-Hexanol was spectroscopic grade (Merck) and methanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 1,4-butanediol, 1,5-pentanediol and 2-methyl-2-propanol (Merck), 1-decanol (Fluka), 1,3-propanediol (BDH), tetraethyleneglycol (Fluka) and cyclohexanol (Eastman Kodak) are proanalysis grade. Acetonitrile and formamide (Merck UVASOLâ ), dichloromethane (Merck) and glycerol triacetate (Fluka) were used as received. 1,2-ethanediol (Fluka), propylene carbonate (Merck) and 1-chloro-naphthalene (Janssen) were chromatographed using a silica gel column. In all cases, the purity was checked by the absorption and ¯uorescence spectra in the UV/VIS range. 2.2. Sample preparation The samples (5 ml) were prepared from ethanol stock solution aliquots. Ethanol was evaporated using a stream of dry nitrogen. Rhodamine 3B perchlorate concentration was kept at 6:8 10 7 M (to avoid dye aggregation and self-absorption/ reemission eects) by addition of each solvent. The
J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
41
samples were kept for 24 h in the dark before the measurements. 2.3. Apparatus Absorption spectra were recorded with a Jasco V-560 UV/VIS spectrophotometer with blank correction and emission spectra were recorded with a Perkin±Elmer LS-50B spectro¯uorimeter, already described [15,20]. In both absorption and emission measurements, 10 mm quartz rectangular cells were used and the temperature was controlled within 1 K using a circulating water stream on the apparatus cell holders. Excitation was selected at 516 nm keeping the optical density at this wavelength below 0.02. Correction of emission spectra was made over the experimental wavelength range using the curve provided by the manufacturer.
3. Results Rhodamine 3B perchlorate (Scheme 1), cationic and ion-pair molecular forms, have nearly indistinguishable visible absorption and emission spectra which present three shoulders of vibronic origin (similarly to other 9-carboxyphenyl rhodamines). The electronic transition behaves as a characteristic p±p and the shoulders relate to the vibronic transitions 2 0, 1 0 and 0 0 by decreasing order of energy [21,22]. The S1 ! S0 emission band is a quasi-specular image of the S1 S0 absorption with corresponding vibronic
Fig. 1. Rhodamine 3B perchlorate visible absorption (A) and emission If spectral intensities (normalized) in 1-decanol and 1,2-ethanediol at T 296 K.
assignment. The visible absorption band spreads from 450 until 620 nm. The spectral overlap between absorption and emission is high and covers 90 nm because the emission of the ®rst singlet begins at 530 nm and reaches 760 nm. Rhodamine 3B perchlorate typical absorption and emission spectra in 1-decanol and 1,2-ethanediol are shown in Fig. 1. The spectral maxima in 1,2-ethanediol are red shifted relatively to 1-decanol. Since the refractive indexes of both liquids are similar, the lowering of energy is attributed to better stabilization due to the static dielectric constant of 1,2ethanediol (see Table 1). The discussion of these relative contributions is presented in the following sections considering both the dipole moment changes as well as the vibrational modes involved upon interaction with the radiation ®eld. 3.1. Solvent dependence
Scheme 1. Molecular structure of Rhodamine 3B perchlorate illustrating one of the possible resonance structures.
The total shift of the spectral maxima is very small and does not exceed 11 nm in all the accessible polarity range covered by the solvents listed in Table 1. In fact, Rhodamine 3B perchlorate is not soluble in hydrocarbons (e.g. 2,2,4-trimethylpentane), aromatics (e.g. toluene), or chlorinated solvents without signi®cant permanent dipole moment, (e.g. carbon tetrachloride). The amplitude of shifts observed in absorption spectra corresponds
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J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
Table 1 Rhodamine 3B perchlorate wavelengths of visible absorption and ¯uorescence emission maxima in various solvents along with refractive indexes and static dielectric constants (T 296 K) Solvent
ka 0:5 (nm)
ke 0:5 (nm)
nD
e
Methanol Ethanol 1-Propanol 1-Butanol 1-Hexanol (H) 1-Decanol (D) 2-Propanol 2-Methyl-2-propanol 2-Butanol Cyclohexanol Water 1,2-Ethanediol (EG) 1,3-Propanediol 1,4-Butanediol 1,5-Pentanodiol Tetraethyleneglycol Glycerol (G) Glycerol triacetate (T) Acetonitrile Formamide Propylene carbonate Dichloromethane 1-Chloronaphthalene
555.4 556.0 557.0 558.0 558.6 559.2 556.0 557.0
577.5 579.2 578.3 579.3 582.1 580.5 579.2 579.4
1.328 1.361 1.386 1.399 1.418 1.437 1.378 1.388
33.6 24.3 20.1 17.4 13.3 8.10 18.3 12.4
556.8 561.0 558.6 561.8
578.8 582.5 583.9 584.0
1.398 1.464 1.333 1.432
15.8 15.0 80.4 37.0
561.2 562.0 561.0 563.2
584.6 582.7 583.0 586.7
1.440 1.446 1.449 1.460
35.0 32.0 25.0 40.2
563.4 560.0
586.6 583.9
1.476 1.430
43.2 7.11
555.6 560.5 559.0
582.6 586.5 585.0
1.344 1.447 1.422
37.5 109 63.1
557.0 565.6
577.7 589.5
1.424 1.633
9.08 4.72
to 325 cm 1 (3.9 kJ mol 1 ) and in emission to 350 cm 1 (4.2 kJ mol 1 ). Since the experimental uncertainty is about 10%, these variations are virtually equal. In Table 1, the experimental peak wavelengths are shown, along with the solvents' refractive indexes and static dielectric permittivities. 3.1.1. Polarization contributions The wave numbers of absorption and emission maxima correlate with the Onsager function f n2 . In Fig. 2A are shown the experimental values and those obtained subtracting from the former the third term of Eq. (1). The correlation improves meaning that the orientational polarization is contained in the experimental data. From the intercepts (Table 2), it is obtained the value of 7.5 kJ mol 1 (1.8 kcal mol 1 ) for the Stokes shift in vacuum. In this way, it was possible to calculate the wave numbers of the absorption and emission maxima with a bilinear regression using Eq. (1) (Fig. 2B). The contributions of each type of polarization were evaluated through the regression parameters that also allow the estimation of dipole moment variation (see Section 3.1.2). The data obtained (Table 2) show that the electronic contribution of the polarization is the dominant one in the interaction with the dielectric environment. Also, the comparison with the hexyl Rhodamine B data shows that the benzoate group does not participate in the p-conjugation, by contrast with
Fig. 2. Absorption and emission spectral maxima of Rhodamine 3B perchlorate obtained in various solvents (cf. Table 1). (A) Experimental values ( ) and calculated by subtraction of the orientational contribution ( ) (Eq. (1), Table 2). (B) Values calculated using Eq. (1) (cf. Table 2) represented as function of the experimental values. The error bars represent uncertainty of instrumental resolution of (0.5 nm).
J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49 Table 2 Correlation between Rhodamine 3B perchlorate absorption and emission wave numbers at the spectral maxima and the Onsager (a) and Debye (b) dielectric functions OBM (Eq. (1))
kv (nm)
aa;e (cm 1 )
ba;e (cm 1 )
Rh 3B (ClO4 ) S1 S0 a 523a (4)b S1 ! S0 e 541 (7)
5209 (527) 5185 (909)
310 (84) 440 (145)
Hexyl Rh B (Cl ) S1 S0 a 524a (13)b 541 (10) S1 ! S0 e
4238 (1510) 4164 (1070)
558 (414) 795 (293)
Regression parameters referring to data on the same transitions in hexyl Rhodamine B chloride [23] are included for comparison. a The wave numbers foreseen for the transitions in vacuum (v) are converted to nanometers. b The standard errors of the parameters estimated are shown in parentheses.
amine alkyl substituents, as will be highlighted in the following sections. 3.1.2. Dipole moment variation From the values of the slopes (b in Eq. (1)), it was possible to estimate the dipole moment variation between the two electronic states (Eq. (3)): ba
be
2 l hca3 0
l1 2 :
3
Here indexes 0 and 1 refer to ground state and ®rst excited singlet respectively. There are a variety of possibilities to estimate a, the more intuitive being the one which assumes that this parameter is related with the molecular dimensions. Rettig and Klock [24] use 40% of the bigger molecular axis for the radius of the Onsager cavity (a), which agrees with the calculated van der Waals radius. We have the Onsager length compatible used a 6:0 A, with the measured dipole strengths and similar to the hydrodynamic radius inferred from ¯uorescence anisotropy data [15]. The value thus obtained for the variation of dipole moment was Dl 1:7 D, close to the experimental one determined for the parent moleculeÐRhodamine B (Dl 1:8 0:2 D, [25]). The dipole moment variation found for other rhodamines (e.g. Rhodamine 110 with Dl 2:81 D [26,27]; Rhodamine 101 with Dl 1:7 D [28] and Rhodamine 6G with
43
the Dl value reported between 4.6 and 5.7 D [29]) allow to envisage a clear dependence on the substitution pattern. In fact, a bilinear correlation as function of both the number of methylene and methyl groups linked to the xanthene skeleton can be drawn. Each methylene and methyl contribute, respectively, with a 0.14 D decrease and a 1.16 D increase in the dipolar moment as compared with the value obtained without substitution. 3.2. Thermochromism The temperature-induced eects of the visible absorption and emission of Rhodamine 3B perchlorate were studied in a selection of solvents. These are abbreviated with capital letters in Table 1. Weakly polar non-protic glycerol triacetate and the two longer chains 1-alkanols with similar dielectric properties were used. Besides, higher polarity solvents, such as 1,2-ethanediol and 1,2,3propanetriol (glycerol) were also chosen. Since they present similar refractive indexes and their static dielectric constants vary considerably, it was expected that polarity eects could be monitored. In the absorption spectra of Rhodamine 3B perchlorate the temperature increase produces a decreasing contribution of the 0 0 vibronic which is accompanied by a progressive red shift and broadening. In the emission, the latter eect dominates the alterations. 3.2.1. Relation between the broadening and Stokes shifts The absorption (and emission) spectra can be decomposed in sums of three Gaussian functions in the frequency domain [22]. This procedure is outlined for the absorption spectra in Appendix A. The vibronic transitions 0 0 and 0 ! 0 are shifted dierently in the various solvents and their contributions to the total spectrum vary with the temperature. By contrast, the associated 1 0 and 1 ! 0 and 2 0 and 2 ! 0 vibronics are affected in less extent. Thus, the quantitative analysis of thermochromic eects was done using the data referring to the 0 $ 0 transitions only. Just as the values of the maximum energy, both the vibronic half-width broadenings show a clear dependence with the temperature increase. The
44
J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
Fig. 3. Thermally induced broadening of the 0±0 vibronic absorption (A) and emission (B) of Rhodamine 3B perchlorate in various solvents represented as function of the temperature. Glycerol (G), 1-decanol (D), 1-hexanol (H), glycerol triacetate (T) and 1,2ethanediol (EG).
results obtained for the 0±0 vibronic, respectively for the case of the absorption and emission, are shown in Fig. 3. It is observed that the nature of the medium aects the broadenings of the absorption spectrum, but leaves (within experimental accuracy) the broadening of the emission spectrum invariant. The thermochromic behavior is interpreted using the relationship between the equilibrium Stokes shifts Seq and the broadenings ra;e (Eq. (2)). In these expressions ra;e are the standard deviations of absorption (a) or emission (e) main Gaussian peaks and S is the Stokes shift considering only the 0±0 vibronic band peaks obtained from absorption and emission spectra using the corresponding compatible decomposition in three
Gaussian functions. A global comparison was achieved with the separate calculation of the ®rst and second members, respectively recast in 1=2 energy units as Fr NA hc r2a r2e and FS 1=2 NA hc 2kB TS . The calculations were made considering the experimental dependence with the temperature. The behavior anticipated from Eq. (2) is observed in lower viscosity media. In fact, the shifts as well as the broadenings show linear dependence with temperature (except for the emission shift in glycerol, object of study in a separate publication [30]). In Fig. 4 are represented the functions Fr and FS . The behavior foreseen is obeyed in what concerns the correlation with the temperature dominated by a slope dependent on the magnitude of the Stokes shift S. The con-
Fig. 4. Broadening (A) and Stokes shifts (B) of the 0±0 vibronic of the spectra of Rhodamine 3B perchlorate in: 1-decanol (D), 1hexanol (H), 1,2-ethanediol (EG), glycerol triacetate (T) and glycerol (G). The values are expressed in terms of the functions Fr and FS (see Eq. (2) and text).
J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
vergent high temperature behavior is evident, while the opposite is observed for the lower temperature data. 3.3. Vibrational structure The 0 $ 0 vibronic half-width can be compared with the variations in the polarization dielectric functions. By contrast with the emission, the absorption half-width does not correlate with the orientational polarization. Other contributions present lead to a less systematic variation. The 0 0 vibronic cannot be described by an interaction of simply dielectric origin, the dominant mechanism being probably related to a mechanical interaction. To corroborate the hypothesis we show in Fig. 5 the representation of the values of the 0 0 vibronic half-widths as function of the relaxational component of the longitudinal modulus of compression Kr =Pa [31]. It is observed that the 0 0 broadening grows with the compression modulus stabilizing at higher values. The broadening of the 0 ! 0 emission generates, however, a good correlation with the electronic polarization. The information contained in the vibronic progressions can be used to assess whether an expansion in the orbital volume is re¯ected in the
45
broadening of the absorption spectra. The ratio of 0 the upper vibronic an relatively to the 0 bands a0 in aromatic rings can be described by Eq. (4) [32]: an mn x2n : a0 m0 n!
4
Here mn is the peak frequency associated with the quantum number n and x is a function of the reduced mass l and vibrational frequency m. R is the normal displacement associated with the nuclear vibration. For absorption, x is given by Eq. (5): 2 1=2 2p lm x R: 5 h Based on the present model, the evaluation of the reduced mass and of the normal displacements associated with the present transitions is limited. However, the vibrational progression can be evaluated and is based on an 800 cm 1 mode m0 with 300 cm 1 increments m. The dependence of x with solvent and temperature variations of the dominant spectral vibronic weights are shown in Fig. 6A. It is noteworthy that the lower x values which are likely to re¯ect the dominant contributions from R, are found in glycerol which is the solvent that presents the saturation eect at the higher compression modulus. The solvent variation of x temperature dependence (Fig. 6B) correlates with the total polarization function f e of the solvents, changing sign at higher dielectric constants. 4. Discussion
Fig. 5. Broadening of the 0±0 vibronic absorption of Rhodamine 3B perchlorate in various media, at T 295 K, identi®ed by their compression module Kr by increasing order of magnitude: 1-decanol, 1-hexanol, 1,2-ethanediol, glycerol triacetate and glycerol.
Alterations on the molecular polarizabilities are revealed from solvent dependence on the vibronic contributions, which in turn are related with the refractive indexes [17±19]. The herein estimated vibronic 0 0 (800 cm 1 ), 1 0 (1100 cm 1 ) and 2 0 (1400 cm 1 ) Franck±Condon absorption transition frequencies mn are in agreement with infrared and Raman spectroscopic data of rhodamine molecules. The xanthene ring breathing modes are assigned in this
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J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
A
B
Fig. 6. (A) Solvent and temperature dependence of the vibrational progression factor x (Eq. (5)). (B) dx=dT correlated with the Onsager reaction ®eld function of total polarization, f e. Lines are regressions of experimental data and bars correspond to standard errors.
frequency range [33,34]. The participation of the amine group modes is compatible, since similar values are observed both in aryl-amine compounds jet spectroscopy [35] as well as in the resonances found in the dielectric relaxation spectroscopy of alkyl amines [36]. A viscoelastic model allows the identi®cation of the non-polar solvation and vibrational relaxation and predicts that the electronic spectral bandwidth will have an increasing value that stabilizes at high viscosities [37,38]. The liquid response induced by the breathing mode of benzene, located around 1000 cm 1 , is compatible with the change in cavity size associated with the orbital expansion [38]. An electrostriction eect results as a consequence of a density increase at the solvation shell [39]. If the polarizing molecule is a cation, the solvent molecules can become nearer than in the case of an anion due to the electrostatic repulsion of the electronic clouds. Thus, if a positive charge is developed at a given atom of a polyatomic molecule upon the electronic transition, it is expected that the solvating molecules will produce a local density increase as a result of the decreased distance towards the new charge center. In water [40,41] the electrostriction pressure P equals the elastic modulus [41,42]. Our results show that large bandwidths occur at high elastic module, making possible the presence of an electrostriction eect associated with mechanical contributions felt in
the process of con®guration averaging. Semiempirical calculations assign to the S1 S0 transition an increase of positive charge on the carbon atom at the 9 position of the xanthene ring at the expense of negative charge increase at the amino groups [26,27]. Thus, an expansion of the molecular cavity due to the enhanced electron density can be assumed. The weight of the 1 0 transition increases with decreasing refractive index as reported for Rhodamine 3B chromophore earlier [22]. However, an opposite behavior can be envisaged from the pattern of 0 0 and 1 0 vibronic contributions to the absorption spectrum. Accordingly, the low dielectric constants associated with high electric ®eld strengths allow that the similar Rhodamine B ¯uorophore will show a pronounced increase in the 1 ! 0 spectral contribution in its electroluminescence spectrum [25]. This fact may be related with the geometrical anisotropy involved in the charge displacement occurring upon the relevant electronic transitions [26,27]. The correlation between the temperature variation of the vibronic function x and the total polarization can thus be understood in terms of the electrostriction eect on the vibrational coordinate involved in the photon absorption. It contains the elongation on the molecular coordinates and re¯ects the change in solute cavity size imposed by the electronic variations. The increase in size is easier in solvents having low modulus of
J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
compression and is more dicult otherwise. A similar conclusion can be inferred from the slope of the temperature dependence at varying polarizations in agreement with the temperature variations associated with the refractive index and thermal expansion (Lorentz±Lorenz equation) and with the dielectric constant (Arrhenius behavior) [43]. The slope is high, positive at low polarizations (and compression moduli), and low at high polarization (and high moduli). The broadening pattern of the 0 0 vibronic transition can be explained if the absorption is accompanied by an expansion of the orbital volume while in the emissive process the inverse process occurs. In absorption, the expansion can be limited by the compression modulus while after emission the response to the decompression reveals a dominant orientational contribution. Recently the activated non-radiative decay of the ®rst excited singlet state of Rhodamine 3B was described as a process controlled by both the frequency dependent hydrodynamic response and the solvent polarity [44]. Since the eect of the viscoelastic response on the reaction coordinate depends on the correlation time of the density ¯uctuations, it is expected that the electrostriction eect, suggested herein, while inducing density variations in the solvation shell can contribute to explain the ¯uorescence quantum yield variations produced under the in¯uence of an external electric ®eld reported earlier [25]. The study of the broadening of the vibronic absorption and emission spectral bands Gaussian functions provides some dynamic information, complementary to that obtained from the analysis of the corresponding energies. The broadening detected in a Gaussian band contains the spread of the distribution by the accessible con®gurations. It is possible to conclude that while the broadening varies linearly with the temperature and becomes quite independent of the solvent medium suggesting fast contributions, the Stokes shifts reveal mainly slow reorientation dynamics. At higher temperatures, the double con®guration coordinate model explains the results obtained from the absorption and emission spectra of Rhodamine 3B perchlorate in a variety of solvents. However, it does not explain the behavior at lower tempera-
47
tures since equilibrated Stokes shifts are considered. In fact, in these cases the Stokes shifts do not relate with the situation of full equilibrium [30]. Kinoshita and Nishi [47] observed similar behavior concerning the ¯uorescence emission of Rhodamine 6G in ethanol also at low temperature and gave evidence for the presence of fast contributions well accounted for by the double con®guration coordinate model. The fast relaxation process was associated with phonon propagation. In this line, Stein and Fayer [48] have also shown that the Rhodamine B spectral shifts in 1-propanol and in glycerol, at low temperature, re¯ect long time orientational components of the solvation. In fact, it was previously observed that the emission spectrum shifts to the red at the expense of components of orientational nature that determine the solvatochromism of the ¯uorescence from ®rst singlet [30]. The wavelength values obtained for transitions in vacuum compare well with those obtained by semi empirical calculations [26,27]. The spectroscopic behavior observed is similar to dierent rhodamines, cyanines and other heterocyclic compounds [13,17±19,46]. Particularly, the independence of the molecular form (cation and ion pair) was also observed in the solvatochromism of (a; x)-amino-polyenes [46]. The interionic distance supports the argument of the solvent separated ion pair as the more stable form of the associated species [26,27]. The correlations with the dielectric functions are also similar to those observed for cyanines [17±19], and for b-carotene [45] where the faster contributions should dominate the non-polar electronic state in polar solvents. Dierent relaxation models account for the energy relaxation. The mechanical model for the solvent response, based on the viscoelastic response of the liquid, can be used along with the dielectric model, the instantaneous normal mode approach and mode coupling theory [37,38,49±51]. In fact, the liquid response induced by an oscillating external electric ®eld is linked to a particular molecular relaxation mechanism, but can also account for the eective friction understood in terms of dipole coupling with the liquid modes. The present work shows identical features for the broadenings, which are less sensitive to the
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J.A.B. Ferreira, S.M.B. Costa / Chemical Physics 273 (2001) 39±49
Acknowledgements This work was supported by Project PRAXIS/ 2/2.1/443/94. Rhodamine 3B perchlorate is a kind gift of Prof. L.F. Vieira Ferreira. J.A.B.F. is grateful to PRAXIS XXI by the ®nancial support through Ph.D. grant BD/3616/94. Appendix A
Fig. 7. Three Gaussian function decomposition of absorption spectra of Rhodamine 3B perchlorate in 1-hexanol at T 285 K (upper spectrum) and T 325 K (lower spectrum). Inset: Semi-logarithmic representation of the same data.
dielectric properties while the reorientation processes shown by the Stokes shifts foresee dynamics associated with the longer time scales, re¯ecting the evolution of the most probable solute±solvent con®guration.
5. Conclusion The spectral pattern of Rhodamine 3B perchlorate was discussed in terms of the in¯uence of dielectric and mechanical contributions. These were monitored through the study of solvent shifts, and vibronic half-width broadenings (associated with the aromatic ring breathing modes) as the dominant aspects to the chromophore interactions. The ¯uorophore solvation process was interpreted through fast and slowly attained con®gurational distributions. OBM theory and the double con®guration coordinate model provide a consistent description in terms of electronic and orientation polarization contributions to the reaction ®eld. The anisotropy associated with charge displacement upon photon absorption can be related to the pattern present in the vibronic structure and the thermochromic variations interpreted invoking the contribution of electrostriction. Accordingly, the increase in the contribution of the 1 0 vibronic to the absorption is re¯ected in the spectrum thermal response through the variation of the normal coordinate displacements.
The spectra decomposition was made in terms of Eq. (A.1): ! 3 X Ai k 1 li 2 1 p exp S k : A:1 2r2i i1 ri 2p The fractions ai (Eq. (A.2)), are the relative contributions of each vibronic band to the spectral distribution: Ai ai P3
i1
Ai
:
A:2
Using each experimental spectrum, the nine global parameters are obtained by a least squares procedure. The number of experimental points in each data ®le is higher than 400 separated by no more than 0.5 nm. A typical result is shown in Fig. 7 for the absorption spectra at two temperatures in 1hexanol. References [1] N. Mataga, T. Kubota, Molecular Interactions and Electronic Spectra, Marcel Dekker, New York, 1970. [2] R.M. Stratt, M. Maroncelli, J. Phys. Chem. 100 (1996) 12981. [3] M. Maroncelli, J. Mol. Liq. 57 (1993) 1. [4] P. Debye, Polar Molecules, Dover, New York, 1929. [5] L. Onsager, J. Am. Chem. Soc. 58 (1936) 1486. [6] C.J.F. B ottcher, Theory of Electric Polarization, Elsevier, Amsterdam, 1973. [7] J. Ma, D.V. Bout, M. Berg, J. Chem. Phys. 103 (1995) 9146. [8] Y.T. Mazurenko, N.G. Bakshiev, Opt. Spectrosc. 28 (1970) 490. [9] R.A. Marcus, J. Chem. Phys. 38 (1963) 1858. [10] R.F. Loring, J. Phys. Chem. 94 (1990) 513. [11] R.A. Marcus, J. Chem. Phys. 43 (1965) 1261. [12] G. van der Zwan, J.T. Hynes, J. Phys. Chem. 89 (1985) 4181.
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