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Influence of the molecular structure and the nature of the solvent on the absorption and fluorescence characteristics of rhodamines
Chemical Physics 130 (1989) North-Holland. Amsterdam
371-378
INFLUENCE OF THE MOLECULAR STRUCTURE AND THE NATURE OF THE SOLVENT ON THE ABSORPTION AND FLUORESCENCE CHARACTERISTICS OF RHODAMINES F. LOPEZ ARBELOA,
I. URRECHA
AGUIRRESACONA
and I. LOPEZ ARBELOA
Departamento de Quimica-Fisica, Facultad de Ciencias. Universidad de1 Pais Vasco - EHlJ, Apartado 644, 48080 Bilbao, Spain Received
13 June 1988; in final form 17 October
1988
A study of the equilibrium of the molecular forms of rhodamine 19 in aqueous and ethanolic solution is carried out by determining the absorption and fluorescence characteristics of the zwitterionic and the cationic forms of the dye. The optical properties of rhodamine 19 are compared with those obtained for rhodamine 6G and also with those previously reported for rhodamine 3B and for the molecular forms of rhodamine B in water and ethanol. Different aspects of the molecular structure of the rhodamines and solvent effects are discussed, as well as their influence on the photophysical properties of the rhodamines. The aggregation of the molecular forms of rhodamine 19 is also studied in water and ethanol.
1. Introduction
to be roughly perpendicular to the planar xanthene ring [ 1, lo], the effect of the dissociation and esterification of the ortho benzoic group can be discussed. Besides, it is also possible to study the influence of the alkyl and/or hydrogen attached to the nitrogen atoms on the optical properties of the xanthene Ksystem. Water and ethanol are chosen as solvents because of their different specific interactions with the rhodamines. Indeed, the interactions of the alcohol with the ethylamino groups are favoured [4,5], whereas water molecules avoid this part of the dye due to the hydrophobic character of the ethyl groups. Furthermore, water molecules tend to solvate the COOgroup more easily than ethanol molecules. The essential role of the solvent on the aggregation tendency of the molecular forms of R19 is also studied.
The absorption and emission properties of the rhodamines are a puzzle in which different structural features [ l-3 ] and medium effects such as solvent interactions [ 4-8 1, pH [ 9- 12 1, temperature [ 5 1, etc., are involved. Consequently, dye lasers with an extensive variety of output characteristics can be obtained [ 11. In the same way, the use of rhodamines as concentrators of solar luminescence in fotovoltaic cells depends on their photophysical properties [ 13- 15 1. In spite of the numerous papers published, the absorption and emission characteristics of the rhodamines continue to be the subject of considerable discussion [ l-l 21. The present work was taken up to generate further information on the influence of the molecular structure and solvent effects on the optical properties of the rhodamines. Absorption and fluorescence spectra of rhodamine 19, R 19, and rhodamine 6G, R6G, are measured in aqueous and ethanolic solution at different pHs and compared with those previously obtained for the molecular forms of rhodamine B [ 4,5 1, RB, and rhodamine 3B [ 6 1, R3B, fig. 1. R 19 has been chosen since it has a xanthene ring similar to that of R6G, but a pendant phenyl ring similar to that of RB, which allows different molecular forms. Considering that the phenyl group of the rhodamines is sterically hindered 0301-0104/89/$03.50 0 Elsevier Science Publishers ( North-Holland Physics Publishing Division )
2. Experimental R19 and R6G were supplied by Kodak (Laser Grade ). Measurements were accomplished in bidistilled water and in ethanol (Merck, pro analysis). Other chemical compounds were Merck Suprapur grade quality. Solutions of R19 were prepared by adding adequate amounts of NaOH and HCl B.V.
F. L&ez Arbeloa et al. /Absorption andfluorescence
372
of rhodamines
C OOEt
R6G
c OOEt
COOH
R3B
RSH+
Fig. 1. Molecular
structure
ofrhodamine
19 (R19H+),
rhodamine
(aqueous and ethanolic ~01s). The hydrogen ion activity in ethanol was taken as paH* = pH - 6, where 6= - 2.8 for ethanol according to Bates [ 16 1. Several buffers were used in order to standardize the pH-meter (Radiometer Copenhagen, model 5 1). For aqueous solutions, paH* = pH was considered. Absorption and fluorescence spectra were recorded on a Shimadzu spectrophotometer (model UV-240) and a Perkin-Elmer spectrofluorimeter (model MPF-66), respectively. The equilibrium of R 19 molecular forms was studied in diluted solution (about 1O-‘j M ) in order to avoid dye aggregation and to reduce the reabsorption and reemission effects [ 17 ] on the fluorescence spectrum. These latter effects were minimized by recording the fluorescence spectrum in a front-face (reflection) configuration with a rectangular cell of 1 mm optical path. The angles between the normal to the cell and the excitation and emission directions were 30” and 60”) respectively. In any case, reabsorption and reemission effects were checked by dilution. The zwitterionic form of RB in ethanol at 2O”C, with a fluorescence quantum yield of 0.70 [9,10], was used as a standard to evaluate the corresponding value of the samples using the calculation method of ref. [ 17 1. The excitation wavelength was 490 nm for all samples.
6G (R6G),
rhodamine
B (RBH
+ ) and rhodamine
3B (R3B)
3. Results and discussion 3.1. Absorption andfluorescence spectra of the rhodamines The absorption and fluorescence spectra of a diluted aqueous and ethanolic solution of R 19 ( 10m6 M) depend on the hydrogen ion activity of the medium. A shift of both spectra to lower energies is observed when the medium hydrogen ion activity increases, fig. 2. Moreover, the molar absorptivity of the dye increases when the hydrogen ion activity increases, fig. 3, whereas the fluorescence quantum yield decreases, fig. 4. When a very high hydrogen ion activity is introduced in the aqueous solution ( paH* < 1.5 ), the absorption intensity of the dye decreases, fig. 3, although the shape and the wavelength of both the absorption and the emission spectra maximum remain constant, fig. 2, as well as the emission quantum yield, fig. 4. This behavior has been attributed to protonation of the amino groups of the rhodamines 131. R 19 is an acid dye whose solution in water and in ethanol must give rise to the following equilibria generated by the dissociation of the carboxyl group [ 9 ] : R19H++H,O=R19’+H,O+, R19H++EtOH=R19’+EtOH:
.
(1)
F. Lipez Arbeloa et al. /Absorption and fluorescence of rhodamines
373
t
520 -
518-
538
516 -
,d.,,, 0
6
4
2
3
12
10
14
16
Pa;
Fig. 2. Influence of the hydrogen ion activity (paH*) on the wavelength of the absorption (A’, 0 ) and fluorescence (A4 0) spectrum maximum of rhodamine 19 in water (full symbols) and in ethanol (open symbols) at 20 “C.
6.0
.
,
.
,
1
,
,
,
.
,
,
*14.0
14
12.0 16
ElOH b.cc_O_____*___~___6 0
1.0**0
2
4
6
8
10
12
Pa;
Fig. 3. Molar absorptivity at the absorption spectrum maximum of rhodamine 19 in aqueous (full symbols) and ethanolic (open symbols) solution at diffrent hydrogen ion activities paH* (20°C).
F. Ldpez Arbeloa et al. /Absorption andfluorescence ofrhodanzines
374 0.8r
.
I
0.6
’
’ 2
0
.
,
’ 4
.
I
.
’ 6
I
I
’ 0
10
’
.
,
I
’
.
12
I
’
1L
.
1.0
16”’
Pa’,
Fig. 4. Fluorescence quantum yield of rhodamine hydrogen ion activities, paH* (20°C).
19 in aqueous
Therefore, the variation of the absorption and fluorescence characteristics of R19 in moderate hydrogen ion activity is due to a molecular form change of the dye between the zwitterionic, R 19’, and the cationic, R19H+, structures. Since the absorption spectra of both molecular forms of R 19 are very similar, the pK, of equilibria ( 1) in the ground state (S,) can be considered as the paH* in the inflection point of the curves of the absorption parameters (E, Aa) in figs. 2 and 3. These results suggest a pK,(S,) =3.15 for the aqueous solution and a pK, (S,) = 9.4 for the ethanolic solution. This latter value is similar to those found for many organic acids in ethanol [ 161. The ground state of R 19 therefore is more acidic in water than in ethanol due not only to the higher basicity but also to the lower dielectric constant and ion solvating ability of the alcohol [ 18 1, which decrease the deprotonation capacity of the carboxyl group. Experimental results suggest that the deprotonation tendency in the first excited state of RI9 in ethanol is higher than that in the ground state in ethanol. Thus, at paH* = 8, the absorption spectrum of the dye corresponds to that of the cationic structure, figs. 2 and 3, while the fluorescence characteristics indicate the participation of the zwitterionic structure in the S, excited state at this paH*, figs. 2 and 4. This behavior is not observed in water. The excited state pK, value has not been determined due
(full symbols)
and ethanolic
(open symbols)
solution
at different
to the uncertainty of whether equilibria (1) are attained in the short fluorescence lifetime of the rhodamines [4,.5]. The absorption and fluorescence characteristics of the zwitterionic and cationic forms of R 19 in water and ethanol at 20°C are summarized in table 1. The spectroscopic parameters obtained also for R6G and those previously reported for the molecular forms of RB [ 4,5], and for R3B [6] in both solvents are included in table 1 for comparison. The results of table 1 show that the fluorescence efficiency of R19 in ethanol depends on the acidity of the medium, which contrasts wit,h results found in the literature [ 1 1. The value obtained for the fluorescence quantum yield of R19” is similar to that reported by Kubin and Fletcher [ 19 1, whereas the 0.80 value that we obtained for R6G in ethanol is smaller than the 0.95 reported by Drexhage [ 11. In order to explain the spectral differences in water and in ethanol among the rhodamines of table 1, both molecular structure (amino group alkylation and ortho benzoic esterilication or deprotonation) and solvent effects should be taken into account. Table 1 shows that the maximum of the absorption and emission spectral band of the rhodamines having diethylamino groups, RB and R3B, appears at about 1000 cm-’ lower energies than their related dyes with monoethylamino groups, R19 and R6G, fig. 1. This change should be due to the effect ofthe amino group
F. Ldpez Arbeloa et al. /Absorption andfluorescence of rhodamines
375
Table 1 Wavelength of the absorption (a) and fluorescence (f ) maximum wavelength (A, AI0.5 nm) and fluorescence quantum yield (@‘) in water and in ethanol at 20” C of the following systems: zwitterionic ( R I9 * ) and cationic (R 19H+ ) forms of rhodamine 6G (R6G), zwitterionic (RB’) and cationic (RBH+) forms ofrhodamine B [4,5], and rhodamine 3B (R3B) (61
19, rhodamine
R19’ R19H+ R6G RB’ RBH+
521.0 524.0 526.0 552.5 557.0
546.5 550.0 550.0 576.0 580.0
0.72 0.62 0.59 0.36 0.31
516.5 526.5 530.0 542.5 553.0
539.0 549.0 551.0 566.0 575.0
0.95 0.82 0.80 0.70 0.58
R3B
557.0
580.0
0.20
555.0
579.0
0.42
substituent the methyl
on the x-system groups attached
of the chromophore at the’2- and 7-carbon
configuration energetically less favoured [ 1 ] and those intermediate structures between “a” or “cl’ and “b” have not been included in fig. 5. On the other hand, the COOR group is juxtaposed above the planar xanthene skeleton since phenylCOOR is sterically constrained to be roughly perpendicular to the chromophore [ 1, lo]. Molecular models show that the negative field of the COO-carboxyl group can mean a restriction of the electron flow of the xanthene n-system at the central 9-carbon atom [4,5] increasing the statistical weight of the mesomerit structure “b” of fig. 5. This effect should be particularly noticeable with the COO- group and decreases in the series COO- Z+ COOH x COOEt. The potential barrier at the central g-carbon atom should produce a shift to higher energies of the absorption and emission band, as well as an absorptivity dimi-
since at-
oms of the R19 and R6G xanthene skeleton do not seem to strongly affect these spectral characteristics according to Drexhage [ 11. The lower inductive effect of the hydrogen atom with respect to the ethyl group [ 201 involves a decrease of the quinonoid character of the xanthene benzene rings in the dyes with monoethylamino groups and, consequently, a higher statistical weight of the mesomeric structure “b” of fig. 5. This electronic configuration involves a loss of the chromophore aromaticity as well as the generation of a potential barrier at the central 9-carbon atom of the chromophore. This explains the shift to higher energies of the R19 and R6G absorption and emission band with respect to those of RB and R3B, respectively. Structures involving an oxonium
COOH R19H+
Fig. 5. Molecular structure “c” of the rhodamines.
of the zwitterionic,
c
b
a
R19’,
and cationic
R19H+,
forms of rhodamine
19. Mesomeric
structures
“a”, “b”, and
376
F. L6pez Arbeloa et al. /Absorption and,fluorescence qf’rhodarnines
nution in the series R 19’, R 19H’ and R6G, table 1, which has also been observed in the series RB’, RBH+ and R3B [ 4-61. The spectral differences between the zwitterionic and the cationic forms of R 19 and RB are smaller in water than in ethanol (table 1 ), because the former solvent tends to solvate the COO- group more easily decreasing the COO- effect on the chromophore rr-system in water. 3.2. PYuorescence quantum yield
qfthe rhodamines
The fluorescence quantum yield of the rhodamines with monoethylamino groups is larger than that of the related dyes with diethylamino groups (table 1). Besides, the fluorescence capacity of the zwitterionic form of R 19 is higher than that of the cationic form, which turns out to be similar to the value obtained for R6G. This is observed in aqueous as well as in ethanolic solution, although the emission capacity of the dyes is higher in ethanol. Results obtained for the molecular forms of RB [ 51 and for R3B [ 61 show a similar tendency. Since the probability of intersystern crossing of the rhodamines is very small, k,,, < 10’ s-’ [ 2 11, these differences should be mainly attributed to changes in the internal conversion (k,,) probability. The non-radiative deactivation process of rhodamine B has been assigned mainly to the rotation motion of the diethylamino groups [ l-3 1. A diminution of the rotation should be expected when the volume of the rotating molecular fragment is increased; but experimental results do not confirm this aspect (table 1 ), i.e. the fluorescence quantum yield of monoethylated dyes (R 19 ‘, R 19H ’ and R6G) is higher than that of the corresponding diethylated compounds (RB ‘, RBH’ and R3B ) . Moreover, the rhodamines with a higher electronic density in the xanthene-nitrogen bond (e.g. a higher participation of the mesomeric structures “a” and “c”) will make the rotation of the amino groups difficult and should be more fluorescent. Table 1 again shows different behavior: those systems with a higher contribution of the mesomeric structure “b” (like monoethylated dyes with respect to diethylated ones or the zwitterionic form with respect to the cationic one) have a higher fluorescence quantum yield. Furthermore, the rotational motion is not related in a simple way with the solvent viscosity; for instance, the rate constant
of the internal conversion of RB in glycerol (q= 1260) is similar to that obtained in ethanol (Y/= 1.2) [4,5]. Though the relation between the rotational motion of the amino groups and the intramolecular quenching process has received considerable experimental confirmation [ l-5 1, a rotational diffusion process hindered by solvent viscosity is too simple a picture to explain the internal conversion process of the rhodamines in solution. Structural and electronic aspects of the dyes as well as solvent effects should be taken into account. Recently [22], the non-radiative deactivation of the rhodamines has been attributed to the formation of a low-lying twisted state with a charge separation analogous to a twisted intramolecular charge transfer (TICT) state proposed by Rettig [ 231. This theory can explain the effect of the amino group alkylation on the fluorescence quantum yield of the rhodamines. Thus, the stronger electron donor capacity of the diethylamino group with respect to the monoethylamino group decreases the TICT-like state energy [ 22 1, depopulating the S, excited state of the dye. The TICT-like state of the rhodamines has not been observed experimentally. To explain the effect of the amino group alkylation on the fluorescence capacity of the rhodamines, an alternative way can be suggested, which also explains the influence of the phenyl group and the solvent on the fluorescence quantum yield. The internal conversion could be due to the perturbation produced in the energy potential of the chromophore n-system when the configuration =N+RR’ is changed to -NRR’. i.e. the displacement of the positive charge in the xanthene-nitrogen bond [4-61. This nitrogen atom change from a planar to a piramidal structure of the amino group means a rotation of the amino group. If one considers the mesomeric structures “a” and “c” of fig. 5 exclusively, the internal conversion process is expected to be controlled by the rotation of the amino groups (opening-closing umbrella-like motion [ 4-61) and, therefore, this model is similar to that of refs. [l-3]. But when the mesomeric structure “b” cannot be neglected, a rotation of the amino group does not necessarily give rise to a deactivation process to So since the energy potential of the xanthene “b” configuration remains unchanged by such a motion (in spite of the higher rotation capacity of
F. Ldpez Arbeloa et al. /Absorption andfluorescence of rhodamines
this structure). In other words, the structure “b” means a restriction of the positive charge displacement at the central 9-carbon produced by a potential barrier. Therefore, the fluorescence efficiency of those systems with a high contribution of the mesomeric “b” structure should be less affected by the rotational motion of the amino groups. This explains the increase of the fluorescence quantum yield of the rhodamines with monoethylamino groups (R 19 * , R19H+ and R6G) with respect to those with diethylamino groups (RB *, RBH + and R3B ), table 1. For the same reasons the activation energy of the radiationless process is higher in diethylamino rhodamines [ I-5 1. The higher fluorescence quantum yield of the zwitterionic form of R 19 with respect to the cationic one and to R6G should be attributed mainly to the larger influence of the COO- group with respect to the COOH and COOR groups on the xanthene x-system. This COO- effect stabilizes the mesomeric structure “b” increasing the fluorescence capacity of the dye. Besides, this interaction also generates an augmentation of the structural rigidity of the dye since the possible twist movement of the ortho benzoic acid group is hindered, that could also be a potential way for the internal conversion process [ 1,22 1. The increase of the acidity of the S, state with respect to the S,, state of R 19 supports the above suggestion. The augmentation of the COOH deprotonation tendency in the S, excited state involves a large interaction of this group with the central 9-carbon atom of the xanthene group in the excited state and consequently an increase of the statistical weight of the mesomeric structure “b” is expected. A linear relationship between @p and log k,c versus the absorption and emission maxima (expressed in wavenumbers) has been observed for the molecular forms of RB in different monoalcohols and other polyfunctional solvents with hydrogen acceptor and donor capacity [ 4,5 1. This suggests that the internal conversion process and the spectroscopic shift for RB have a similar origin, supporting the present theory. The solvent has also an important influence on the internal conversion process of the rhodamines, i.e. the fluorescence quantum yield of both molecular forms of R 19 and R6G is about 30% higher in ethanol than in water, table 1. This behavior can be attributed to
377
different reasons. First of all, the ethylamino groups can rotate more freely in aqueous solution [ 1,6 ] than in ethanol due to the hydrophobic character of the ethyl groups, whereas specific interactions of ethanol molecules with the amino groups are favoured [461. Secondly, the N-H vibration, suggested by Drexhage [ 1,3 ] as a possible internal conversion process of R6G, should be hindered by specific ethanol interactions [ 4-6 1. 3.3. Aggregation characteristics of the rhodamines Changes of the absorption spectrum shape are observed when the concentration of the molecular form of R 19 is increased in aqueous solution from 10e6 to 2X 10e5 M. This concentration effect has also been observed for aqueous solutions of R6G [ 24,251, RB [ 26,27 ] and R3B [ 26,281, being the first variations of the spectrum attributed to the dimer formation of the dye. The dimerization constant at 20°C evaluated by means of the method of ref. [29], is about 4400 (standard concentration: 1 M) for both molecular forms of R 19. Very different behavior is observed in ethanol. No changes in the absorption spectrum shape are observed when the concentration of each molecular form of R 19 is increased up to 2 x 10W3 M in ethanolic solution. However, when the dye concentration is increased from 1Ov6 up to 2 x 10m3 M at the natural paH* in ethanol, a shift of the absorption spectrum is observed as fig. 6 shows. The zwitterionic absorption spectrum is obtained at natural paH* in diluted ethanolic solutions of the dye, whereas the spectrum of the 2x lop3 M dye concentration at the natural paH* looks like that of the cationic structure, fig. 6. This is due to a change of the molecular form of R19 with increase of dye concentration, which should be attributed to the concentration dependence of the dissociation degree of COOH [9-l 2 1. Therefore, a marked difference is observed between the aggregation of the molecular forms of R 19 in aqueous solution and ethanolic solution. The hydrophobic nature of the ethyl group is responsible for the large aggregation of the dye in water. However, ethanol molecules have a larger affinity for the rhodamines since they can solvate at different sites of the solute and, as a consequence, the self-association
/ 1
F. L&e~pz Arbeioa et a/./Absorption and,fluorescence qfrhodarnine.~ 14 J I. Mpez Arbeloa and K.K. Rohatgi-Mukherjec.
iI\
I
I
i.
1
’
i
’
!
/
\
i
J
\ / : “1, ’
i
‘, \
I
I
43”
‘1 !
!
’
/
’
530
480
1
580
A (nm)
Fig, 6. Absorption spectrum of rhodamine 19 in ethanol under different conditions: IO-‘M at natural paH* ( -.-.), 10e6 M in acidic medium f-) and 2x lo-‘M at natural paH* f - - -)_
is avoided at least at the moderate tions used in this work.
dye concentra-
Acknowledgement We wish to thank Professor E. Dominguez of the Chemistry Department for her helpful comments. The authors also want to thank the Universidad de1 Pais Vasco - EHU for their financial support. One of us (FLA) would like to thank the Gobierno Vasco for awarding him a grant.
References [ 11 K.H. Drexhage. in: Dye lasers, ed. F.P. Schafer (Springer, Berlin, 1977) ch. 4. [ 21K.H. Drexhage. Laser Focus 9 ( I973 ) 35. [ 3 ] K.H. Drexhage. J. Res. Nat. Bur, Std. 8O.a ( t 976) 42 1.
Chem. Phys. Letters 128 f 1986) 474. [ 5 j I. Mpcz Arbeloa and K.K. Rohatgi-Mu~erjee. Chem. Phys. Letters 129 ( 1986) 607. [6] I. Lopez Arbeloa and K.K. Rohatgi-Mukherjee, to be published. [7] M.J. Snare, F.E. Treloar, K.P. Ghiggino and P.J. Thistlethwaite, J. Photochem. 18( 1982) 335. [ 81 T. Govindanunny, Appl. Phys. 23 ( 1980) 253. [9]J. Ferguson and A.W.H. Mau. Chem. Phys. Letters 17 (1972) 543. [ lo] P.J. Sadowski and G.R. Flemming. Chem. Phys. Letters 57 (1978) 526. [ II] M. Faraggi. P. Peretz. I. Rosenthal and D. Weinraud. Cbem. Phys. Letters 103 ( 1984) 3 10. [ 121 I. Lopez Arbeloa and P. Ruiz Ojeda, Chem. Phys. Letters 79(1981)347. [ 131 W.H. Weber and J. Lambe. Appl. Opt. I5 ( 1976) 2299. [ 141 J.S. Batchelder, A.H. Zewail and T. Cole, Appl. Opt. I8 ( 1979) 3090. [ 1.51B.B. Bhownik, R. Chaudhuri and K.K. Rohatgi-Mukherjee. Indian J. Chem. 26A ( 1987 ) 95. [ 161 R.G. Bates. The determination of pH (Wiley. New York, 1964). [ f 7 11.Lopez Arbeloa, J. Photochem. 14 ( i980f 97. [ 18 J C. Reichardt, Solvent effects in organic chemistry (Springer, Berlin, 1979) p. 45. [ 191 R.F. Kubin and .4.N. Fletcher. J. Luminescence 27 ( 1982) 455. [ 201 J. March, Advanced organic chemistry, 3rd Ed. (Wiley, New York. 1985). [2 I] A. Penzkofer and M. Falkenstein. Opt. Quant. Electron. 10 ( 1978) 399. [ 22 ] M. Vogel, W. Rettig, R. Sens and KH. Drexhage. Chem. Phys. Letters 147 ( 1988) 452. [23] W. Rettig. Angew. Chem. Intern. Ed. 25 (1986) 971. [ 241 C. Bojarski and G. Obermuller, Acta Phys. Polon. A 50 ( 1976) 389. [25] F. Lopez Arbeloa, 1. Llona Gonzalez, P. Ruin Qjcda and I. Lopez Arbeloa, J. Chem. Sot. Faraday Trans. II 78 ( 1982) 989. [26] G. Obermuller and C. Bojarski, Acta Phys. Polon. A 52 (1977) 431. [27] 1. Lopez Arbeloa and P. Ruiz Ojeda, Chem. Phys. Letters 87 (1982) 556. 1281 M.M. Wong and Z.A. Schelly. J. Phys. Chem. 78 (1974) 1891. [ 291 I. Lopez Arbeloa, J. Chem. Sot. Faraday Trans. II 77 ( 198 1) 1725.
371-378
INFLUENCE OF THE MOLECULAR STRUCTURE AND THE NATURE OF THE SOLVENT ON THE ABSORPTION AND FLUORESCENCE CHARACTERISTICS OF RHODAMINES F. LOPEZ ARBELOA,
I. URRECHA
AGUIRRESACONA
and I. LOPEZ ARBELOA
Departamento de Quimica-Fisica, Facultad de Ciencias. Universidad de1 Pais Vasco - EHlJ, Apartado 644, 48080 Bilbao, Spain Received
13 June 1988; in final form 17 October
1988
A study of the equilibrium of the molecular forms of rhodamine 19 in aqueous and ethanolic solution is carried out by determining the absorption and fluorescence characteristics of the zwitterionic and the cationic forms of the dye. The optical properties of rhodamine 19 are compared with those obtained for rhodamine 6G and also with those previously reported for rhodamine 3B and for the molecular forms of rhodamine B in water and ethanol. Different aspects of the molecular structure of the rhodamines and solvent effects are discussed, as well as their influence on the photophysical properties of the rhodamines. The aggregation of the molecular forms of rhodamine 19 is also studied in water and ethanol.
1. Introduction
to be roughly perpendicular to the planar xanthene ring [ 1, lo], the effect of the dissociation and esterification of the ortho benzoic group can be discussed. Besides, it is also possible to study the influence of the alkyl and/or hydrogen attached to the nitrogen atoms on the optical properties of the xanthene Ksystem. Water and ethanol are chosen as solvents because of their different specific interactions with the rhodamines. Indeed, the interactions of the alcohol with the ethylamino groups are favoured [4,5], whereas water molecules avoid this part of the dye due to the hydrophobic character of the ethyl groups. Furthermore, water molecules tend to solvate the COOgroup more easily than ethanol molecules. The essential role of the solvent on the aggregation tendency of the molecular forms of R19 is also studied.
The absorption and emission properties of the rhodamines are a puzzle in which different structural features [ l-3 ] and medium effects such as solvent interactions [ 4-8 1, pH [ 9- 12 1, temperature [ 5 1, etc., are involved. Consequently, dye lasers with an extensive variety of output characteristics can be obtained [ 11. In the same way, the use of rhodamines as concentrators of solar luminescence in fotovoltaic cells depends on their photophysical properties [ 13- 15 1. In spite of the numerous papers published, the absorption and emission characteristics of the rhodamines continue to be the subject of considerable discussion [ l-l 21. The present work was taken up to generate further information on the influence of the molecular structure and solvent effects on the optical properties of the rhodamines. Absorption and fluorescence spectra of rhodamine 19, R 19, and rhodamine 6G, R6G, are measured in aqueous and ethanolic solution at different pHs and compared with those previously obtained for the molecular forms of rhodamine B [ 4,5 1, RB, and rhodamine 3B [ 6 1, R3B, fig. 1. R 19 has been chosen since it has a xanthene ring similar to that of R6G, but a pendant phenyl ring similar to that of RB, which allows different molecular forms. Considering that the phenyl group of the rhodamines is sterically hindered 0301-0104/89/$03.50 0 Elsevier Science Publishers ( North-Holland Physics Publishing Division )
2. Experimental R19 and R6G were supplied by Kodak (Laser Grade ). Measurements were accomplished in bidistilled water and in ethanol (Merck, pro analysis). Other chemical compounds were Merck Suprapur grade quality. Solutions of R19 were prepared by adding adequate amounts of NaOH and HCl B.V.
F. L&ez Arbeloa et al. /Absorption andfluorescence
372
of rhodamines
C OOEt
R6G
c OOEt
COOH
R3B
RSH+
Fig. 1. Molecular
structure
ofrhodamine
19 (R19H+),
rhodamine
(aqueous and ethanolic ~01s). The hydrogen ion activity in ethanol was taken as paH* = pH - 6, where 6= - 2.8 for ethanol according to Bates [ 16 1. Several buffers were used in order to standardize the pH-meter (Radiometer Copenhagen, model 5 1). For aqueous solutions, paH* = pH was considered. Absorption and fluorescence spectra were recorded on a Shimadzu spectrophotometer (model UV-240) and a Perkin-Elmer spectrofluorimeter (model MPF-66), respectively. The equilibrium of R 19 molecular forms was studied in diluted solution (about 1O-‘j M ) in order to avoid dye aggregation and to reduce the reabsorption and reemission effects [ 17 ] on the fluorescence spectrum. These latter effects were minimized by recording the fluorescence spectrum in a front-face (reflection) configuration with a rectangular cell of 1 mm optical path. The angles between the normal to the cell and the excitation and emission directions were 30” and 60”) respectively. In any case, reabsorption and reemission effects were checked by dilution. The zwitterionic form of RB in ethanol at 2O”C, with a fluorescence quantum yield of 0.70 [9,10], was used as a standard to evaluate the corresponding value of the samples using the calculation method of ref. [ 17 1. The excitation wavelength was 490 nm for all samples.
6G (R6G),
rhodamine
B (RBH
+ ) and rhodamine
3B (R3B)
3. Results and discussion 3.1. Absorption andfluorescence spectra of the rhodamines The absorption and fluorescence spectra of a diluted aqueous and ethanolic solution of R 19 ( 10m6 M) depend on the hydrogen ion activity of the medium. A shift of both spectra to lower energies is observed when the medium hydrogen ion activity increases, fig. 2. Moreover, the molar absorptivity of the dye increases when the hydrogen ion activity increases, fig. 3, whereas the fluorescence quantum yield decreases, fig. 4. When a very high hydrogen ion activity is introduced in the aqueous solution ( paH* < 1.5 ), the absorption intensity of the dye decreases, fig. 3, although the shape and the wavelength of both the absorption and the emission spectra maximum remain constant, fig. 2, as well as the emission quantum yield, fig. 4. This behavior has been attributed to protonation of the amino groups of the rhodamines 131. R 19 is an acid dye whose solution in water and in ethanol must give rise to the following equilibria generated by the dissociation of the carboxyl group [ 9 ] : R19H++H,O=R19’+H,O+, R19H++EtOH=R19’+EtOH:
.
(1)
F. Lipez Arbeloa et al. /Absorption and fluorescence of rhodamines
373
t
520 -
518-
538
516 -
,d.,,, 0
6
4
2
3
12
10
14
16
Pa;
Fig. 2. Influence of the hydrogen ion activity (paH*) on the wavelength of the absorption (A’, 0 ) and fluorescence (A4 0) spectrum maximum of rhodamine 19 in water (full symbols) and in ethanol (open symbols) at 20 “C.
6.0
.
,
.
,
1
,
,
,
.
,
,
*14.0
14
12.0 16
ElOH b.cc_O_____*___~___6 0
1.0**0
2
4
6
8
10
12
Pa;
Fig. 3. Molar absorptivity at the absorption spectrum maximum of rhodamine 19 in aqueous (full symbols) and ethanolic (open symbols) solution at diffrent hydrogen ion activities paH* (20°C).
F. Ldpez Arbeloa et al. /Absorption andfluorescence ofrhodanzines
374 0.8r
.
I
0.6
’
’ 2
0
.
,
’ 4
.
I
.
’ 6
I
I
’ 0
10
’
.
,
I
’
.
12
I
’
1L
.
1.0
16”’
Pa’,
Fig. 4. Fluorescence quantum yield of rhodamine hydrogen ion activities, paH* (20°C).
19 in aqueous
Therefore, the variation of the absorption and fluorescence characteristics of R19 in moderate hydrogen ion activity is due to a molecular form change of the dye between the zwitterionic, R 19’, and the cationic, R19H+, structures. Since the absorption spectra of both molecular forms of R 19 are very similar, the pK, of equilibria ( 1) in the ground state (S,) can be considered as the paH* in the inflection point of the curves of the absorption parameters (E, Aa) in figs. 2 and 3. These results suggest a pK,(S,) =3.15 for the aqueous solution and a pK, (S,) = 9.4 for the ethanolic solution. This latter value is similar to those found for many organic acids in ethanol [ 161. The ground state of R 19 therefore is more acidic in water than in ethanol due not only to the higher basicity but also to the lower dielectric constant and ion solvating ability of the alcohol [ 18 1, which decrease the deprotonation capacity of the carboxyl group. Experimental results suggest that the deprotonation tendency in the first excited state of RI9 in ethanol is higher than that in the ground state in ethanol. Thus, at paH* = 8, the absorption spectrum of the dye corresponds to that of the cationic structure, figs. 2 and 3, while the fluorescence characteristics indicate the participation of the zwitterionic structure in the S, excited state at this paH*, figs. 2 and 4. This behavior is not observed in water. The excited state pK, value has not been determined due
(full symbols)
and ethanolic
(open symbols)
solution
at different
to the uncertainty of whether equilibria (1) are attained in the short fluorescence lifetime of the rhodamines [4,.5]. The absorption and fluorescence characteristics of the zwitterionic and cationic forms of R 19 in water and ethanol at 20°C are summarized in table 1. The spectroscopic parameters obtained also for R6G and those previously reported for the molecular forms of RB [ 4,5], and for R3B [6] in both solvents are included in table 1 for comparison. The results of table 1 show that the fluorescence efficiency of R19 in ethanol depends on the acidity of the medium, which contrasts wit,h results found in the literature [ 1 1. The value obtained for the fluorescence quantum yield of R19” is similar to that reported by Kubin and Fletcher [ 19 1, whereas the 0.80 value that we obtained for R6G in ethanol is smaller than the 0.95 reported by Drexhage [ 11. In order to explain the spectral differences in water and in ethanol among the rhodamines of table 1, both molecular structure (amino group alkylation and ortho benzoic esterilication or deprotonation) and solvent effects should be taken into account. Table 1 shows that the maximum of the absorption and emission spectral band of the rhodamines having diethylamino groups, RB and R3B, appears at about 1000 cm-’ lower energies than their related dyes with monoethylamino groups, R19 and R6G, fig. 1. This change should be due to the effect ofthe amino group
F. Ldpez Arbeloa et al. /Absorption andfluorescence of rhodamines
375
Table 1 Wavelength of the absorption (a) and fluorescence (f ) maximum wavelength (A, AI0.5 nm) and fluorescence quantum yield (@‘) in water and in ethanol at 20” C of the following systems: zwitterionic ( R I9 * ) and cationic (R 19H+ ) forms of rhodamine 6G (R6G), zwitterionic (RB’) and cationic (RBH+) forms ofrhodamine B [4,5], and rhodamine 3B (R3B) (61
19, rhodamine
R19’ R19H+ R6G RB’ RBH+
521.0 524.0 526.0 552.5 557.0
546.5 550.0 550.0 576.0 580.0
0.72 0.62 0.59 0.36 0.31
516.5 526.5 530.0 542.5 553.0
539.0 549.0 551.0 566.0 575.0
0.95 0.82 0.80 0.70 0.58
R3B
557.0
580.0
0.20
555.0
579.0
0.42
substituent the methyl
on the x-system groups attached
of the chromophore at the’2- and 7-carbon
configuration energetically less favoured [ 1 ] and those intermediate structures between “a” or “cl’ and “b” have not been included in fig. 5. On the other hand, the COOR group is juxtaposed above the planar xanthene skeleton since phenylCOOR is sterically constrained to be roughly perpendicular to the chromophore [ 1, lo]. Molecular models show that the negative field of the COO-carboxyl group can mean a restriction of the electron flow of the xanthene n-system at the central 9-carbon atom [4,5] increasing the statistical weight of the mesomerit structure “b” of fig. 5. This effect should be particularly noticeable with the COO- group and decreases in the series COO- Z+ COOH x COOEt. The potential barrier at the central g-carbon atom should produce a shift to higher energies of the absorption and emission band, as well as an absorptivity dimi-
since at-
oms of the R19 and R6G xanthene skeleton do not seem to strongly affect these spectral characteristics according to Drexhage [ 11. The lower inductive effect of the hydrogen atom with respect to the ethyl group [ 201 involves a decrease of the quinonoid character of the xanthene benzene rings in the dyes with monoethylamino groups and, consequently, a higher statistical weight of the mesomeric structure “b” of fig. 5. This electronic configuration involves a loss of the chromophore aromaticity as well as the generation of a potential barrier at the central 9-carbon atom of the chromophore. This explains the shift to higher energies of the R19 and R6G absorption and emission band with respect to those of RB and R3B, respectively. Structures involving an oxonium
COOH R19H+
Fig. 5. Molecular structure “c” of the rhodamines.
of the zwitterionic,
c
b
a
R19’,
and cationic
R19H+,
forms of rhodamine
19. Mesomeric
structures
“a”, “b”, and
376
F. L6pez Arbeloa et al. /Absorption and,fluorescence qf’rhodarnines
nution in the series R 19’, R 19H’ and R6G, table 1, which has also been observed in the series RB’, RBH+ and R3B [ 4-61. The spectral differences between the zwitterionic and the cationic forms of R 19 and RB are smaller in water than in ethanol (table 1 ), because the former solvent tends to solvate the COO- group more easily decreasing the COO- effect on the chromophore rr-system in water. 3.2. PYuorescence quantum yield
qfthe rhodamines
The fluorescence quantum yield of the rhodamines with monoethylamino groups is larger than that of the related dyes with diethylamino groups (table 1). Besides, the fluorescence capacity of the zwitterionic form of R 19 is higher than that of the cationic form, which turns out to be similar to the value obtained for R6G. This is observed in aqueous as well as in ethanolic solution, although the emission capacity of the dyes is higher in ethanol. Results obtained for the molecular forms of RB [ 51 and for R3B [ 61 show a similar tendency. Since the probability of intersystern crossing of the rhodamines is very small, k,,, < 10’ s-’ [ 2 11, these differences should be mainly attributed to changes in the internal conversion (k,,) probability. The non-radiative deactivation process of rhodamine B has been assigned mainly to the rotation motion of the diethylamino groups [ l-3 1. A diminution of the rotation should be expected when the volume of the rotating molecular fragment is increased; but experimental results do not confirm this aspect (table 1 ), i.e. the fluorescence quantum yield of monoethylated dyes (R 19 ‘, R 19H ’ and R6G) is higher than that of the corresponding diethylated compounds (RB ‘, RBH’ and R3B ) . Moreover, the rhodamines with a higher electronic density in the xanthene-nitrogen bond (e.g. a higher participation of the mesomeric structures “a” and “c”) will make the rotation of the amino groups difficult and should be more fluorescent. Table 1 again shows different behavior: those systems with a higher contribution of the mesomeric structure “b” (like monoethylated dyes with respect to diethylated ones or the zwitterionic form with respect to the cationic one) have a higher fluorescence quantum yield. Furthermore, the rotational motion is not related in a simple way with the solvent viscosity; for instance, the rate constant
of the internal conversion of RB in glycerol (q= 1260) is similar to that obtained in ethanol (Y/= 1.2) [4,5]. Though the relation between the rotational motion of the amino groups and the intramolecular quenching process has received considerable experimental confirmation [ l-5 1, a rotational diffusion process hindered by solvent viscosity is too simple a picture to explain the internal conversion process of the rhodamines in solution. Structural and electronic aspects of the dyes as well as solvent effects should be taken into account. Recently [22], the non-radiative deactivation of the rhodamines has been attributed to the formation of a low-lying twisted state with a charge separation analogous to a twisted intramolecular charge transfer (TICT) state proposed by Rettig [ 231. This theory can explain the effect of the amino group alkylation on the fluorescence quantum yield of the rhodamines. Thus, the stronger electron donor capacity of the diethylamino group with respect to the monoethylamino group decreases the TICT-like state energy [ 22 1, depopulating the S, excited state of the dye. The TICT-like state of the rhodamines has not been observed experimentally. To explain the effect of the amino group alkylation on the fluorescence capacity of the rhodamines, an alternative way can be suggested, which also explains the influence of the phenyl group and the solvent on the fluorescence quantum yield. The internal conversion could be due to the perturbation produced in the energy potential of the chromophore n-system when the configuration =N+RR’ is changed to -NRR’. i.e. the displacement of the positive charge in the xanthene-nitrogen bond [4-61. This nitrogen atom change from a planar to a piramidal structure of the amino group means a rotation of the amino group. If one considers the mesomeric structures “a” and “c” of fig. 5 exclusively, the internal conversion process is expected to be controlled by the rotation of the amino groups (opening-closing umbrella-like motion [ 4-61) and, therefore, this model is similar to that of refs. [l-3]. But when the mesomeric structure “b” cannot be neglected, a rotation of the amino group does not necessarily give rise to a deactivation process to So since the energy potential of the xanthene “b” configuration remains unchanged by such a motion (in spite of the higher rotation capacity of
F. Ldpez Arbeloa et al. /Absorption andfluorescence of rhodamines
this structure). In other words, the structure “b” means a restriction of the positive charge displacement at the central 9-carbon produced by a potential barrier. Therefore, the fluorescence efficiency of those systems with a high contribution of the mesomeric “b” structure should be less affected by the rotational motion of the amino groups. This explains the increase of the fluorescence quantum yield of the rhodamines with monoethylamino groups (R 19 * , R19H+ and R6G) with respect to those with diethylamino groups (RB *, RBH + and R3B ), table 1. For the same reasons the activation energy of the radiationless process is higher in diethylamino rhodamines [ I-5 1. The higher fluorescence quantum yield of the zwitterionic form of R 19 with respect to the cationic one and to R6G should be attributed mainly to the larger influence of the COO- group with respect to the COOH and COOR groups on the xanthene x-system. This COO- effect stabilizes the mesomeric structure “b” increasing the fluorescence capacity of the dye. Besides, this interaction also generates an augmentation of the structural rigidity of the dye since the possible twist movement of the ortho benzoic acid group is hindered, that could also be a potential way for the internal conversion process [ 1,22 1. The increase of the acidity of the S, state with respect to the S,, state of R 19 supports the above suggestion. The augmentation of the COOH deprotonation tendency in the S, excited state involves a large interaction of this group with the central 9-carbon atom of the xanthene group in the excited state and consequently an increase of the statistical weight of the mesomeric structure “b” is expected. A linear relationship between @p and log k,c versus the absorption and emission maxima (expressed in wavenumbers) has been observed for the molecular forms of RB in different monoalcohols and other polyfunctional solvents with hydrogen acceptor and donor capacity [ 4,5 1. This suggests that the internal conversion process and the spectroscopic shift for RB have a similar origin, supporting the present theory. The solvent has also an important influence on the internal conversion process of the rhodamines, i.e. the fluorescence quantum yield of both molecular forms of R 19 and R6G is about 30% higher in ethanol than in water, table 1. This behavior can be attributed to
377
different reasons. First of all, the ethylamino groups can rotate more freely in aqueous solution [ 1,6 ] than in ethanol due to the hydrophobic character of the ethyl groups, whereas specific interactions of ethanol molecules with the amino groups are favoured [461. Secondly, the N-H vibration, suggested by Drexhage [ 1,3 ] as a possible internal conversion process of R6G, should be hindered by specific ethanol interactions [ 4-6 1. 3.3. Aggregation characteristics of the rhodamines Changes of the absorption spectrum shape are observed when the concentration of the molecular form of R 19 is increased in aqueous solution from 10e6 to 2X 10e5 M. This concentration effect has also been observed for aqueous solutions of R6G [ 24,251, RB [ 26,27 ] and R3B [ 26,281, being the first variations of the spectrum attributed to the dimer formation of the dye. The dimerization constant at 20°C evaluated by means of the method of ref. [29], is about 4400 (standard concentration: 1 M) for both molecular forms of R 19. Very different behavior is observed in ethanol. No changes in the absorption spectrum shape are observed when the concentration of each molecular form of R 19 is increased up to 2 x 10W3 M in ethanolic solution. However, when the dye concentration is increased from 1Ov6 up to 2 x 10m3 M at the natural paH* in ethanol, a shift of the absorption spectrum is observed as fig. 6 shows. The zwitterionic absorption spectrum is obtained at natural paH* in diluted ethanolic solutions of the dye, whereas the spectrum of the 2x lop3 M dye concentration at the natural paH* looks like that of the cationic structure, fig. 6. This is due to a change of the molecular form of R19 with increase of dye concentration, which should be attributed to the concentration dependence of the dissociation degree of COOH [9-l 2 1. Therefore, a marked difference is observed between the aggregation of the molecular forms of R 19 in aqueous solution and ethanolic solution. The hydrophobic nature of the ethyl group is responsible for the large aggregation of the dye in water. However, ethanol molecules have a larger affinity for the rhodamines since they can solvate at different sites of the solute and, as a consequence, the self-association
/ 1
F. L&e~pz Arbeioa et a/./Absorption and,fluorescence qfrhodarnine.~ 14 J I. Mpez Arbeloa and K.K. Rohatgi-Mukherjec.
iI\
I
I
i.
1
’
i
’
!
/
\
i
J
\ / : “1, ’
i
‘, \
I
I
43”
‘1 !
!
’
/
’
530
480
1
580
A (nm)
Fig, 6. Absorption spectrum of rhodamine 19 in ethanol under different conditions: IO-‘M at natural paH* ( -.-.), 10e6 M in acidic medium f-) and 2x lo-‘M at natural paH* f - - -)_
is avoided at least at the moderate tions used in this work.
dye concentra-
Acknowledgement We wish to thank Professor E. Dominguez of the Chemistry Department for her helpful comments. The authors also want to thank the Universidad de1 Pais Vasco - EHU for their financial support. One of us (FLA) would like to thank the Gobierno Vasco for awarding him a grant.
References [ 11 K.H. Drexhage. in: Dye lasers, ed. F.P. Schafer (Springer, Berlin, 1977) ch. 4. [ 21K.H. Drexhage. Laser Focus 9 ( I973 ) 35. [ 3 ] K.H. Drexhage. J. Res. Nat. Bur, Std. 8O.a ( t 976) 42 1.
Chem. Phys. Letters 128 f 1986) 474. [ 5 j I. Mpcz Arbeloa and K.K. Rohatgi-Mu~erjee. Chem. Phys. Letters 129 ( 1986) 607. [6] I. Lopez Arbeloa and K.K. Rohatgi-Mukherjee, to be published. [7] M.J. Snare, F.E. Treloar, K.P. Ghiggino and P.J. Thistlethwaite, J. Photochem. 18( 1982) 335. [ 81 T. Govindanunny, Appl. Phys. 23 ( 1980) 253. [9]J. Ferguson and A.W.H. Mau. Chem. Phys. Letters 17 (1972) 543. [ lo] P.J. Sadowski and G.R. Flemming. Chem. Phys. Letters 57 (1978) 526. [ II] M. Faraggi. P. Peretz. I. Rosenthal and D. Weinraud. Cbem. Phys. Letters 103 ( 1984) 3 10. [ 121 I. Lopez Arbeloa and P. Ruiz Ojeda, Chem. Phys. Letters 79(1981)347. [ 131 W.H. Weber and J. Lambe. Appl. Opt. I5 ( 1976) 2299. [ 141 J.S. Batchelder, A.H. Zewail and T. Cole, Appl. Opt. I8 ( 1979) 3090. [ 1.51B.B. Bhownik, R. Chaudhuri and K.K. Rohatgi-Mukherjee. Indian J. Chem. 26A ( 1987 ) 95. [ 161 R.G. Bates. The determination of pH (Wiley. New York, 1964). [ f 7 11.Lopez Arbeloa, J. Photochem. 14 ( i980f 97. [ 18 J C. Reichardt, Solvent effects in organic chemistry (Springer, Berlin, 1979) p. 45. [ 191 R.F. Kubin and .4.N. Fletcher. J. Luminescence 27 ( 1982) 455. [ 201 J. March, Advanced organic chemistry, 3rd Ed. (Wiley, New York. 1985). [2 I] A. Penzkofer and M. Falkenstein. Opt. Quant. Electron. 10 ( 1978) 399. [ 22 ] M. Vogel, W. Rettig, R. Sens and KH. Drexhage. Chem. Phys. Letters 147 ( 1988) 452. [23] W. Rettig. Angew. Chem. Intern. Ed. 25 (1986) 971. [ 241 C. Bojarski and G. Obermuller, Acta Phys. Polon. A 50 ( 1976) 389. [25] F. Lopez Arbeloa, 1. Llona Gonzalez, P. Ruin Qjcda and I. Lopez Arbeloa, J. Chem. Sot. Faraday Trans. II 78 ( 1982) 989. [26] G. Obermuller and C. Bojarski, Acta Phys. Polon. A 52 (1977) 431. [27] 1. Lopez Arbeloa and P. Ruiz Ojeda, Chem. Phys. Letters 87 (1982) 556. 1281 M.M. Wong and Z.A. Schelly. J. Phys. Chem. 78 (1974) 1891. [ 291 I. Lopez Arbeloa, J. Chem. Sot. Faraday Trans. II 77 ( 198 1) 1725.