2203
J . Phys. Chem. 1991, 95,2203-2208 12, but the dihydroxyporphyrin again appeared as a sole photoproduct as in the absence of Fe-Cyt-c. The fact that no reaction occurred between Fe-Cyt-c and 02'-under our experimental conditions suggests that either 02'-or preferably Fe-Cyt-c is trapped near a p-CD cavity. The fate of the superoxide ions remains unclear, although the final product is most probably H202. The role of singlet oxygen as an oxidizing agent leading to isoporphyrin and dihydroxyporphyrin is questionable. The electron transfer from the ground-state porphyrin to the singlet oxygen, which is an uphill reaction by 0.154.45 eV according to Whitten et should be discarded. It is also noteworthy that the short lifetime of singlet oxygen in water, due to nonradiative deactivation (1 ps)" via OH vibrational relaxation, precludes its reaction with dilute electron donor (the ZnTSPP concentration in the photolysis experiments was 2.0 X M). In organic solvents, the singlet oxygen lifetime is hundreds times longer, thus enabling its reaction with porphyrin as shown by Matsuura et a1.6' in the photooxidation of MgTPP. This reaction, which leads finally to the ring-opening (60) Wilkinson, F.; Brummer, J . G. J . Phys. Chem. Ref. Doto 1981, 10, 809. (61) Matsuura, T.; Inoue, K.; Ranade, A. C.; Saito, I. Phorochem. Phorobiol. 1980, 31. 23.
of the macrocycle and the formation of bilinone, proceeds via addition of the singlet oxygen to the pyrrole double bond rather than electron transfer. Finally it is interesting to point out the resemblance of the shapes of the absorption spectra of two groups of intermediates, ZnP'+, 'ZnP*, ZnPim2+,and ZnP(OH)iso*+on one hand and ZnP(OH)2, ZnP2-, and phthalocyanin on the other hand. The first group possesses usually two well-defined absorption bands, a broad one above 650 nm and a sharper one at ca. 450 nm. A glance at Chart I shows that ZnP'+ and the two isoporphyrin derivatives possess the same degree of conjugation. The same is true for the second group of intermediates. In other words it appears40 that the shape of the absorption spectra of metalloporphyrins derivatives is influenced mainly by the degree of conjugation and less by the nature of the peripheral functional groups or by the oxidation-reduction valency. Acknowledgment. We are grateful to Prof. A. Henglein for enabling us to use the Van de Graaff generator, the 6oCo source, and other devices of the Bereich Strahlenchemie of the HahnMeitner Institut, Berlin. Many thanks to Dr. P. Neta from the National Institute of Standards and Technology for very helpful discussions.
Photophysics of Rhodamines. Molecular Structure and Solvent Effects F. Ldpez Arbeloa, T. Ldpez Arbeloa, M. J. Tapia Estevez, and I. Ldpez Arbeloa* Departamento Quimica- Fisica, Vniversidad del Pa% Vasco-EHV, Apartado 644, 48080- Bilbao, Spain (Received: May 22, 1990; In Final Form: September 21, 1990)
The absorption and emission (fluorescence spectra and radiative decay curves) characteristics of rhodamine 3B and of the molecular forms of rhodamine B are determined in several waterlethano1mixtures. The results are compared to those previously obtained for the related monoethylamine-substitutedrhodamines, i.e. rhodamine 6G and the molecular forms of rhodamine 19. Molecular structural factors (alkylation of the amino groups and the protonation or esterification of the carboxyphenyl group) and solvent effects (specific solutesolvent interactions)are investigated. The internal conversion mechanism of rhodamines is discussed.
Introduction The photophysical properties of rhodamine dyes in solution are a puzzle in which factors such as the molecular structure of the dye (substitutions at the a m i n ~ l and - ~ carboxyphenyle7 groups) and environment effects (nature of solvent*-'3, pH'3-15,temper( I ) Drexhage, K. H. Dye Laser. Schiifer. F. P., Ed.;Springer: Berlin, 1973; p 144. (2) Drexhage, K. H. J . Res. Notl. Bur. Stand., Secr. A 1976, 80A, 421; Laser Focus 1973, 9, 35. (3) Vogel, M.; Rettig, W.; Sens, R.; Drexhage, K . H . Chem. Phys. Lea. 1988. 147,452. (4) Ldpez Arbeloa, F.; Urrecha Aguirresacona, 1.; Epez Arbeloa, 1. Chem. Phys. 1989, 130, 371. (5) Tredwell, C. J.; Osborne, A. D. J . Chem. Soc., Farodoy Trons. 2 1980, 76, 1627. (6) Drake, J. M.; Morse, R. 1.; Steppel, R. N.; Young, D. Chem. Phys. Lerr. 1975, 35, 18 I . (7) Ldpez Arbeloa, F.; Lbpez Arbeloa, T.; Gil Lage, E.; Ldpez Arbeloa, 1.; De Schryver, F. C. J . Phoiochem. Phorobiol., in press. (8) Ldpez Arbeloa, 1.; Rohatgi-Mukherjee, K. K. Chem. Phys. Lerr. 1986, 128, 474. (9) Ldpez Arbeloa, 1.; Rohatgi-Mukherjee, K. K. Chem. Phys. Leu. 1986, 129, 607. (IO) Rosenthal, I.; Peretz, P.; Muszkat, K . A. J . Phys. Chem. 1979, 83, 350.
ature, etc.) are involved. Although these properties have been extensively studied, understanding their influences continues to be controversial due to the difficulty of determining each of them separately. Since rhodamines are used as active media in dye lasers,' one of the most important photophysical characteristics of rhodamines is the fluorescence quantum yield. The rate constants of the internal conversion from the SI excited state of rhodamines increase strongly upon alkylation of the amino g r o u p ~ , ' - ~ leading ,~*'~ to important decreases in the fluorescence lifetimes ( 7 ) and quantum yields (4). Rhodamines with rigid amino groups (rhodamine 101) or rhodamines in frozen solutions show fluorescence quantum yields close to 1
.'"'
( 1 I ) Snare, M. J.; Treolar, F. E.; Ghiggino, K. P.; Thistlethwaite, P. I. J . Phorochem. 1982, 18, 335. (12) Casey, K. G.; Quitevis, E. L.J . Phys. Chem. 1988, 92, 6590; h o c . SPIE-Ini. Soc. Opi. Eng. 1988, 910, 144. (13) Sadkoswki, P. J.; Fleming, G. R. Chem. Phys. Lerr. 1978, 57, 526. (14) Ldpez Arbeloa, I.; Ruiz Ojeda, P. Chem. Phys. Lerr. 1981, 79, 347. (15) Faraggi, M.; Perezt, P.; Rosenthal, 1.; Weinraub, D. Chem. Phys. Lett. 1984, 103, 310. (16) Osborne, A. D.; Winkworth, A. C. Chem. Phys. Lerr. 1982,85,513. Osborne, A. D. J . Chem. Soc., Foradoy Trons. 2 1980, 76, 1638. (17) Kubin, R. F.; Fletcher, A. N. J . Lumin. 1982, 27, 455.
0022-3654191 12095-2203%02.50/0 0 1991 American Chemical Society
2204 The Journal of Physical Chemistry. Vol. 95, No. 6. 1991
LBpez Arbeloa et al.
On the basis of these results, intramolecular rotation of the amino fragments was first proposed as the main pathway for the nonradiative deactivation from the SI excited state of rhodamines.1*2*5Thus, rhodamines in which amino group rotation is allowed (i.e. rhodamine B and Fast Acid Violet 2R) exhibit temperature-dependent fluorescence lifetimes and quantum y i e l d ~ l -related ~ , ~ to the xanthenemamine double-bond character. However, the nonradiative deactivation of rhodamines via a simple intramolecular rotation of the amino groups has recently been q u e s t i ~ n e d . ~ , Experimental ~.~~J~ observations suggest a more complicated internal conversion process in rhodamines: (i) the rate constant of internal conversion, k,,, of rhodamine B does not correlate in a simple way with solvent viscosity, since a k,,.yalue was obtained in water similar to that in ethylene glycol;9 (11) the 7 and 4 values of rhodamines with monoethylamino groups are higher than those of rhodamines with diethylamino group^,^^^ although both the rotation volume and the xanthenezamine double-bond character are smaller in the former rhodamines; (iii) the effect of the carboxyphenyl group, remote from the amino groups, on the internal conversion p r o c e s ~ e s cannot ' ~ ~ ~ ~ be adequately explained. In the last few years, a new pathway for internal conversion in rhodamines and other xanthene dyes3J8via a twisted intramolecular charge-transfer (TICT) has been proposed. A TICT state was first suggested to explain the dual fluorescence observed for p-aminobenzonitrile derivatives.21 The TICT-state formation in rhodamines is characterized by an electron transfer from the amino groups to the xanthene ring followed by a rotation between them, in connection with the aforementioned mechanism. The TICT-state formation therefore depends on the electron donor-acceptor capacities of the partners and on the solvent p ~ l a r i t y . ~ q IThe ~ - ~ TICT-state ~ population competes with the radiative deactivation of the SIexcited state. Since rotational motion is also involved in the TICT-state formation, this process would also depend on the rigidity of the system3 and on the solvent viscosity.3J2J8 The decrease in T and 4 of rhodamines upon alkylation of the amino groups is then explained by the increase of the electrondonating capacity of the amino groups. Some authors3 attribute these differences to the fact that the TlCT state is populated from the SIstate for rhodamines with diethylamino groups, while the TICT state for rhodamines with monoethylamino groups is energetically higher than the SI ~ t a t eand , ~ it is not populated from the SIexcited state. However, fluorescence from the TICT state of rhodamines has not yet been observed, which could be due to a very rapid nonradiative deactivation to the So ground state and/or a forbidden radiative tran~ition.~ An alternative mechanism for internal conversion of rhodamines has recently been c o n ~ i d e r e d . ~ > It ~can - ~ explain the variation of the k,, value for rhodamines with monoethylamino groups (R19 and R6G) in which the TlCT state is not thought to be f ~ r m e d . ~ This mechanism correlates internal conversion with a change in the amino groups from a planar structure
R1B -
xanthene =N+
\
to a pyramidal one xanthene*-
.N*P \
the so-called open-closed umbrella-like m o t i ~ n . ~This , ~ structural change involves disruption of the xanthene-amine double bond to form a xanthene-amine single bond (now the amino groups (18) Vogel, M.; Rettig. W.; Sens, R.; Drexhage, K. H. Chem. Phys. Leu. 1988.147. 461. (19) Grabowski, Z. R. Acra Phys. Pol. 1987,A71, 743. (20) Rettig, W. Angew. Chem., Inr. Ed. Engl. 1986,25,971;Appl. Phys. 1988. ~,194s. 46 -I. . (21) Cazeau Dubroca, C.; Peirigna, A.; Ait Lyazidi. S.; Nouchi, G. Chem. Phys. Lert. 1983.98,51 I . Rotkiewicz, K. Specrrochim. Acta 1986,42A. 575. Yang, Y.; Eisenthal, K. B. J . Chem. Phys. 1982,77,6076. Su,S.G.;Simon, J . D. J . Chem. Phys. 1988,89,908. ~
..
Figure l. Molecular structure of both molecular forms of rhodamine B and of rhodamine 38.
can freely rotate, in analogy to the first mechanism) and a displacement of the rhodamine positive charge from the amino N atom to the xanthene ring (in connection with the TICT mechanism). Although this mechanism could be related to the previous ones, some conceptual differences can be distinguished: (i) not all the amino group rotations cause radiationless deactivations but only those involved in a disruption of the *-electron system in the xantheneamine double bond; (ii) there is a change in the electron density of the T system in the xantheneGamine bond, but not a real charge-transfer state. Consequently, all interactions that stabilize the chromophore positive charge, and therefore the electron flow of the 7r system, would decrease the nonradiative deactivation probability of rhodamines.' In the present work, we study the photophysical properties of rhodamines with diethylamino groups (RB and R3B) in solution. Rhodamine B presents two molecular forms related by protonation of the carboxyphenyl group (Figure 1): the zwitterion form, RB*, with the PhCOO- group and the cationic one, RBH', with the PhCOOH group. Rhodamine 3B, R3B+, is the ethyl ester derivative of RB (with the PhCOOEt group). Comparison of the experimental results for RB', RBH', and R3B+ allows us to determine the effect of the PhCOOR group on the photophysical properties of rhodamines!-7 The effect of amino group alkylation is also studied by comparing the present results to those previously obtained for the related rhodamines with monoethylamino groups, R19 and R6G.' In order to study the influence of the nature of the solvent on the photophysics of rhodamine^,^,^ water, ethanol, and several water/ethanol mixtures are used as solvents. These solvents are chosen because of their varying capabilities of solvating rhodamine dyes. The diethylamino groups are solvated better by ethanol than by water molecules because of the hydrophobic solvation of the amino ethyl substituents by the ethyl part of the alcohol. Thus, ethanol is attracted by the ethylamino fragment of the dye and is a better solvent for rhodamines than ~ a t e r . This ~ . ~is supported by the much higher tendency of rhodamines to aggregate in aqueous than in ethanolic solution^.^^-^^ Water molecules, however, solvate the PhCOO- group better. Therefore water/ ethanol mixtures are ideal media for studying specific solutesolvent interactions in rhodamines. Experimental Section Rhodamines B and 3B were supplied by Kodak (Laser Grade) and used without further purification. The acid and the zwitterion molecular forms of RB were obtained at pH = 2 and pH = 8, respectively, in water (pK, = 3.114)and at pH = 4 and pH = 10, respectively, in ethanol by adding appropriate amounts of NaOH and HC1 to the solutions. Water was doubly distilled, and absolute ethanol (Merck, pro-analysis) was supradried by d i ~ t i l l a t i o n . ~ ~ The dye concentration was always
