The triplet states in lactone form of rhodamine 101

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JOURNALUI

LUMINESCENCE

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Journal of Luminescence 6O&6 I

994) 474 478

The triplet states in lactone form of rhodamine 101 Jerzy Karpiuk Polish .4 (as/cHic of .Ss,enss’s. l,i.s (flute of Phisieal ( heimsire. Foopr:aka 44 52. 1)1—224 U (irons . l’olwul

Abstract An intense phosphorescence of the rhodamine 101 lactone was ascribed to the iwitterion form generated in excited state. The ratio of fluorescence to phosphorescence intensities of the zwitterion 1:3) suggests that both excited states arc produced according to their spin statistical factors. The participation of the triplet state in nonradiative deactivation of the singlet states of rhodamines is postulated.

1. Introduction Rhodamines are extensively used as active media in dye lasers. The rhodamines containing a nonestrifled carboxyiphenyl group (e.g. rhodamine B or rhodamine 101) may participate in equilibria between different molecular forms, three main of

which are lactone (neutral form), zwitterion and cation (ionic forms). The existence of a dye in a delinite molecular form depends mainly on solvent polarity and proticity (pH). The zwitterion form is favoured by polar protic solvents whereas the lactone dominates in nonpolar and polar aprotic media. The cations occur under acidic conditions. Most studies on rhodamines have been carried out on zwitterion and cation forms of these dyes, mainly using the solutions of salts of the dyes in protic solvents. In these media hydrogen bonding with the solvent molecules may play an important role in stabilisation of the ionic forms. One of the main problems in photophysics of rhodamines that still remain unclear is a thermally activated intramolecular deactivation of the singlet states of these dyes. Various mechanisms have been postulated in

order to explain the nature of this non-radiative process. Drexhage [1] suggested that torsional motion of the substituents at amino nitrogen is involved in the deactivation reaction. The hypothesis has subsequently been extended by Vogel et al..

who postulated the transition to non-emissive twisted

intramolecular

charge

transfer

states

(TICT) to be responsible for the non-radiative deactivation [2]. One of the main structural arguments which supported the TICT hypothesis for rhodamine dyes was the lack of the temperature dependence of the fluorescence quantum yield and lifetime for ethyl ester derivative of rhodamine 101. According to the TICT model the rigidly fixed amino group cannot yield the TICT states, because the twisting motion is impossible. Rhodamine 101 is a distinguished representative among the rhodamine dyes. It was synthesized in early seventies and due to its very high fluorescence quantum yield (about 1.0 in EtOH) was a spectacular example supporting the theory postulating that torsional motions of amino group is responsible for thermally activated deactivation of rhodamines. Such a large value indicates also a negligible

0022-231 3/94.$07.0() u~1994 Elsevier Science B.V. All rights reserved .S.S~DI 0022-2313(931 E03 78-B

J. Karpiuk / Journal of Luminescence 60&61 (1994) 474 —4 78

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In this paper we report on the role of triplet states in photophysics of the lactone of rhodamine 101 (LR1O1) as well as on their participation in thermally activated non-radiative deactivation of excited singlet state of the zwitterion of rhodamine 101 generated upon excitation of the lactone form of the dye.

zwitterion 2. Results and discussion N

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I. The molecular formulae,

quantum efficiency of the transition to the triplet state in EtOH. The dye was postulated as a standard of fluorescence quantum yield [3]. Recently we have studied the photophysics of the lactone forms of rhodamines, see Scheme 1 [4,5]. These molecular forms exhibit in the ground state an entirely different electronic structure as compared with that of ionic forms. The lack of an elongated it-electronic system that is characteristic for ionic forms causes the solutions of lactones to be colourless. In nonpolar solvents the lactones emit a single broad luminescence band (L) that comes from highly polar charge transfer state (L~T) produced in an electron transfer reaction in the excited state (dipole moment 25 D). In more polar solvents a second fluorescence band appears (Z) revealing the dissociation of the C—O lactone bond. The excited singlet state of the zwitterion form (Zr) is stabilized by increase of the solvent polarity. Temperature measurements have shown that the excited state of the Z form is produced in all solvents; it is, however, quenched in less polar media. The studies on photophysics of the lactones may therefore deliver substantial information contributing to our understanding of the nature of nonradiative processes in rhodamines.

The synthesis of LR1OI was described elsewhere [5]. Only specially purified, freshly distilled or

dried solvents were used as we found a large effect of protic impurities on the quantum yield of luminescence and the relative intensities of both bands. Fluorescence spectra of LR1O1 are very strongly solvent dependent. At room temperature the compound emits in less polar solvents a single broad L band that shifts significantly to the red with increase of the solvent polarity. In medium polarity solvents a second band appears thus revealing the dissociation of the C—O lactone bond resulting in a zwitterion form (Z) in excited state. Increase in solvent polarity causes gradual disappearance of the short-wave band and the fluorescence spectrum seems to be composed of only one (Z) band. This is accompanied by a strong increase in the observed quantum yield of the long-wave luminescence. The decay curves in L band were monoexponential. The decay curves in Z band in the solvents, where Z fluorescence is displayed with relatively high quantum yield (butyronitrile, pyridine), measured at fluorescence maxima could be fitted with a single decay time. The significant shortening of these decay times with decrease of solvent polarity confirms the connection of the quenching mechanism with the dielectric properties of the surrounding medium [5]. The population efficiency of the zwitterion form in excited state was found very similar in different solvents [5] and equal on average 0.23 ±0.03. The maximal observed luminescence yield of the L form (i.e. not corrected for the population efficiency of the L~Tstate) was 0.034 (in diethyl ether). From these two efficiencies one can conclude that either there is a significant radiationless deactivation of

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wavelength [nm] Fig. 1. l.uminescencespectraofrhodamine 101 lactonein MTHF at'~arioustemperatures:la)at293K(continuouslincl:lbla122~ K {dotted line): (c) at 143 K (dashed line). The intensity ratio of particular spectra m the tigure does not correspond to the ratio of quantum yields. The band at shorter wavelengths (l.} comes from the L*~ state and the band at longer v,avelengths IZ) from the Z* s{ale

the L*~ state or a n o t h e r state is p r o d u c e d in the initial phase of the reaction in excited state. Decreasing the t e m p e r a t u r e of the solvent has an effect on the s p e c t r u m that is a n a l o g o u s to increase of the solvent polarity. In fact, the dielectric constant of the solvent strongly increases with the decrease of the t e m p e r a t u r e and the p o l a r L*v state [source of the L band) is better stabilized. Fig. 1 shows the luminescence spectra of LRI01 in M T H F at r o o m t e m p e r a t u r e and at lower temperatures. Decrease of t e m p e r a t u r e causes a large increase of the intensity of the Z b a n d followed by increase of the decay time of the Z form. T e m p e r a t u r e m e a s u r e m e n t s prove that the zwitterion form is p r o d u c e d in all solvents, it is, however, q u e n c h e d in less p o l a r media. The luminescence emitted from a perfectly glassed M T H F solution of LR101 consists of 3 b a n d s tFig. 2). The b a n d at 455 nm was ascribed to the fluorescence from the L*T state, shifted at this temp e r a t u r e t o w a r d s s h o r t e r wavelengths a c c o r d i n g to the change of the dielectric p r o p e r t i e s of M T H F . The b a n d with m a x i m u m at 559 nm comes from the zwitterion built up on the dissociation in the exci-

ted state. The shift of this m a x i m u m t o w a r d s shorter wavelengths as c o m p a r e d with the liquid solution m a y be c o n n e c t e d with n o n - p l a n a r c o n f o r m a tion of the zwitterion p r o d u c e d in rigid medium. As calculated by a PC M o d e l m o l e c u l a r mechanics p r o g r a m , the c o n f o r m a t i o n of the xanthene part in lactone molecule in the g r o u n d state is slightly bent in c o n t r a s t to that of zwitterion existing in protic media, where it is planar. The b a n d with m a x i m u m at 672 nm was identified as the p h o s p h o r e s c e n c e of the LR101. The excitation s p e c t r u m of the p h o s p h o r e s c e n c e was the same as that of Z fluorescence and m a t c h e d the a b s o r p t i o n s p e c t r u m of the lactone. The occurrence of similar p h o s p h o r e s c e n c e s was discovered for lactones of o t h e r r h o d a m i n e s studied. A question of p r i m a r y i m p o r t a n c e is the c h a r a c t e r of the triplet state being a source of this p h o s p h o r e s c e n c e , i.e. does the p h o s p h o r e s c e n c e come from (i) the lactone charge transfer triplet state, 0il a local triplet state within the lactone molecule or (iiil a triplet state of the Z form? A very large energy difference between the C T fluorescence and the p h o s p h o r e s c e n c e rules out the possibility Ill. The o p t i o n {ii) can be eliminated by c o m p a r i s o n of the p h o s p h o r e s c e n c e s of

J. Karpiuk / Journal of Luminescence 6O&6/ (1994) 474 —4 78

460

500

600 wavelength [mn]

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Fig. 2. Luminescence spectra of rhodamine 101 lactone in MTHF glass at 93 K: (a) total luminescence (continuous line): (b) phosphorescence (dashed line). L, Z and P denote fluorescence from the L~Tstate, fluorescence from the Z~’state, and phosphorescence from the triplet state of the zwitterion form, respectively.

lactone of rhodamine B (LRB) and its analogue differing from LRB by substituting the oxygen atom in the phthalide ring by the nitrogen atom [6]. The latter compound displays also the CT luminescence, does not, however, undergo the dissociation in the excited state and emits an intense, slightly structured phos-

created according to their spin statistical factors. Phosphorescence of LR1O1 in aprotic media mdicates significant stabilization of triplet states under these conditions as compared with solvents yielding possibility of hydrogen bonding. The results described here and in Ref. [5] allow

phorescence with the maximum at 425 nm, which we

us to postulate the following picture of the photo-

ascribe to a local triplet of this molecule. We believe therefore that the observed phosphorescence of LR1OI comes from the triplet state of the Z form. Analogy with the LRB allows for supposition that the emitting triplet state is of (it,ir*) nature [5]. The total luminescence quantum yield of LRIOI in MTHF glass was equal to 1.0 within the error limits. This means that all non-radiative processes are frozen under these conditions and that all species produced in the excited states are envisaged by their luminescence. High quantum yield of the phosphorescence indicates significant participation of the triplet states in the photophysics of LR1O1. Preliminary measurements of transient absorption [6] show that the triplet states are generated with high efficiency also at room temperature. The ratio of fluorescence to phosphorescence intensities of the zwitterion (1:3) in glass allows for supposition that the singlet and triplet states of the Z form are

physics of the LRIO1. The molecule of LRIO1 can be considered as being composed of two mutually perpendicular it-electronic subsystems which are approximately non-interacting in the ground state: the xanthene part and the phthalide part. The mitializing step after absorption of the photon is a rapid electron transfer from the amino group to the phthalide system that yields ion radicals of both parts of the molecule. This ion radical pair can then relax on two routes: (i) vibronic relaxation to the L~Tstate or (ii) charge recombination resulting in the zwitterion form in the excited singlet and triplet states. The independence of the population efficiency of the zwitterion on the solvent might suggest that the partition in two reaction paths occurs beyond the control of the medium, and thus faster than the medium could respond. The partition must be controlled by intramolecular factors, most probably of vibronic nature.

478

.1. Kai~iuk Journal of 1.umisie,scencc 60&O/ ( /994: 474 4’S

The value ~ indicates that partition into singlet and triplet states occurs at the moment of creation of the excited zwitterion form. This will be justified if we assume that the excited Z form is created in a “back” charge transfer reaction from the phthalide to the xanthene part of the molecule. It finds support in the large difference of the dipole moments of the Lc~Tand the Zr states [5]. The discovery of the phosphorescence of the zwittenon form of rhodamine 101 enables us to view in a new way the question of the thermally activated deactivation of the excited singlet states of rho-

damines. Neglecting the role of the triplet state in other studies was somewhat justified because of low quantum yield of intersystem crossing in these compounds observed in protic solvents. The possible source of this stabilization of the zwitterion form against the transition to the triplet state in rhodamines in protic solvents may be connected with lack of significant mixing of singlet and triplet states in planar it-system of the zwitterion and large energy gap between the Z~and the T~(or T7) state. Especially. little population of significant out-of-plane vibrations might for account this effect. Thecase vibrations that can be account taken into in the of rhodamines are those connected with the “pyramidal” out-of-plane motions of the nitrogen atoms or the bending vibrations of the xanthene system caused by folding this part of the molecule on the

symmetry axis of the xanthene part (butterfly like

motion). In the study of rhodamine 101 adsorbed on the surfaces of organic crystals, Kemnitz et al. [7] have observed radiationless deactivation of the singlet state and found that addition of water removes the observed quenching. In view of our results the folding of the xanthene skeleton (butterfly motion) on irregular sites of the crystal surface would contribute to the enhanced spin orbit coupling and facilitate the transition to the triplet state. Water enables the RIOl molecules to create hydrogen bonding and thus stabilizes the zwitterion form both in the excited and in the ground state. The transition to the triplet state may be a natural explanation of the thermally activated radiationless deactivation in rhodamines. Very efficient quenching of the Z form in less polar solvents implies large rate constant for the intersystem crossing. The influence of the polarity on the k 15~.can be explained

by assuming that the strong coupling of the Z~’and the triplet state participating in the radiationless transition is caused by a favourable Franck --Condon factor for intersystem crossing that may result

from small energy gap between these states. The triplet state being active in the thermally activated deactivation may be described as the T 2 state. The increase of the solvent polarity would directly influence the energy gap by either lowering the Z~state or rising up the triplet state and thus stabilizing the zwitterion against the transition to the triplet state. The increase of the polarity causes the Z15 state to approach the L55 state, and connected with relative insensitivity of the transition energy (Z~’ Z51) (small dipole moment change on transition) to the polarity change results in lowering the energy of the Z~state against the L0 state. On the other hand, rising up the triplet level responsible for the radiationless deactivation in more polar medium may be connected with the )n.rc*) nature of this state. Such a nature of the triplet would also explain the absence of the singlet—triplet transitions in rhodamines in protic solvents. The hydrogen bonding mustincon3(n,ic*) state especially the siderably rise up the case when the n orbital is localized on the atom directly participating in the hydrogen bonding. —

Acknowledgements This work was supported in part by KBN Grant 203659101 and by the Fundusz PONT. Fundacja Stefana Batorego is acknowledged for sponsoring part of the travel costs connected with participation of the author at ICL’93. References [I]

K.H. Drexhage. in: Topics in Applied Physics. ed. F.P

Schafer (Springer, Berlin, 1977) p. 143.

[2] M. Vogel, W. Rettig, R. Sens and K.H. Drexhage, (‘hens. Phys Lctt 147 (1988) 461 [3] T. Karstens and K. Kobs. J. Phys. (‘hem. 84(1980( 1871. [4] J. Karpiuk, ZR. Grabowski and F.C. De Schryvcr. Proc.

Indian Acad. Sci. ((Them. Sci.( 104 (i992( 133. [5] J. Karpiuk, ZR. Grabowski and F.(’. De Schryver. J. Phys.

(‘hem., in press. [6] J Karpiuk and Z R Grahowski. in preparation

[7] K. Kemnitz, N. Tamai, I. Yamazaki. N. Nakashima and

K. Yoshihara, J. Phys. (‘hem. 91(1987(1423.