J. Phys. Chem. 1992,96,9817-9820
9817
Laser Flash Photolysis Studies of Novel Xanthene Dye Derivatives Elena Klimtchuk, M. A. J. Rodgers, and D. C. Neckem* Center for Photochemical Sciences.’ Bowling Green State University, Bowling Green, Ohio 43403 (Received: May 13, 1992; In Final Form: August 6, 1992)
The transient phenomena in a laser flash photolysis study of a new series of xanthene dyes in ethanol solution at room temperature are described. The difference between the triplet- and ground-state ahsorption spectra of tetrabromohydrofluorescein(TBHF) or tetraiodohydrofluorescein (TIHF) shows peaks around 360 and 580 nm. Tetrabromohydrocyanofluorescein (TBHCF) and tetraiodohydrocyanofluorescein (TIHCF) demonstrate triplet-triplet absorption with peaks around 340 and 460 nm. The corresponding anion radicals D,‘ which are produced by electron-transfer reactions from aromatic amines, have absorption maxima at 410 nm. Triplet-state lifetimes and electron transfer kinetics are reported. The role of the 9-CN substituent is discussed.
Experimental Section
constant at high laser power. The value at the plateau was used for the calculation of cw The triplet quantum yield was determined from the slope of the curve in the linear part by comparing with the known standard quantum yield of Rose Bengal (0.76).12 Electrochemical measurements were made by using cyclic voltammetry and square-wave ~oltammetry’~ on a Bioanalytical Systems Model 300 A electrochemical analyzer. Square-wave voltammetry measurements were made at a frequency of 15 Hz with a step height of 4 mV and squarewave amplitude of 25 mV. A commercially available carbon electrode (Bioanalytical Systems) was used as the working electrode. A Pt wire constituted the counter electrode. A saturated calomel electrode (SCE) served as the reference electrode. All electrolyte solutions were degassed with dry nitrogen for at least 10 min before experiment. An electrolyte solution of 0.1 M tetra-n-butylammonium perchlorate (TBAP) in EtOH was used to measure the redox potentials of the dyes. The reference standard was 0.1 M TBAP in EtOH.
The dyes, TIHF, TBHF, TIHCF, and TBHCF, were a gift of Dr. Jianmin Shi, who had synthesized them according to the method of Shi and N e ~ k e r s . Their ~ purity was confirmed as greater than 99% by TLC. The electron donors, diphenylamine, N,N-diethylaniline, triethanolamine, and N-methylpyrollidinone, were purchased from Aldrich and distilled or recrystallized before use. The solutions of dyes (10”-10-5 M) in ethanol (Aldrich, HPLC grade) were deaerated by bubbling with argon. All measurements were conducted at ambient temperature, 23 f 2 OC. Ground-state absorption spectra were measured with one of two diode array spectrophotometers (Perkin-Elmer, Lambda Array 3840, and Hewlett-Packard 8451). The laser flash photolysis experiments were camed out by using the second harmonic (532 nm) of a Q-switched Nd:YAG laser (Quantel YG660) with a pulse width of approximately 10 ns. The computer-controlled absorption spectrometer used to detect the optical density changes in the sample after laser excitation was based on the apparatus described by Rodgers.’O Transient species were monitored at 90° with respect to the laser beam by using a 150-W xenon arc lamp, a Spex 1681 (0.22 m) monochromator, and a Hamamatsu R928 photomultiplier tube in the configuration of Beck.” A LeCroy 9450 digital oscilloscope (400 MHz bandpass) was employed to convert the signal to digital form and transfer it to a microcomputer for signal averaging and analysis. Nonlinear regression analysis of the data was performed by using the Marquardt least-squares algorithm. Sample solutions were contained in 1-cm square quartz cuvettes and were bubbled continuously with argon during the experiment. The argon was passed through an oxygen trap (Alltech Associates) prior to bubbling in order to reduce the concentration of oxygen to less than 0.1 ppm. The m and triplet quantum yield were determined by measuring the dependence of the triplet-state absorbance on laser intensity. The value of the optical density at 580 nm corresponding to the dye triplet state grows in and then becomes
Results and Discussion a. 9-H Subetituted Dyes. If one compares the new compounds with the common xanthene dyes, the substitution of H at C-9 (TBHF and TIHF) has only a minor influence on the absorption spectrum. The A, values for eosin (M = 2 carboxyphenyl, X = Br) and erythrosin (M = 2 carboxyphenyl, X = I) are 523 and 532 nm, respectively. Replacing carboxyphenyl with CN, which creates a resemblance to a cyano-substituted quinomethine, mults in a shift of almost 100 nm to the red in every case; see Table I. The effect of the 9-CN group is to allow the formation of a r-electron system highly delocalized throughout the whole molecular structure. The significant “red” shift (about 100 nm in the absorption spectra) is noticed by comparing the spectrum of 9-H derivatives to that of TBHCF and TIHCF, respectively. The inclusion of the CN group in the ?r-electron system of the dye also leads to an increase in the triplet yield as compared with the corresponding compound where no cyano exists. In fact, the quantum yield of triplet state formation increases from 0.87 for TIHF to 0.99 for TIHCF. Irradiation of a deaerated TIHF solution in ethanol gives the transient absorption recorded at 1.O c(s after excitation shown in Figure 1. The transient spectrum of TBHF recorded 1.O after excitation also demonstrated positive absorptions at 360 and 580 nm. By analogy with the behavior of xanthene dyes,6Jel6 these bands can be ascribed to the triplet-triplet absorption of the dye, D. Bleachings in the 430-540-nm region, corresponding to the disappearance of the D ground-state bands, were also observed. These were not observed in air-saturated solutions owing to the high efficiency to oxygen quenching of the dye triplet state. At 360 and 580 nm under relatively low laser power and at lW5 M dye, the triplet decay was a first-order process with 7 = 100 and 25 for TBHF and TIHF, respectively. The triplet lifetime was strongly dependent on the concentration of the dye, indicating the important role of the self-quenching process. Triplet lifetime
Introduction The xanthene dyes have long been used as photoinitiators of polymerization in the visible region of the spectrum, as sources of singlet ~ x y g e n ,and ~ . ~as photosensitizers in certain biological systems: The photochemical properties of the xanthenes, transient phenomena involving a number of these dyes and physicochemical properties of the corresponding free radicals have been studied e~tensively.~-* Recently, several novel xanthene dye derivatives (I) with H and CN groups at the g-position have been synthesized? These derivatives differ from the commercial xanthenes, all of which have substituted carboxyphenyl groups in the 9-position (Table I). In the present work the transient phenomena and the effect of substitution on the photoinduced electron-transfer reactions of these novel xanthene derivatives (D) have been studied.
0022-365419212096-9817$03.00/0 0 1992 American Chemical Society
9818 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
Klimtchuk et al.
TABLE I: Properties of Novel Xantbeoe Derivatives H
M
x
H
x I
M = H, CN; X = Br, I
TBHF
TIHF
M
H
X h,(EtOH), nm ( s ) ~ Xfl(EtOH),nm (9,nQ9 X,,(EtOH), nm (77 K)9 9f19 4,
Br
H I
530 (39 300) 539 (2.33) 660 0.52 0.25
536 (91 200) 548 (0.65) 676 0.13 0.87
TBHCF CN
TIHCF CN
Br 626 (51 400) 638 (1.45) 755 0.20 0.53
I 638 (80000) 654 (0.69) 777 0.02 0.99
TABLE 11: Extlnctioa Coefficients of tbe Triplet State and Radical Anions of Dyes and Triplet Decay Rate Constants TBHF TIHF TBHCF TIHCF en, M-l cm-I 2.4 X lo3 (580 nm) 9.0 X 10' (580 nm) 4.4 x 10' (340 nm) 6.3 X lo3 (340 nm) cR-., M-I cm-l 1.22 X lo4 (410 nm) 1.45 X lo4 (410 nm) 9.65 X 10' (410 nm) 1.7 X 10' (410 nm) k2, s-' 0.9 x lol 2.0 x 103 2.4 X 10' 1.5 x 105 1.7 x 109 1.2 x 10'0 kn,M-' S-' k,, M-I s-I 3.7 x 108 1.4 x 109 2.0 x 108 1.0 x 109
I
I
400
500
600
4.01
Wavelength (nm)
A
1 480 360
320 1
7
440
400
Wavelength (nm)
Figure 2. Transient absorption spectra recorded from a M TIHF in EtOH with delays ( 0 ) 0.9, (0) 11.1, (M) 26.1, and (0) 43.5 p.
3 0.1
'2
-e; 3
a0"
0.0
300
500
600
Wavelength (nm)
Figure 1. Ground-state absorption (a) and T-T absorption (b) spectra of TIHF in EtOH taken 1.0 ps after laser excitation.
also depends on triplet concentration. At high laser dose and low dye concentration the triplet decay trace included a second-order component that could be attributed to triplet-triplet annihilation. Using the measured triplet extinction coefficients (see below) allowed estimation of the rate constants of T-T annihilation (km) (Table 11). k2 was determined from the 0 dye concentration intersection of the plot of the inverse of triplet lifetime versus dye concentration. The triplet decay at 580 nm is accompanied by growth in the absorption at 410 nm (Figure 2). The nature of the growing absorption can be understood from the following experiments. The triplet state was found to be quenched by several electron donors, mainly aromatic amines. Figure 3, for example, shows the spectrum observed after quenching of the TIHF triplet by triethanolamine. Excellent agreement between the bimolecular rate constants of triplet decay at 580 nm and radical growth at 410
350
390
430
470
Wavelength (nm)
Figure 3. Absorption spectrum obtained from M TIHF in EtOH in the presence of lo-' M triethanolamine. Recorded 12 ps after laser
excitation.
nm was obtained for all dyes. Figure 4 demonstrates this for TIHCF (see below). The peak position at 410 nm is in excellent agreement with that assigned to the dye anion radical (D*-) of the other xanthene dye^.*^.''-^^ We conclude that the reaction between 3D*and triethanolamine is an electron-transfer process
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9819
Studies of Xanthene Dye Derivatives
0
2
4
8
6
'
0
7
1
i o 1 2
[Diphenylamine].I d , M
Figwe 4. Kinetic correspondence of triplet decay ( 0 )and radical growth (m) as a function of donor (diphenylamine) concentration (TIHCF).
\ 320
400
]
480
Wavelength (run)
Figure 6. Absorption spectrum obtained from M TIHCF in EtOH in the presence of lo-' M diphenylamine. Recorded 2.2 after laser excitation.
500
600
700
Wavelength (rim)
320
400
480
Wavelength (nm)
Figure 5. Absorption (a) and T-T absorption (b) spectra (1.0 ps) of TIHCF in EtOH.
that results in reduction of the dye. One canonical structure of the single-electron-transfer reduced species is shown (11). U
M
H
I1
the following processes (eqs 1-4) could be anticipated in the cases of TIHF and TBHF (D): D + ID* 3D* (1)
On the basis of reactions of other xanthene
--
3D* 3D* + D 3D*
+ ,D*
D
products products
(2) (3) (4)
Reactions 3 and 4 correspond to triplet quenching by dye ground state (kq = k,) and triplet-triplet annihilation, which may be an
energy-transfer or an electron-transfer process (km = k4). We were unable to observe the absorption spectrum of the dye cation radical (D'+) because of the intense ground-state absorption in the area around 450 nm where the absorption of xanthene dye cation radicals typically occurs. b. 9-CN Subetituted Dyes. The dyes containing the CN group at C-9 position show absorption spectra that have a strong red shift compared to that of the other xanthenes, Figure 5a. The two CN-substituted dyes, TBHCF and TIHCF, demonstrate very similar flash photolysis behavior. Figure 5b shows the transient absorption spectrum obtained immediately after flashing a deaerated 1 X lC5M solution at 532 nm. The transient absorption bands at 340 and 450 nm decay following an exponential law with T = 35 (TBHCF) and 7 ps (TIHCF), and the same lifetimes are obtained for the recovery of the ground-state absorptions monitored at 626 and 638 nm for TBHCF and TIHCF, respectively. Since these intermediates could be quenched by reaction with electron donors (see below), the depicted signals were attributed to the dye triplet states. Intense ground-state bleaching signals masked transient absorptions in the region around 600 nm; therefore we used 340 nm to monitor the triplet decay rate constants of TBHCF and TIHCF. Triplet decay was a first-order process over the dye concentration range 2 X 10"-2 X M and was independent of laser intensity. Thus, we observed no triplet-triplet annihilation processes with the 9-CN substituted dyes. This may be due to the insufficiency of the absorbed energy at 582 nm for the 9-CNcontaining dyes, since their optical densities at 532 nm are much less than for 9-H dyes at the same concentration. The upper dye concentrations were limited to 20 pM in order to avoid the aggregation processes, which are well known for xanthene dyes.*O This same reason, coupled with the fact that the intrinsic 3D* lifetimes of the 9-CN dyes were relatively short, precluded significant ground-state concentration effects on the triplet lifetime. Furthermore the ground-state bleaching signal recovered the original baseline, indicating no long-lived product from minor reactions was formed. The absence of reduction properties for these dyes was confirmed by cyclic voltammetry and squarewave voltammetry, which showed no peaks corresponding to electrochemical oxidation of the dyes. Flashing of a solution containing 1 X M TBHCF (or TIHCF) and 1 X lo-' M diphenylamine (QH) showed rapid loss of the TI absorption with concomitant production of another transient absorption (Figure 6), which was attributed to the anion radical (P-). Figure 4 demonstrates the correspondence of triplet decay at 340 nm and growth of absorption of the radical anion at 410 nm. It was difficult to observe the characteristic absorption due to the oxidized product, the diphenylamine cation radical (A670 nm)?' because there is still a 'tail" of the dye ground-state bleaching spectrum in this region, but we have indirect evidence
-
9820
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
TABLE In: Oueacbiag of Dye Triplet States by N,"-Metbylrniline TBHF
TIHF
TBHCF
TIHCF
E(D'-ID),
-0.95
(V (=E)) AG, eV k,, M-1 s-I
-0.36 -0.28 -0.54 -0.46 1.5 x 107 2.4 x 107 1.5 x 109 8.9 x 108
-0.99
-0.53
-0.57
of this process. The growth lifetime for the transient at 410 nm is the same as for another transient, that at 670-690 nm. We conclude that the quenching of the triplet by QH is due to electron transfer from QH to the triplet of the dye with production of semireduced dye (DO-) and semioxidized amine (QH"): 3D*
+
OH
-
3.1[D*-, OH-']
D.-
D
+ OH
(5)
i
+ OH*+
Our study of triplet decay kinetics has demonstrated that under our conditions this is the only reaction that serves as the decay channel of the excited dye molecule 3D* and it is the only way to generate Do- for the 9-CN dyes. The free energy change AG (Table 111) for the electron-transfer process in the excited state was calculated from the Rehm-Weller equation22 AG = E,,(QH/QH'+)
- Erd(D'-/D) - Z e 2 / t a - E,,,
(6)
where E,,(QH/QH'+) is the oxidation potential of the electron donor (amine), E,d(D'-/D) is the reduction potential of the electron acceptor (dyes), and E,,, is the triplet excitation energy. The reduction potentials of the dyes were determined versus the standard calomel electrode (SCE) in ethanol at 298 K by a square-wave voltammetry method (see Experimental Section). These are listed in Table 111. The oxidation potential of the amine is +0.7 eV (versus SCE).23 The Coulombic energy -Ze2/ta is considered to be negligible compared to the overall magnitude of the AG in present systems. For all of the dyes the triplet decay rate constant increased on the addition of donor. The observed fmt-order rate constants were linear with respect to quencher concentration. Bimolecular rate constants for the electron-transfer process are collected in Table 111. As could be expected the rate constant of triplet quenching is higher for the CN-substituted dyes (Table 111) due to the strong electron-acceptor properties of CN group that lead to charge separation in the dye molecule and promote electron transfer from the donor. The formed dye anion radicals can participate in recombination reactions or be protonated by protons released from the amine cation radical with bleaching of the dye. The latter has been observed in a study of dye bleaching under steady-state photolysis condition^.^^ The radical decay for H-substituted dyes (TBHF and TIHF) is a second-order process under a wide range of donor concentrations ( l p - 5 X lW2 M), which indicates that the reaction is between radicals D' and QH'+ only. On the contrary, the decay kinetics of the anion radicals of the 9-CN dyes (TBHCF and TIHCF) depend on donor concentration. At low concentrations
Klimtchuk et al. of amine (
