8516
--
J. Phys. Chem. 1995, 99, 8516-8518
Electronic Relaxation in a Series of Cyanine Dyes: Evidence for Electronic and Steric Control of the Rotational Rate Sean Murphy and Gary B. Schuster*3'
Department of Chemistry, Roger Adams Laboratory, University of Illinois, Urbana, Illinois 61801 Received: January 6, 1995; In Final Form: March 14, 1995@
The photophysical properties of a series of substituted indocarbocyanine dyes was examined by fluorescence and time-resolved absorption spectroscopy. The excited singlet-state lifetimes vary with the identity of the substitutent. The lifetime is controlled primarily by rotation about a carbon-carbon bond. The substituents affect this process in several ways.
Introduction In connection with our investigations of intra-ion pair electron transfer between tetraaryl borates and cationic 5,5'-substituted1,3,3,1',3',3'-hexamethylindocarbocyanines (X-Cy), we prepared the series of dyes shown in Chart 1. Cyanine dyes are wellknown as photosensitizers' and are used extensively as saturable absorbers for lasers.* Their synthesis from para-substituted anilines allows a wide variety of structures with groups in the 5 and 5' position^.^ In the course of our studies, we measured the lifetimes and fluorescence quantum yields for this series and found these values to vary systematically with the nature of the substituent. We previously reported a detailed examination of the deactivation processes for the electronically excited cyanine with X = H.4 The main decay routes for the singlet state are fluorescence (kfl), and a torsional rotational about one of the central carbon-carbon bonds (krOt). This torsion presumably gives a perpendicular state that can go to the mono cis-cyanine (Cy+& shown in Chart 1, or return to the original, all-trans cyanine. Typically, relaxation via rotation accounts for ca. 90% of the deactivation of the excited cyanine at room temperature. Therefore, this is the most important process controlling the lifetime of the dye. Intersystem crossing (kist) in cyanine dyes normally contributes insignificantly to deactivation of the singlet state in fluid solution unless aided by heavy atoms either covalently5 or ionically6 bonded to the dye. The radiative and nonradiative relaxation reactions of cyanines are outlined in Scheme 1.
'.' T .-0
e
0.8
-t 0.6 1 = 0.4
!
p
0.2
0.0 400
450
550
500
600
650
700
Wavelength (nm)
Figure 1. Absorption and emission spectra of H-Cy hexafluorphosphate in benzene.
CHART 1: Series of Substituted Cyanines
-+
N 2
I
All Trans Cyanine
1.
+
Results and Discussion The substituents on the 5 and 5' positions of the cyanines influence the electronic properties of the dye. Typical absorption and emission spectra are shown in Figure 1. The singlet energy of the dye (Em) is taken as the point where the normalized absorption and emission spectra overlap. Generally, the shape of these spectra are independent of the substituent, but their maxima are shifted. The data, collected in Table 1, show that the maxima for the absorption and emission spectra of the cyanine dyes as their PF6- salts in benzene solution do not follow a simple trend that depends on the electron withdrawing power of the substituents (the Hammett constant, up,Table 1). Compared with X = H, the trifluoromethyl group has a very small shift to higher energy and the methyl and 'Current address: School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332. @Abstractpublished in Advance ACS Abstracts, May 1, 1995.
0022-3654/95/2099-85 16$09.00/0
Mono-Cis Cyanine X = OMe, Me, H, CF,, NO,, SO,CF,
SCHEME 1: Photophysics of Cyanine Dyes C y ' h v
[CY']'
kfl
Cy'+ hv
[CY+13
trifluoromethylsulfone groups have moderate shifts to lower energy. It is only the methoxy- and nitro-substituted dyes that show large low-energy shifts. It appears that the singlet energies, as well as the absorption maxima of these dyes, are 0 1995 American Chemical Society
Electronic Relaxation in a Series of Cyanine Dyes
J. Phys. Chem., Vol. 99, No. 21, 1995 8517
TABLE 1: Photophysical Data for Cyanines X Me0 Me
H CF3 NO2 CF3S02
(nmY 583 570 558 556
labs
(-0.28) (-0.14) (0.00) (0.53) (0.81) (0.93)
I,, (nm)
EOO(eV)
600 583 572 570
2.10 2.15 2.20 2.21
(PSIb 310 300 250 290
594 5 80
2.1 1 2.16
710‘ 510
581 568
5
@fl
(%I 3.3 4.7 4.7 6.2 13‘ 10
kn (s-’) 1.1 x 108 1.6 x lo8 1.9 x lo8 2.1 x 108
krot
3.1 3.2 3.8 3.2
(s-’) x 109
x 109 x 109 x 109
c
c,d
1.8 x 108 2.0 x 108
0.7 x 109 1.8 x 109
All values measured in benzene with PF6 counterion unless otherwise noted. The estimated errors for the lifetimes and quantum yields are &lo% and for the rate constants &20%. Measured with dl-10-camphor sulfonate anion in benzene. 5 x lo8 s-] has been subtracted from the nonradiative rate to account for intersystem crossing (see text).
TABLE 2: Results from Semiempirical Calculations on X-Cy+ X
% change Cs-C9 bond order
% change C2-C8 bond order
c 2 - C ~ bond length
valence electrons on C9
OMe Me H“ CF3 NO2 S02CF3
f0.15 f0.07 0.00 11.3661 -0.29 -0.44 -0.66
-0.52 -0.22 0.00 r 1.3421 +0.82 f1.2 f1.9
1.3985 1.3981 1.3978 1.3968 1.3962 1.3952
3.877 3.876 3.875 3.870 3.868 3.863
a
Number in brackets is the calculated bond order.
shifted by the n-electron character of the substituents. Those substitutents with extended n-conjugating systems have reduced singlet energies. The substitutents on the dye also affect their excited singlet lifetimes (ts)and fluorescence quantum yields (Qfl). The lifetimes were determined in benzene solution by time-resolved absorption spectroscopy following excitation with an 18 ps pulse at 532 nm. The decay of the singlet-singlet absorption and the recovery of the absorption for the ground-state all-trans isomer follow the same single exponential kinetic law. The data are collected in Table 1. In most cases all but ca. 5% of the trans form of the cyanine is reformed within 10 ns after the laser pulse. The unrecovered ground-state absorption is attributed to formation of the mono-cis cyanine dye, which is observed as a relatively long-lived transient in the absorption spectrum. The NO2-substituted cyanine is an e ~ c e p t i o n .The ~ time-resolved absorption experiment shows that ca. 40% of the trans NO2-substituted cyanine is not re-formed within 10 ns, and no absorption bands characteristic of the mono-cis isomer are observed. These observations are attributed to an increased rate of intersystem crossing to the triplet in this case. Nitro groups are well known enhancers of triplet formation. From the quantum yield of triplet formation we estimate that kist = 5 x lo8 s-I for this compound. The zs and @fl of the cyanine dyes are determined in large part by the fluorescence and rotational rate constants (kfl and krot,respectively). As is seen from the data in Table 1, kfl decreases (compared with X = H) when there are electron donating substituents on the dye, and are the same, within experimental error, for electron-withdrawing substituents. We performed semiempirical calculations using an AM1 Hamiltonian to examine the effect of substitutents on the bondorders of ground state cyanine dyes. The results are listed in Table 2. According to the calculation, the bond order, bond length, and valence electron distribution change systematically with the substituent. As the substituent becomes more electron withdrawing, the C S - C ~bond order decreases while the C2Cg bond order is increased. Likewise, the C2-CS bond length becomes shorter, and the valence electron density on C9 decrease due to greater positive charge on that atom. A key experimental observation from the examination of these cyanine dyes is that all substituents increase their singlet lifetimes. The primary reason for the increase in lifetime is a
d
d
CY
Figure 2. Resonance forms of cyanine dyes. decrease in the rotational rate constant. Our previous examination of the effect of solvent viscosity on the lifetime of the cyanine dye with X = H reveals that one component of this effect is due to the greater size of the substituted dyes. Rotational relaxation requires movement of the dye through the solution. The “larger” the rotating group, the more this process will be retarded by the solvent. Thus we attribute part of the change in krot to this simple steric effect. Clearly, however, krot is not controlled only by steric effects of the dye. The dramatic decrease in rotational relaxation for N02- and CF$302-substituted cyanines cannot be explained fully by the increase in size of the substituent. A second effect must be operating which we attribute to the strong electron-withdrawing power of these groups. This affects bond localization on the central carbon-carbon bonds of cyanine. Four possible resonance structures for the cyanine are shown in Figure 2 . The left-hand structures should be predominant, but for strongly electron-withdrawing groups, the resonance forms on the righthand side will have greater contribution. Since the Cz-Cg bond acquires more double-bond character, it will hinder the rotation and thus increase the lifetime, as observed. There may be a third, experimental, factor controlling the relaxation rate in the N02-substitued cyanine. In this case k,, is significantly lower than for the other dyes even though it is only the second strongest electron-withdrawing group and not as bulky as the trifluoromethyl sulfone group. It is possible that the counterion plays a role in controlling the lifetime of this dye. In this case the PF6- salt is insufficiently soluble in benzene for lifetime measurements and was therefore replaced by the considerably larger camphor sulfonate anion. We have
Murphy and Schuster
8518 J. Phys. Chem., Vol. 99, No. 21, 1995 previously observed an increase in lifetime for cyanines with some large tetraarylborate counter ion^.^^,^
Conclusion The spectroscopic and photophysical properties of indocarbocyanine dyes in solution are very sensitive to the nature of substituents on the aryl rings. This appears primarily to be a consequence of two factors. First, the excited-state lifetime is controlled mainly by rotation about a carbon-carbon double bond. In solution, larger substitutents sweep out a greater volume and inhibit this process. Second, molecular orbital calculations reveal that the electronic properties of the substituent affect the carbon-carbon bond order in a systematic way. These findings reveal that it is possible to adjust the properties of excited cyanine dyes by adjustment of their substituents.
Experimental Section General Methods. Absorption spectra were recorded with a Varian Cary 1E W - v i s spectrophotometer in quartz cuvettes with 1 cm path length. Fluorescence spectra of cyanine borates in benzene solution were recorded with a Spex Fluorolog F1 11. Quantum yields for the fluorescence of cyanine borates were determined on freshly prepared solutions by comparison with the fluorescence of cyanine hexafluorophosphate in benzene solution (a = 0.047). The synthesis of the cyanine dyes described in this work is reported el~ewhere.~ Excited-State Lifetimes. The excited-state lifetimes of the cyanines were measured as the hexafluorophosphate salt in benzene solution. The exception is nitrocyanine, which is very insoluble as the hexafluorophosphate salt so a dl- 10-camphor
sulfonate anion was used. The transient absorption of the cyanine was monitored with a neodymium:YAG laser.g The change in optical density data (AOD) showed excited state absorption, bleaching recovery and stimulated emission. These intensities vs time fit a single exponential, and the lifetimes were wavelength independent.
References and Notes (1) Chaterjee, S.; Davis, P. D.; Gottschalk, P. D.; Kurz, M. E.; Sauerwein, B.; Yang, X.; Schuster, G. B. J. Am. Chem. SOC.1990, 112, 6329. Strumer, D. M. Synthesis and Properties of Cyanine and Related Dyes. Special Topics in Heterocyclic Chemistry; Wissberger, A,, Taylor, E. C., Eds.; John Wiley and Sons: New York, 1977. (2) Snavely, B. B. Proc. lnstrum. E. E. E. 1969, 57, 1374. (3) The synthesis is described in: Murphy, S.; Yang, X.; Schuster, G. B. J. Org. Chem., submitted for publication. (4) (a) Sauenvein, B., Murphy, S.; Schuster, G. B. J. Am. Chem. SOC. 1992, 114, 7920-7922. (b) Murphy, S.; Sauerwein, B.; Drickamer, H. G.; Schuster, G. B. J. Phys. Chem., in press. (5) Kuz’min, V. A,; Darmanyan, A. P.; Shirokova, V. I.; Al’perovich, M. A.; Levkoev, I. I. Izv. Akad. Nauk SSSR, Ser. Khim., Engl. Transl. 1978, 501. (6) Sauenvein, B.; Schuster, G. B. J. Am. Chem. SOC.1991, 95, 1903. (7) A complication with the NOz-substituted Cy is that the hexafluorophosphate salt is extremely insoluble in benzene, and thus only the absorption and emission spectra could be measured with this salt. The hexafluorophosphate was exchanged for 10-camphorsulfonate in order to make this dye soluble in benzene, and this salt was used for the time-resolved spectral measurements. (8) Yang, X.; Zaitsev, A,; Sauerwein, B.; Murphy, S.; Schuster, G. B. J. Am. Chem. SOC.1992, 114, 793-794. (9) The picosecond spectrometer has been described previously: Zhu, Y.; Kofed, R. S,; Devadoss, C.; Shapley, J. R.; Schuster, G. B. lnorg. Chem, 1992, 31, 3505. Jp950112Q
