Fluorescence lifetime and electron transfer in cyclobutenediylic dyes

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16 September 1988

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COMMENT FLUORESCENCE LIFETIME AND ELECTRON TRANSFER IN CYCLOBUTENEDIYLIC DYES Rock-Yee LAW Xerox Webster Research Center, 800 Phillips Road, 114/39D, Webster, NY 14S80, USA Received 28 April 1988

The fluorescence spectra of “cyclobutenediylic dyes”, DMA and DEAF, reported recently by Rehak and Israel were found to be different from those observed in our laboratory. The detailed procedures for which multiple fluorescence emission bands were recorded in the solutions of these compounds are reported and the probable cause of the discrepancy is discussed.

1. Introduction Squaraines are 1,3-disubstituted products synthesized by condensation of one equivalent of squaric acid with two equivalents of amine. Because of the unique electronic arrangement in these compounds, the nomenclature of them had not been systematic; for example, they have been named as cyclotrimethine dyes [ 111, substituted 3-oxo-l-cyclobutenolates [ 21, cyclobutenediylium dyes [ 3 1, cyclobutenediylic dyes [ 41, etc. In 1981, Schmidt proposed the name squaraine for this class of compounds [ 5 1. We find the Schmidt nomenclature system very systematic and compounds of a variety of substitution, either at the nitrogen or in the phenyl ring can be named unambiguously. This system has been used in all our publications in this area [ 6-101. Recently, Rehak and Israel reported a study entitled “Fluorescence lifetime and electron transfer in cyclobutenediylic dyes” [ 41. The fluorescence spectral data of four squaraines, namely DMA, DEAF, DBA and DBT were reported. Single emission bands with high fluorescence quantum yields were observed. Although the high er values are consistent with our findings, the single emission bands recorded for DMA and DEAF in different solvents are different from the multiple emission bands observed in our laboratory [ 6,9,10]. The aim of this Comment is to report the detailed conditions used in our laboratory to record the multiple emission bands of

squaraines and to discuss the probable cause of the discrepancy.

RI

RZ

CH3

H

DEAF QHg

OH

DMA

DS*

CaH5CH3

H

DST

C6HsCH3

CH3

2. Instrumentation and techniques Absorption spectra were recorded with a Cary 17 spectrophotometer. Fluorescence spectra were taken on a Perkin-Elmer MPF-66 fluorescence spectrophotometer, which was interfaced with a professional computer, model 7700 from Perkin-Elmer. A 1,1,2-trichloroethane solution of tetra-tert-butyl metal free phthalocyanine ( z 1.2~ 1O-3M ) was used in place of the RhB 101 quantum counter solution so that spectral responses ranging from 500 to 750 nm could be corrected. Curves a and b of fig. 1 show that excitation correction curve and emission correction curve respectively obtained from the phthal-

0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )

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16 September 1988

WAlYELErKn L”nl,

Fig. 1. Excitation correction, emission correction and ex/em test curves obtained from a tetra-tert-butyl metal free phthalocyanine quantum counter solution.

ocyanine quantum counter solution. After both excitation and emission correction factors were stored, the spectral correction factors between 500 to 750 nm were tested (curve c, fig. 1 ), our result shows that spectral responses between 510 and 750 nm were corrected with a signal-to-noise ratio better than + 1.5% between 5 10 and 7 10 nm and about ? 3% between 7 10 and 7 50 nm. The correction procedures used in this work were identical to those described by Duggan, Dicesare and Williams [ 111. Although the range of correction is narrower in this work as compared to that reported by Duggan and co-workers, in which a carbocyanine dye quantum counter solution was used, we found our phthalocyanine quantum counter solution has a lower signal-to-noise ratio in the spectral correction factors and highly stable for repetitive uses. In fact, the current quantum counter solution has been used for &2 years and no apparent deterioration in performance is evident. Fluorescence quantum yields were determined in a corrected mode by comparing with the emission of sulforhodamine 101 inethanol (&=l.O [12]).Arefractive index correction was made for each solution [131.

550

so0

650 WAVELENGTHlnm)

700

3

Fig. 2. Corrected fluorescence excitation and emission spectra of DMAinCHlCll([DMA]z3X10-‘M).

at & 648,657 and z 700 nm are observed. The spectral characteristics of DMA in methylene chloride are identical with those reported earlier where a MPF44A fluorescence spectrophotometer equipped with a DSCU-2 unit was used [ 91. The three bands are designated to the a, p, and y band as shown in fig. 2. From structure-property relationships, solvent effect and temperature effect studies, as well as results obtained from mixed solvent experiments, we showed previously that the a band is the emission from the Franck-Condon-excited state of squaraine, the p band is the emission of the excited state of the solute-solvent complex and the y band is the emission of a relaxed excited state. Details of the spectral assignment has been reported earlier [ 9 1. Fig. 3 shows the corrected fluorescence excitation and emission spectra of DMA in chloroform. Again, multiple emission bands are observed. The emission spectrum is do,minant by the a band and the change

3. Comparison of results and discussion Fig. 2 shows the corrected fluorescence excitation and emission spectra of DMA in methylene chloride. The excitation spectrum was found to be independent of monitoring wavelengths and was identical with the absorption spectrum. Three emission bands 358

Fig. 3. Corrected fluorescence excitation and emission spectra of DMAinCHCIS([DMA]=3~10-‘M).

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methylene chloride solution of DMA was chosen for our initial study because of the similar emission intensity of the a and the p band. The fluorescence decay curve at z 640 nm was found to fit well with a biexponential decay function with a x2 value of 1.2 [ 151. The lifetimes of the excited states measured are 4 and 0.7 ns. From the absorption spectrum of DMA in methylene chloride, the radiative decay rate ofDMAiscalculatedtobe1.83x10ss-1 [16].Since the &of DMA in methylene chloride is 0.65, the calculated lifetime is 3.5 ns. We accordingly assigned the 4 ns emission to excited state of DMA and the 0.7 ns emission to the excited state of the solute-solvent complex. Recently, we studied the ground state conformation of the solute-solvent complex of squaraine by proton NMR spectroscopy [ 171. Results generally show that the planarity of squaraine is distorted in the complexation process. Since rotation of the carbon-carbon bond between the phenyl ring and the four-membered ring has been shown to be the major radiationless decay process( es) of the excited state of squaraine [ 91, and rotational de-excitation is known to be facilitated by geometrical distortion [ 181, the assignment of the short-lived species to the excited state of the solute-solvent complex seems reasonable. Further fluorescence decay measurements at longer wavelengths could not be performed however, due to the sharp drop of the photoresponse of the PMT tube. An improvement in instrumentation is needed for future efforts. The lifetime of DMA in chloroform was measured to be 6.1 ns by Rehak and Israel [ 41, From the calculated radiative decay rate and the quantum yield datum in this work, the calculated lifetime is x4.4

in emission composition from methylene chloride to chloroform is attributable to solvent effect. Identical trend was observed earlier in our solvent effect study on the fluorescence emission of a model squaraine in a large variety of organic solvents [9]. The observation of multiple emission bands in this work is contrasting to that reported by Rehak and Israel where only a single emission band was observed [ 41. A comparison of the spectral data of DMA is given in table 1. We would like to point out that, in addition to the difference observed in the emission spectra, the I,,, of DMA in chloroform also appears to be different, by > 15 nm as compared to our values and the value reported by Sprenger and Ziegenbein

PI. In Rehak and Israel’s paper, they indicated that their emission spectra were corrected by a Kortum Vth 8 photothermoelement. Correction factors obtained by a photothermoelement are usually corrected in either 5 or 10 nm intervals and are different from the continuous correction (every 1 nm) obtained in this work by using a phthalocyanine quantum counter solution. It thus appears that the single emission band recorded by Rehak and Israel is the result of the close overlapping of the multiple emission bands and the inadequate resolution of the spectral correction factors in their instrument #I, Further support of the multiple emission of DMA comes from recent lifetime measurements. A diluted #’A single emission band was also recorded by Dr. F. Winnik of Xerox Research Centre of Canada from a DMA methylene chloride solution on a Spex Fluorolog 212 fluorescence spectrometer, which was calibrated by a standard NBS Tungsten lampatevery5nm[14]. Table I Absorption and fluorescence emission spectral data of DMA

_

Solvent

A,,, (nm) loge (cm-i M-i) & (nm)

4f ref.

CHzClz

CHCl,

CHCl,

CHC&

628 5.49 648 657 N 700 0.65

624 5.48 646 656 (shoulder) ~695 0.80 this work

643.5 5.45 674

6.28

191

0.68 [41

121

359

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16 September 1988

helpful discussion, Mr. Shu Huelin and Professor M.A. Winnik of the University of Toronto for providing the lifetime data of DMA.

References

WA”ELENGTHhrn,

Fig. 4. Correction fluorescence emission spectra of DEAF in different solvents ([DEAF] %3X IO-‘M).

ns. Without clearly knowing the wavelength in which the decay was measured, it is hard to access whether the difference between the experimental value and the calculated value is genuine or the lifetime of other species in solution was being determined. The lifetimes of DEAF were reported to be in the range of 2.4 to 4.6 ns, depending on the solvent used [ 41. However, as seen in fig. 4, DEAF exhibits multiple emission bands in various solvents also. In view of this complication, the lifetime data would not be meaningful, unless detailed experimental conditions such as wavelength studied, quality of the mono-exponential, etc., were reported. Acknowledgement I thank Dr. F. Winnik of Xerox Research Centre of Canada for communication of her results and

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[ 1 ] A. Treibs and K. Jacob, Angew. Chem. Intern. Ed. Engl. 4 ( 1965) 694. [2] H.E. Sprenger and W. Ziegenbein, Angew. Chem. Intern. Ed. En& 5 (1966) 894. [ 31 H.E. Sprenger and W. Ziegenbein, Angew. Chem. Intern, Ed. Engl. 6 (1967) 553; 7 (1968) 530. [4] V. Rehak and G. Israel, Chem. Phys. Letters 132 (1986) 236. [ 51A.H. Schmidt, in: Oxocarbons, ed. R. West (Academic Press, New York, 1980) [ 6 I K.Y. Law, Am. Chem. Sot. Polymer Preprint 27 (2) ( 1986) 312. [7] KY. Law andF.C. Bailey, Can. J. Chem. 64 (1986) 2267. [8] KY. Law and F.C. Bailey, J. Imaging Sci. 31 (1987) 172. [9] KY. Law, J. Phys. Chem. 91 (1987) 5184. [lo] KY. Law, Am. Chem. Sot. Polymer Preprint 28 (2) (1987) 74. [ 1I ] J.X. Duggan, J. Dicesare and J.F. Williams, ASTM Spec. Techn. Publ. 822 (1983) 112, and references therein. [ 121 T. Karstens and K. Kobs, J. Phys. Chem. 84 (1980) 1871. [ 131 J.N. Demas and G.A. Grosby, J. Phys. Chem. 75 (1975) 991. [ 141 F. Winnik, private communication ( 1987) [ 151 D.V. O’Connor and D. Phillips, Time-correlated single photon counting (Academic Press, New York, 1984) pp. 18Off. [ 161 W.R. Ware and B.A. Baldwin, J. Chem. Phys. 40 (1964) 1703. [ 171 K.Y. Law, submitted for publication. [ 181W. Rettig and R. Gleiter, J. Phys. Chem. 89 ( 1985) 4676.