134
J. Phys. Chem. 1994,98,134-I31
Photophysics of Dithiacarbocyanine Dyes: Subnanosecond Relaxation Dynamics of a Dithia-2,2‘-carbocyanineDye and Its 9-Methyl-SubstitutedMeso Analog+ Nick Serpone’ Laboratory of Pure and Applied Studies in Catalysis, Environment and Materials, Department of Chemistry and Biochemistry, Concordia University, Montreal, Canada H3G IM8
M. R. V. Sahyun Graphics Research Laboratory, 3M Center, St. Paul, Minnesota 55144 Received: July 19, 1993; In Final Form: November I , 1993”
The subnanosecond relaxation dynamics of two dithiacarbocyanine dyes, 3,3’-didodecyldithia-2,2’-carbocyanine bromide (dye I) and 9-methyl-3,3’-dihexadecyldithia-2,2’-carbocyanine perchlorate (dye 11),were examined in dichloromethane solutions by picosecond transient absorption spectroscopy. Contrary to earlier reports, intersystem crossing to an isomerized excited state, together with internal conversion, plays a major role in the radiationless deactivation of excited dyes. Torsional motion of the polymethine chain is prerequisite to both processes. Intersystem crossing occurs at a rate comparable to that of internal conversion but is faster than fluorescence by an order of magnitude: kist = 2.3 X lo9 s-l (I) and 1.8 X 1 O l o s-l (11); aisc= 0.55 f 0.05 (dye I) and 0.63 f 0.04 (dye 11); 781 241 f 55 ps (dye I) and I30 ps for the meso-substituted dye II. No ground-state geometrical isomers formed in our time window ( 1 1 2 ns); we have inferred that isomerization takes place from the longer lived triplet state manifold of these dyes.
-
Introduction The photophysics of cyanine dyes are of interest owing to the dyes’ utility as spectral sensitizers in silver halide photography,’ in biomedical applications,2 and in laser physics.j Electron transfer is generally implicated in spectral sensitization; however, much remains to be learned about the other photophysical processes of these interesting molecules which may compete with photoinduced electron transfer or stimulated emission. For example, it has been known for some time4,5 that solution fluorescence quantum yields for cyanine dyes are very low (see also ref 6). Fluorescencepolarization studies7Jlater established that radiationlessdeactivation,which competes with fluorescence and with spectral sensitization by electron or energy transfer, involves torsional motion about bonds of the polymethine chain of the dye. Recent instrumental advances in time-resolved spectroscopies have allowed the process of spectral sensitization to be probed. Insofar as the fluorescencequantum yield of the cyanine dyes is greatly enhanced on adsorptionto an inert surface,4q9fluorescence lifetime measurements have been the probe of c h o i ~ e . ~ Laser -’~ flash photolysis coupled with transient absorption spectroscopy has also been used with microsecond,*J3nan~second,’~J~ and picosecond16time resolution. These studies have confirmed the general picture that torsional motion of the polymethine chain leads to deactivation and that restriction of this motion accounts for enhanced fluorescence efficiency of cyanine dyes in the adsorbed state. Early workers5J4considered the possibility that radiationless deactivation might in part involve intersystem crossing of the excited cyanine chromophore. However, the absence of a significant heavy atom effect on fluorescencequantum yield led OBrien and co-workers5 to conclude that formation of the triplet state of the dye was, in fact, a very minor deactivation pathway for cyanine dyes of photographic interest. Low intersystem crossing quantum yields (& 10-2-104) seem to
characterize many polymethine dyes in nonviscous solutions? Eske and Naqvi” echoed this sentiment and inferred that all the excitationenergy was dissipated by fluorescence and radiationless internal conversion for the compounds they examined, namely 1,l’-diethyl-2,2’-carbocyanine iodide (pinacyanol) and 1,l’diethyl-4,4‘-carbccyanineiodide (kryptocyanine)which are both minimally fluorescent in fluid solution. More recently, Krieg and Redmond2estimated the intersystem crossing quantum yield for 3,3’-diethyldithia-2,2’-carbyanine iodide as 0.004at ambient temperature in ethanol, an order-of-magnitude agreement with an earlier estimate by Chibisov.6J8 The purpose of the present study was to extend previous investigationsinto the photophysics of representative carbocyanine dyes using the laser flash photolysis-transient absorption spectroscopy technique, with the specific aim of clarifying the role of intersystem crossing in the deactivation of these dyes. For our study, we selected the 3,3’-didodecyldithia-2,2’-carbocyanine bromide (dye I) and its meso-substituted analogue 9-methyl-
CH3
CH 3
Dye I
-
* All correspondence to this author at Concordia University. t This is Paper No. F195 from the Materials Research Group, Laboratory of Pure and Applied Studies in Catalysis, Environment and Materials, Concordia University and is ContributionNo. IIL-I 13 from the Information, Imaging and Electronics Sector Laboratories, 3M Center. Abstract published in Aduance ACS Abstracts, December 15, 1993. 0022-3654194 f 2098-0134W4.50/0
Dye I1 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No.3, 1994 735
Photophysics of Dithiacarbocyanine Dyes 1.0,
I
1.0 a, V C
m
n U
L a I
400
450
5 0 0 550 6 0 0 650 WAVELENGTH (nm)
U
700
425
Absorption spectra of (a) dye I and (b) dye 11, both 5 X M in methylene chloride, 2-mm cell. (c) Emission spectrum of dye I in methylene chloride; excitation wavelength 522 nm. Figure 1.
3,3’-dihexadecyl-2,2’-carbocyanineperchlorate (dye 11),both of which are readily soluble in methylene chloride and in which the monomeric form is favored.lg The principal difference between the two dyes examined here was expected to be the consequence of the meso-alkyl group in dye II. Introduction of meso-alkyl substitution is known to destabilize the all-trans isomer of cyanine dyes in solution.20This is normally the preferred ground-state isomer in, for example, dye I comparedto the mono-cis isomer13J4,21 which we accordingly expect to be preferred in dye II. Significant consequences on the dynamics of processes gated by torsional motion about the polymethine chain might be expected. The presence of the long alkyl chains, on the other hand, was not expected to affect appreciably the photophysics of the dyes in comparison to, for example, N-ethyl analogs examined previously.2 Experimental Section
Results and Discussion Absorption spectra of the two dyes 5 X 1C5M in methylene chloride (2 mm cell) are shown in Figure 1, a and b. The 19-nm blue shift of the absorption maximum for dye II relative to that for dyeLis consistent withassignmentofthe ground-stategeometry of dye II as From these spectra, the natural lifetimes, mat, of 7.3 ns for dye I and 6.7 ns for dye II can be estimated in the usual manner.z3 The fluorescencespectrum for dye I (SI SO)is also depicted in Figure IC;the emission is Stokes-shifted to the red by ca. 1700 cm-l, reflecting a slight distortion of the emissive SIstate relative to the SOground state. Transient absorption spectra recorded for a similar solution of dye I at various delay times after the 532-nm laser excitation (- 30 ps fwhm pulse) are shown in Figure 2. The salient features are transient absorptionbetween 425 and 525 nm, nearly complete bleaching of the ground state centered at -567 nm which only
-
525 575 625
675
Wavelength (nm)
Q,
0
C
z m
ffl
n < I
.-C I
0,
Ol
C
m L 0
425 445 465 485 505 525 Wavelength (nm)
0 08 m
The dyes 3,3’-didodecyldithia-2,2’-carbocyaninebromide (dye I) and meso-methyl substituted 3,3’-dihexadecyldithia-3,3’carbocyanine perchlorate (dye II) were kindly provided by Profs. E. Barni and E. Pelizzetti (Universita di Torino, Italy). Methylene chloride was spectral grade. Absorption spectra were recorded on a Shimadzu 265 UV/vis spectrophotometer. The luminescence spectrum of dye I was taken on a Perkin-Elmer MPF 44B spectrofluorometer;excitation wavelength was 522 nm. Transient absorption spectra were determined by picosecond laser flash spectroscopy utilizing a passively mode-locked 30-ps (fwhm) Nd:YAG laser and using a pumpprobe technique;excitation wavelength was 532 nm, energy ca. 2.5 mJ/pulse; the cell was a 2-mm quartz cell. Additional details of the picosecond laser setup and details of the procedures employed have been describedelsewhere.22 All experimentswere carried out in air-equilibrated solutions as oxygen would have no effect on the events2observed under our experimental conditions.
475
g
uus-002-
ffl
-007.-C
h, -0130) C
m
0
.-n 37
“ L L
I
630 650 670 Wavelength (nm) Figure 2. (A) Transient absorption spectra of dye I following 30-ps (fwhm), 532-nm laser pulse; decay times as indicated. (B) Exploded view of transient absorption in the wavelength region 425-525 nm illustrating the decay of SI;note the residual absorption at 483 nm (see text). (C) Exploded view of transient absorption illustrating the decay of amplified stimulated emission from the SIstate of dye I.
590
6io
recovers to the extent of ca. 50% on the time scale of the experiment, and amplified stimulated emission (ASE) in the spectral regime of dye fluorescence (see Figure IC). On closer inspection, the transient absorption is seen to comprise two components, one centered at -455 nm and a second, longerlived (120 ns) band at -483 nm. The kinetics of decay of the 455-nm transient and the ASE at 620 nm, as well as the recovery of the ground state at 569 nm are portrayed in Figure 3. These kinetics are all singly exponential with characteristic lifetimes, rob,which are within experimental error of each other 241 f 55 ps and within experimental error of the major component of the biexponentialdecay of the samechromophoreobserved in ethanol.2 The shorter wavelength component of the transient absorption is accordingly assigned to the emissive SIstate of the dye. This assignment puts an upper limit of ca. 0.03 on the fluorescence quantum yield of dye I to be compared to estimates of @f 0.1 1
-
Serpone and Sahyun
I36 The Journal of Physical Chemistry, Vol. 98, No. 3, 1994 9)
V
C
m
a L 0 v)
a
a
-IO!
0
'
I
'
I
'
I
'
I
'
I
'
I '
500 1000 1500 2000 2500 3000 Time (ps) Figure 3. Kinetics of decay of light absorbing transient@)observed in dye I at 455 nm (upper trace); of ASE at 620 nm (middle trace); and recovery of ground-stateabsorption (lower trace)at 569 nm. Data points are experimentally observed changes-in-absorbance; solid and dashed curve are fits to single exponential, Le., pseudo-first-order kinetics with lifetimes as indicated.
for theN,N-diethyl analogue in gelatin,lOca. 0.1 in either ethanol or chloroform solution,2 and 0.07 in methanol." There is no evidence in Figure 2 of the appearance of a new absorption within 3 ns that could be attributed to a geometric isomer of dye I; the mono-cis isomer is expected to exhibit a band at ca. 550 nm.2J3 At the same time, absorption spectroscopy showed that no permanent changes resulted in the spectral distribution of absorption (cf. Figure la) after multiple p u m p probe cycles of the dye solution; that is, no permanent chemical changes resulted in the chromophore under our conditons. The ionization product, i.e., the radical dication, formed electrolytically from similar chromophores is known to absorb in the 450-470nm regime.24 However, the laser photon energy (-2.35 eV, or 532 nm) used in our experiments is not energetic enough to ionize the dye, even if biphotonic excitation occurred. We therefore dismiss this possibility for the 483-nm transient. Insofar as ground-state absorption does not recover within 10ns of excitation, the long-lived transient absorption must therefore be assigned to an excited state of the dye with lifetime substantially longer than T , , ~ ~ .The triplet excited state of the dye is the only reasonable assignment, despite the earlier inference of OBrien and coworkers5and the assignment of a long-lived transient at 630 nm to a triplet state derived by energy-transfer sensitization from this chromophore.2 The quantum yield for intersystem crossing, @is, under our conditionsis accordingly0.55 i 0.05, substantially greater than previously reported estimates.2Js The quantum yield of cis-trans isomerization of mesounsubstituted carbocyaninedyes is generally thought to be low11~25 though not all authors2T6seem to concur. Under our conditions, photoisomerization of dye I would occur on a time scale much longer than the time window of the experiment (30 ps to 12 ns). From studies in ethanol solutions, Steiger and co-workers13noted that isomerization was essentially complete in the cyanine dyes within the 30-ps duration of their lamp pulse. Given our assignment of the long lived species formed on photoexcitation of dye I as a triplet excited state, we must conclude that no significant isomerization of the dye occurs within the -240 ps lifetime of SI;isomerization must accordingly occur within the triplet-state manifold of the dye as speculatively proposed earlier by Ehr1i~h.l~ Similar transient absorption spectra are shown in Figure 4 for dye II. In this case, the transient absorption to shorter wavelengths of the bleached ground-state absorption, assignable to SI of dye 11, cannot be resolved into short- and long-lived components. A weak transient absorption at X 1 600 nm forms in parallel with the decay of the shorter wavelength transient. The triplet state of the 3,3'-diethyldithia-2,2'-carbocyanine chromophore formed by energy transfer to the all-trans ground state of this dye has
425 475 525 575 625 675 Wavelength (nm) Transient absorption spectra of dye II following 30-ps (fwhm), 532-nm laser pulse; delay times as indicated.
Figure 4.
TABLE 1: Rate Constants for the Deactivation of the SI State of Dyes I and I1 parameter ns rob#, PS
kr,s-I kh,S-'
ki,, 9-l
dye I 7.3 241 h 55 1.4 X lo* (2.3 i 0.3) X (1.7f0.7) X
dye II 6.7
lo9 lo9
530 h 10 1.5 x 108 (1.8 h 0.4) X 1O'O (1.0f 0.9) x 10'0
been reported in this region of the spectrum.2 We appropriately assign the longer wavelength transient to an all-trans triplet state derived by intersystem crossing and isomerization of dye II,No ASE is observed. Most of the decay of the transient centered at -470 nm and recovery of the ground state during the experiment occurs within the pulse duration of the pump laser. We therefore can only place an upper limit of ca. 30 f 10 ps on fobs, the lifetime of the SIstate of this dye. From the amount of ground-state bleaching persisting at times longer than 500 ps after excitation, we infer an intersystem crossing quantum yield of 0.63 i 0.04. From the usual expression for the intersystem crossing rate constant, kist = @ix/70bsr the definition of the fluorescence rate constant, kr = rnat-l,and robs = (kr + kix + kit)-', where kic is the rate constant for radiationless internal conversion of S1to the ground state, we estimate the rate constants for deactivation of SI of dyes I and 11; they are summarized in Table 1. The sum of the nonradiative decay rate constants kic + kisc = (4.0 f 0.6) X lo9 s-I for dye I is in good agreement with the estimate of the sum of rate constants for nonradiative decay processes given by Krieg and Redmond2for the 3,3'-diethyl analog in ethanol, namely 2 X lo9 s-I; these processes are identified as internal conversion and isomerization. In our interpretation, kiCand ki, respond in parallel fashion to the introduction of the meso-alkyl group into the chromophore. If both processesreflect torsional motion of the polymethinechain, it is not surprising that the more sterically strained cis compound relaxes an order of magnitude faster than its trans counterpart. However, this point of view implies that torsional relaxation is prerequisite to both internal conversion and intersystem crossing. Molecular orbital c a l c ~ l a t i o nhave s ~ ~ shown that the principal consequenceof photoexcitation in a carbocyanine dye is transfer of electron density from the 8 and 8' C-atoms to the 9-position, thereby conferring diradical character on the C 8 4 9 and C9C8' bonds. Molecular relaxation by rotation through 90" about, for example, the C 8 4 9 bond localizes the unpaired electrons in a pair of orthogonal but unsymmetrical, "disjoint"26molecular orbitals. In this configuration,considerablesinglet-triplet mixing is to be expected, and the molecule can further relax with comparable facility to products of singlet or triplet multiplicity. Accordingly, geometrical isomerization of the dye correlates with intersystem crossing as portrayed schematically in Figure 5 for dyeI. Therelativeenergeticsofcis and transstates will be reversed
The Journal of Physical Chemistry, Vol. 98, No. 3, 1994 131
Photophysics of Dithiacarbocyanine Dyes
AS‘
n
0
e
(c8-cg)
Figure 5. Hypothetical correlation diagram for the deactivation of photoexcited dye I by torsional motion about the C8
