J. Phys. Chem. 1994,98,4511-4516
4511
Photophysical Properties of (Dimethy1amino)anthraquinones: Radiationless Transitions in Solvent and Polyelectrolyte Media Guilford Jones, 11,' and Zhiming Feng Department of Chemistry, Boston University, Boston, Massachusetts 02215 William R. Bergmark' Department of Chemistry, Ithaca College, Ithaca, New York 14850 Received: August 30, 1993;In Final Form: February 18,1994'
Substitution of 9,lO-anthraquinone with dialkylamino groups in the 2(6) positions results in either highly solvent-sensitive fluorescence emission from an intramolecular charge transfer state or, for a symmetrical substitution pattern, an intensely absorbing triplet transient. For the 2-dimethylaminoderivative1,the fluorescence quantum yield is strongly dependent on solvent polarity, consistent with the imposition of a nonradiative decay channel involving motion of the amine moiety that is favored in more polar media. The emission yield is uniformly low for the symmetrical bis-substituted derivative 2 due to a competitive intersystem crossing which yields strongly absorbing triplet transients that decay in the microsecond regime. The binding of dyes to poly(methacry1ic acid) (PMAA) is particularly efficient for the polyelectrolyte in its globular or "hypercoiled" conformation in water (acid form, p H 3.0). For bound dye the yield of fluorescence for 1is increased significantly; the triplet states for 2 are especially long-lived and resist excited-state quenching.
Introduction Substituted anthraquinones constitute an important class of dyes that strongly absorb in the 400-600-nm range. They are used in a variety of textile and other commercial applications.' Placement of an amine or similar electron-donatingfunction (e.g., -OH)on the anthraquinone skeleton results in excited-state structures with charge-transfer character that are responsible for intense transitions in the visibleS2Substitution with primary and secondary amine groups for many anthraquinones leads to structures having weak fluorescence and poor intersystemcrossing yields in more polar media,3 due to a rapid radiationless transition of excited dye singlets involving carbonyl groupamine interaction and hydrogen bonding to ~ l v e n t . ~ The dialkylamine functional group is prominent in dyes such as coumarins and xanthenes5 but has been little used for anthraquinone dyestuffs. The present report includes photophysical data for a pair of diemthylamine-substituted anthraquinones, 1 and 2. The results point to the importance of a "rotatory" decay mode for the intramolecular charge transfer (ICT) singlet state, whose emission intensity is remarkably solvent dependent. For symmetrically substituted 2, photophysical parameters are altered in that fluorescenceemission and internal conversion of singlets no longer dominate, but moderate yields of intersystem crossing (dye triplets) are observed. The observed effects of medium have been extended to the incorporation of dye in an aqueous polyelectrolyte, poly(methacrylic acid) (PMAA). A number of investigations have documented host-guest interactions for hydrophobic moieties and the globular form of this polymer in water.6 In contrasting ways for 1 and 2, dye-polymer binding leads to marked changes in fluorescence yields, lifetimes, and triplet properties. Our efforts are directed in part to an understanding of how dyes such as the anthraquinones are bound in aqueous media to various kinds of polymers (particularly polyelectrolytes, and including humic *Abstract published in Aduance ACS Abstracts, April 1, 1994.
0022-365419412098-4511$04.50/0
substances in natural aquatic systems'). It is expected that systematic alteration of photophysical and photochemical properties will provide spectral signatures of dye binding in unusual macromolecular environments.*
Experimental Section Acetonitrile (HPLC grade, Fisher) was used as received. Cyclohexane, ethyl ether, benzene, toluene, isobutyric acid, acetone, methanol, ethyl acetate, and acetic acid were obtained from Baker (Analyzed). Ethanol, 2-propanol,DMF, and DMSO were obtained from Aldrich. Poly(methacry1ic acid) (PMAA) was prepared by AIBN (2,2'-azobis(isobutyronitri1e)) initiated polymerization of methacrylic acid9 (freshly distilled under reduced pressure) in DMF with continuous nitrogen purging (60 OC, 12 h) and purified by multiple precipitation from methanol on addition of ethyl ether. The weight-average molecular weight of the principal PMAA sample was determined by flow viscometryg to be 25 000. Aqueous polymer solutions were colorless and displayed no absorption or emission above 250 nm. Samples of the anthraquinone dyes were prepared in the group of Professor T. Mukai of Tohoku University: for 2-(dimethylamino)-9,1O-anthraquinone(l),red crystals were obtained (m.p. 184-186 OC; literaturevalue, 185-186 OC1o);ll2,6-bis(dimethylamino)-9,1O-anthraquinone(2), was recrystallized from methanol, m.p. 300-307 OC (material sublimes) (Scheme 1). The purity of dye samples was confirmed by NMR, TLC (silica gel, ethyl acetate), and HPLC. Absorption spectra were obtained using a Beckman DU-7 spectrophotometer and emission properties measured using a Model 48000 phase-shift fluorometer from SLM Instruments. Measurements of pH were made on a Fisher Accumet pH meter. For measurement of fluorescencequantum yields, rhodamine 3B in ethanol (@f = 0.45)12 was used as the standard. Fluorescence lifetimes were determined by the phase and modulation method using the SLM fluorometer.13 Excitation wavelengths (Lc) were 0 1994 American Chemical Society
Jones et al.
4512 The Journal of Physical Chemistry, Vol. 98, No. 17, 1994
TABLE 1: Solvent Effects on Photophysical Properties of 1’
SCHEME 1
Me.N I
Me
0 1
0 2
COpH
PMAA
selected toward the blue side of the visible anthraquinone absorption bands, and the total emission was detected without an emission monochromator. Cutoff filters (550and 575 nm) were used to prevent Raman scattered light from entering thedetection chamber. An aqueous solution of glycogen (scatterer) was used as standard. Modulation frequencies in the range of 2-1 50 MHz were used. Flash photolysis experiments were carried out using the Nd/ YAG laser system (X/3,355 nm excitation, ca. 50 mJ/7 ns pulses) and detection methods previously described.14 Dye solutions of ca. 20-25 pM concentration were used in 2.2 cm X 1.0 cm rectangular Pyrex cells. Irradiations were conducted at 20 ‘C, and the solutions were deaerated by purging with argon (ca. 20 min). Transient decays were recorded and treated with a kinetics program that inspected fits to first and second order and multiexponential decays. An extinction coefficient for T-T absorption, e ~ for , dye 2 in acetonitrile solvent was determined by comparison of the optical densities (extrapolated to zero time) at one of the peak bleaching wavelengths and the peak absorption wavelength of the transient spectrum and referenced to the known extinction coefficient for ground-state dye at the bleach wavelength ( € 3 ~ )= 2.4 X lo4M-’ cm-’). This method is based on the assumption that the observed ground-state bleaching that is sustained in the microsecond regime is solely due to the conversion of the dye into its triplet state and that absorption by ground and triplet dye species are not significantly overlapping. Use of this assumption was supported in part by the observation that the decay at the triplet peak and the recovery of the bleached dye have the same lifetimes. The extinction coefficient for T-T absorption (€490 = 1.7 X lo4 M-’ cm-1) obtained in this way for 2 in acetonitrile was used to estimate the triplet quantum yield, @T, for the dye in several media based on a comparative method,I5 using benzophenone as reference (@T = 1.0,ET = 6500cm-l M-l at 520 nm) for flash photolysis a t 355 nm of a 0.5 mM solution of benzophenone in acetonitrile.I6 Molecular orbital calculationswere carried out using MOPAC (version 6.0, October, 1990,PM3 Hamiltonian, with geometry optimization and configuration interaction)” installed in the QUANTA program (Polygen software) on a Silicon Graphics Indigo 2 work station.
media
X, (nm)
cyclohexane PMAA(pH 3) R / D = 1000b ether toluene benzene ethylacetate acetone glycerol acetonitrile DMF DMSO HCONH2 i-PrOH EtOH H2O isobutyricacid acetic acid
442 474 449 459 461 457 464 489 467 471 479 488 467 468 488 460 474
z
(lo3)
**
Xr(nm)
@f
6.2 7.1
559 560
0.41 0.15
6.9 7.0 6.9 7.3 7.6 8.2 7.2 7.3 7.2 7.9 7.6 7.6 4.9 7.4 8.0
566 568 574 602 622 625 631 632 637 d 622 627 630 579 612
0.39 0.21 0.27 0.052 0.011 0.010 0.0062 0.0039 0.0021 d 0.0017 0.0008 0.0017 0.0004 0.0007
a
0 0 (0.08) 0.27 0.54 0.59 0.55 0.71
0 0 0 0 0.08
0.75 0.88 1.00 0.97 0.48 0.54 1.09
0.19 0 0 0.71 0.76 0.83 1.17
0.64
1.12
“Dye concentration = 10 pM; bX= 460 nm; for solvatochromic parameters, r* and a,see: Kamlet, M. J.; Taft, R. W. J . Org. Chem. 1983, 48, 2877.b R / D = 1000 corresponds to [PMAA residue] = 10 mM. c Solvatochromic parameter for PMAA estimated from this study. No emission observed.
TABLE 2
Solvent Effects on Photophysical Properties of 2’
media
A. (nm)
cyclohexane ether benzene toluene ethyl acetate PMAA(pH 3) R/D = lOOOb acetone acetonitrile DMF DMSO HCONH2 i-PrOH EtOH H20 isobutyric acid acetic acid
398 420 430 427 424 433
~(10’) 4.1od 14.5 16.4 15.3 15.0 11.4
427 429 434 437 443 431 431 434 427 439
14.7 14.2 14.5 14.9 14.9 14.6 14.4 7.51 13.5 13.8
Xr(nm)
@f
(528)d (O.O1l)d 558 0.070 564 0.040 565 0.048 573 0.067 589 0.0099 591 602 605 618 648 629 637 641 61 1 634
0.074 0.049 0.048 0.022 0.00092 0.0043 0.0020 0.0019 0.0018 0.00017
A*
a
0 0 0.27 0 0.59 0 0.54 0 0.55 0 (0.68)c 0.71 0.75 0.88 1.00 0.97 0.48 0.54 1.09
0.08 0.19 0 0 0.71 0.76 0.83 1.17
0.64
1.12
Dyeconcentration= 10pM;bx=460nm. R / D = 1OOOcorresponds to [PMAA residue] = 10 mM. Solvatochromic parameter for PMAA estimated from this study. Solubility very limited, approximate values. (1
I
ethvl ether
Results and Discussion Dependence of Dye Absorption and Emission Properties on Solvent. The effect of solvent polarity on the photophysical propertiesof anthraquinones 1 and 2 was determined for a variety of media (Tables 1 and 2). Absorption bands were shifted to the red for more polar solvents (up to about 50 nm for both dyes), consistent with significant relative stabilization of the chargetransfer excited states in more polar media. Somewhat larger red shifts of emission wavelengths (80-120 nm) were observed as well, a pattern often found for organic structures having “pushpull” substitution by electron donor (amine) and adceptor (quinone) groups.ls Notably, the pattern of emission properties diverges for the two dyes in that relatively robust emission was observed for 1 in nonpolar media, along with a falloff of fluorescence intensities with increased medium polarity (Figure
o.o+ps+ 480
520
560
600 640 Wavelength (nm)
680
720
Figure 1. Emission spectra for 10 pM 1 in different media, Lx= 460 nm; for PMAA aqueous solution, pH = 3.0 and R/D = 1000.
1). In contrast, fluorescence yields are uniformly low for 2 for solvents with varied polarity. In terms of emission energies (separation of relaxed excited- and ground-state species), the effects for both dyes were quite regular in that plots of vf vs the polarity-polarizability parameter, **,I9 were linear for solvents ranging in polarity from cyclohexane to DMSO (Figure 2). The dramatic reduction in emission yield for 1 with increased solvent polarity is quite similar to the behavior of 7-(dialkylamino)-
Photophysical Properties of (Dimethy1amino)anthraquinones I
19.0 i o
aa. PMAA
15.0
I
I ” ” I ” ” I “ ” I 0.25 0.50
0.75 I .oo n* Figure 2. Emission frequency for 2 (circles) and 1 (stars) in different solventsvs the solvent polarity/polarizability parameter, T * . Thesolvents in order of decreasing polarity: DMSO, DMF, acetonitrile, acetone, ethyl acetate, ether and cyclohexane.
0.00
SCHEME 2 W
i
-
M Me e
0.
0.
IC1
TIC1
SCHEME 3
coumarin dyes.538.20 The commonly accepted mechanism18 involves excitation to a low-lying polar excited state with intramolecular charge-transfer (ICT) character. The excited species proposed for 1 and 2 can be modeled in a simple way by reference to canonical structures (Scheme 2) in which charge has been transferred from an aniline or amine moiety to a naphthaquinone unit. The dynamics of the relaxation of the ICT state (more polar than the ground state by some 10 D18) to lower energy in more polar media has been examined for the coumarins using picosecond fluorescence methods.21 The sharp reduction in yield for the moderately polar fluorescent ICT state is understood in terms of the dominance of an additional rapid nonradiative decay channel that is favored for more polar media. According to one model, dyes having dialkylamine “free rotors”, evolve to a twisted charge transfer state (TICT, Scheme 2); the TICT species is typically nonfluorescent and believed to have a fully charge-separated, zwitterionic structure. Nonradiative return to the ground state is highly favored at the twisted geometry where ground and excited potential surfaces are close in energy.18.20.22 We sought some verification of these predicted changes in electronic structure for ground and excited anthraquinones through MO calculations. Results are shown in part in Scheme 3 in terms of calculated charge densities for C, 0,and N atoms at key sites for ground and lowest singlet excited states (SOand S1). Notably for the excited state, electron density is shifted away from the nitrogen substituent group consistent with the ICT model; hydrogens on the CH3 groups also take on positive charge (+0.02-0.04each) through a mechanism akin to hyperconjugation. A similar change in polarity is indicated for 2 in that low-lying singlet states are configured as degenerate locally excited species in which each nitrogen is in turn involved in an excitation with charge shift to the ring system. The resulting
The Journal of Physical Chemistry, Vol. 98, No. 17, I994 4513
TABLE 3 Fluorescence Lifetimes, Decay Rate Constants and Intersystem Crossing Quantum Yields for 1 and 2 CH3CN 1.1 PMAA/H@ 7.1 toluene 7.4 2 CHpCN 3.3 PMAA/HzO” 2.1 i-PrOH toluene 2.8 a R/D = 5000. From ref 25. 1
5.5 22 28 18 4.5
8.8 1.2 1.1 2.8 4.8
17
3.4
0.08b
0.089
0.049 0.040
low-lying, highly polarizable,singlet states are described by linear combinations of the two configurations representing the local excitations.23 There are several peculiarities in the profile of solvent effects that should be emphasized. For protic (H-bonding) solvents, the correlation of emission frequencies and yields (with, for example, the Kamlet-Taft CY parameter,lg associated with hydrogen bond donor ability, Tables 1 and 2) is not regular. Water and formamide are most effective in stabilizing both Franck-Condon (absorption shift) and relaxed ICT excited states (emission shift) for 1and 2. In addition, the polar protic solvents (water, alcohols and carboxylic acids) lead to the lowest yields of emission, but, again, not in a regular pattern regarding their H-bond donating ability (Le., CY parameters). Emission energies likewise do not correlate in such a way as to suggest that a simple “gap law” controls nonradiative decay rate. These results support the notion that interaction of the aminoanthraquinones with H-bonding solvents is very specific with regard to solvent structure and that the interactions are complex and likely to be cooperative (e.g., H-bond donation to the carbonyl group and solvent dipole interaction with the amine function). The result is a moderate scattering of the data for spectral band shifts (excited state energies) and emission yield^.^.^^,^^ Triplet Formation for Aminoanthraquinones. For the dyes in several media, fluorescence lifetimes were determinedusing phaseshift fluorometry.13 Radiative and nonradiative decay constants ( k fand kndr respectively) were computed from fluorescence yield and lifetime data (Table 3), based on the relations: kf = @f/n and knd= (1 - @f)/Tf. The two dyes showed interesting contrasts. For 1, the solvent-dependentradiationless decay that we associate with the ICT TICT transformation results in a very large value for kndina polar medium (e.g., an 8-fold larger rate constant for acetonitrile vs toluene). For solvents for which the emission yield was
