J. Phys. Chem. 1995,99, 8504-8509
8504
Polarized Light Spectroscopy of Dihydropyrrolopyrroledione in Liquids and Liquid Crystals: Molecular Conformation and Influence by an Anisotropic Environment Peter Edman and Lennart B.-A. Johansson" Department of Physical Chemistry, University of UmeB, S-901 87 UmeB, Sweden
Heinz Langhals Institute of Organic Chemistry, University of Munich, Karlstrasse 23, 0-80333 Munich, Germany Received: December 9, 1994; In Final Form: March 10, 1995@
Different phenyl derivatives of dihydropyrrolopyrrolediones (DPP) have been examined by means of polarized absorption and fluorescence spectroscopy. The derivatives were 3,6-bis(3,5-di-tert-butylphenyl)-2,5dihydropyrrolo[3,Cc]pyrrole- 1,4-dione (BDPP), 3,6-bis(2-methoxyphenyl)-2,5-dimethylpyrrolo[3,4-c]pyrrole1,4-dione (MMDPP), 3,6-bis(2-methoxyphenyl)-2-hydro-5-methylpyrrolo[3,4-c]pyrrole1,Cdione (MHDPP) 3 ,Cc]pyrrole- 1,Cdione (HHDPP). Intramolecular hydrogen and 3,6-bis(2-methoxyphenyl)-2,5-dihydropyrrolo[ bonds can form between the DPP core and the phenyl groups of MHDPP and HHDPP. The Stokes shift (ca. 10-70 nm) and the bandshape of absorption and fluorescence spectra depend strongly on possibilities of intramolecular n-electronic overlapping of the DPP core and the phenyl groups. Different conformations of the DPP and aryl planes are likely present. The rate of transfer between these conformations is rapid, which is supported by the monoexponential photophysics observed for all derivatives. The lifetime varies between 5.5 and 9 ns in different liquid solvents, as well as in a lyotropic nematic liquid crystal. The fluorescence quantum yields and Forster radii are reported. The wavelength dependence of the limiting fluorescence excitation and emission anisotropies have been studied. Except from MMDPP and MHDPP, the SO SI bands constitute one direction of the transition dipoles corresponding to the same limiting anisotropy of ro = 0.38. Second rank order parameters of the ground and excited state were determined for the DPP derivatives solubilized in a macroscopically aligned lyotropic nematic liquid crystal. Taken together, the experimental results suggest that the molecular symmetry of HHDPP is the same in the ground and the first excited states, contrary to the other derivatives.
-
The 3,6-diaryl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-diones (DPP) constitute a new class of fluorescent molecules. Although the first DPPs were synthesized 20 years ago,' most work with DPPs dates from recent year^.^-^ The methods of DPP synthesis have been improved, and new derivatives have been prepared which are photostable and highly soluble in many organic With the exception of X-ray studies and reported absorption and fluorescence ~ p e c t r a ,no ~ . spectroscopic ~ studies of DPP molecules are available in the literature. The DPPs are of potential interest as pigments, laser dyes, materials for the storage of digital data,8 textile dyes, or fluorescent probes in biosystems. In the latter case, they might serve as reporter molecules of dynamics, structure, and function of lipid membranes, peptides, and proteins. A fluorescence experiment reveals direct information about the probe molecules but informs only indirectly about the particular system where they reside. This is the major drawback of any probe technique. However, by a critical selection of the probe for a specific question, it may still be possible to extract important molecular information. For example, probe molecules whose photophysical and spectral properties depend strongly on local polarity are less well suited for fluorescence depolarization experiments, where an analysis in terms of molecular motions, order parameters, and/or molecular separations is desirable. On the other hand, such probes may be better suited for estimations of local polarity. In fluorescence depolarization studies, it is desirable to work with highly fluorescent molecules having strong and pure electronic transitions with known directions of the transition @
Abstract published in Advance ACS Abstracts, May 1, 1995.
dipoles in the spectra and in the molecular frame. Furthermore, in order to avoid perturbations of structure and/or activity in biological and biophysical applications it is desirable to use relatively small fluorescent groups. This work reports on basic light spectroscopic properties of carefully selected derivatives of 3,6-diaryl-2,5-dihydropyrroolo[3,4-c]pyrrole-l,4-diones. The influence of an anisotropic environment on intramolecular interactions between the DPP core and the aryl groups is studied.
Materials and Methods The preparation of 3,6-bis(3,5-di-tert-butylphenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-l,4-dione(BDPP) is given in ref 2. 3,6Bis-(2-methoxyphenyl)-2,5-dihydropyrrolo[ 3,4-c]pyrrole-1,4dione (HHDPP) and 3,6-bis(2-methoxyphenyl)-2-hydro-5methylpyrrol0[3,4-~]pyrrole-1,4-dione (MHDPP) were preparted according procedures given in refs 3 and 4. 3,6-Bis-(2methoxyphenyl)-2,5-dihydro-2-methylpyrrolo[3,4-c]pyrrole1,4dione (MMDPP) was prepared as follows. A mixture of 250 mg (0.72 mmol) of 3,6-bis-(2-methoxyphenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-l,4-dione(HHDPP), 700 mg (5.07 mmol) of K2CO3, and 25 mL of DMF is stirred for 5 min at 120 OC. Then a mixture of 130 mg (0.72 "01) of p-toluenesulfonic acid methyl ester and 10 mL of DMF is added dropwise within 20 min at this temperature. The reaction is then immediately quenched by the addition of 200 mL of cold water. The solid is collected by vacuum filtration through a G4 glass filter. The precipitate is dried, dissolved in a small amount of chloroform, and purified by column separation with tert-butyl methyl ether
0022-365419512099-8504$09.00/0 0 1995 American Chemical Society
DPPs as a New Class of Fluorescent Molecules over silica gel (70 x 1.5 cm). Yield: 150 mg (57%) dark red crystals mp 253-254 “C from tea-butyl methyl ether. Rf(si1ica gel, tert-butyl methyl ether): 0.27. IR (KBr): v = 2940, 2835 (C-H), 1653 (C=O), 1611, 1594 (C=C), 1594, 1543, 1490, 1467, 1437, 1383, 1253 (C-0), 1140, 1110, 1015, 769, 749 cm-I. UV (CHC4): A, (log E ) = 945 (4.290), 473 (4.300) nm. Fluorescence (CHC13): A = 527 nm. ‘H NMR (CDC13): 6 = 3.15 (s, 3H, N-CH3), 3.89 (s, 3H, 2’-O-CH3), 4.00 (s, 3H, 2”-0-CH3), 6.95-7.06 (m, 2H, 3’-H, 3’-H, 3”-H), 7.107.22 (m, 2H), 7.45-7.53 (m, 2H), 7.60 (dd, lH, 6’-H, J = 7.61, J = 1.72), 9.20 (dd, lH, 6”-H, J = 7.93, J = 1.67), 9.60 (s, lH, N-H). I3C NMR (CDC13): 6 = 28.89 (q, N-CH3), 55.58 (q, 2’-0-CH3), 55.97 (4, 2”-0-CH3), 108.97 (s, C-3a, C-6a), 111.49 (d, C-3’, C-3”), 116.61 (s, C-l”), 117.18 (s,C-1’), 121.15 (d, C-5’), 122.06 (d, C-5”), 131.51, 132.84, 133.51 (d, C-4’, C-4”, C-6’, C-6”), 144.35 (s, C-6), 146.53 (s, C-3), 157.10 (s, C-2’), 157.29 (s, C-2”), 161.68 (s, C-4), 162.04 (s, C-1). MS (70 eV): mlz (%) = 362 (100) M+, 348 (2),347 (2) M+-CH3, 345 (l), 333 (4) M+-OCH3, 3.11 (4), 316 (3), 303 (3), 288 (2), 276 (3), 269 (12), 248 (3), 214 (2), 200 (3,186 (3,181 (S), 172 (3), 159 (4), 148 (15), 135 (15). Anal. Calcd for C21H18N204 (362.4): C, 69.60; H, 5.00; N, 7.72. Found: C, 69.79; H, 5.00; N, 7.55. Potassium dodecanoate (laurate) was synthesized as described in ref 9. Dimethyl sulfoxide (Fluka) and 1,2-propanediol (Merck) were used as received. The lyotropic nematic liquid crystalline phase was composed of potassium laurate/KCl/DzOin the amounts 34.00/2.30/63.70 % by weight. The liquid crystals were macroscopically aligned in a magnetic field of 5.9 T for about 20 h at 295 f 1 K. The steady-state fluorescence spectra and anisotropies were obtained using a SPEX Fluorolog 112 instrument (SPEX Ind., NJ), equipped with Glan-Thompson polarizers. The spectral bandwidths were 1.8 and 3.7 nm for the excitation and emission monochromators, respectively. The fluorescence spectra were corrected. The fluorimeter was calibrated by using a standard lamp from the Swedish National Testing and Research Institute, Boris. A PRA 3000 system (Photophysical Research Assoc. Inc., Canada) was used for single-photon-countingmeasurements of the fluorescence decay. The excitation source is a thyratrongated flash lamp (model 51OC, PRA) filled with deuterium gas and operated at about 30 kHz. The excitation wavelengths were selected by interference filters (Omega/Saven AB, Sweden) centered at 430 nm (HBW = 8 nm), 460 (8.8), 470 (9.3), and 477 (4.7). The fluorescence emission was observed above 470 and 550 nm through a long pass filters (Schott, Germany). In order to avoid reabsorption, the maximum absorbance was 10.08 for all samples. The time-resolved polarized fluorescence decay curves were measured by repeated collection of photons during 200 s, for each setting of the polarizers. The emission polarizer was fixed and the excitation polarizer rotated periodically. In each experiment the decay curves fil(t) and F l ( t ) were collected. The subscripts II and I refer to an orientation of the emission polarizer parallel and perpendicular with respect to the excitation polarizer. From these a sum curve
+ 2GF,(t)
s(t) = F,,(t)
and a difference curve
were calculated. The correction factor, G, was obtained by normalizing the total number of counts fir and F l collected in
J. Phys. Chem., Vol. 99, No. 21, 1995 8505 fil(t)
and Fl(t), respectively, to the steady-state anisotropy, r,,
as
G = (1 - r,)(l
+ ~~J’F,,(FJ-’
The fluorescence decay curves were deconvoluted on a PC (33MHz, 16MB) by using a nonlinear least-squares analysis based on the Levenberg-Marquardt algorithm. Linear dichroism (LD) spectra were recorded on a JASCO 5-720 instrument supplemented with an Oxley device (Bemhard Halle, Germany) and the absorption spectra on a GBC 920 spectrophotometer supplemented with Glan-Thompson polarizers (Bemhard Halle, Germany). The Forster radius (Ro)was determined in the following way. The corrected fluorescence spectrum, F(Y),and the molar absorptivity, ~ ( v ) ,were determined. These were used to calculate the Forster radius form
1
9000(ln l O ) ( ~ ~ ) @ l i j ROf
= 128dn4NA
Here Y, NA, n, and 0 denote the wavenumber of light, Avogadro’s number, the refractive index of the medium, and the fluorescence quantum yield, respectively.I0 In the equation ( K ~ is ) a mean value of the orientational part of a dipole-dipole interaction. In order to calculate the Forster radius it is convenient to choose ( K ~ )= 2/3 as a reference state. This average value is relevant in a three-dimensional system for the interaction between rapidly rotating dipoles and is often referred to as the dynamic limit or fast case. We denote this Forster radius by Ro. The fluorescence quantum yield, 0,was determined as follows. Fluorescein in water (pH = 12) and the perylene dye (S- 13, N,”-bis-( l-hexylheptyl)perylene-3,4:9,lO-tetracarboxylic bisimide, RN 110590-84-6) were used as a standards with a quantum yields of 0.93” and 1.0. The fluorescence quantum yield was calculated from
’
= ‘ref
F[1 -exp(-A,,,
In lo))?
Fref[l - exp(-A In 10)]nref2
Here, F denotes the integral of the corrected fluorescence spectrum and A is the absorbance at the excitation wavelength. The radiative lifetime, TO, was calculated by using the modified Strickler-Berg equation.I2
Basic Theory Linear Dichroism. Some lyotropic nematic liquid crystals align spontaneously in a strong magnetic field and form macroscopically uniaxial systems with the optic axis being parallel to the applied field. The orientation of chromphoric molecules solubilized in such systems is uniaxially anisotropic. If linearly polarized light propagates perpendicular to the optic axis, so that its electric field vector is parallel and perpendicular with this axis, the difference in absorbance defines the linear dichroism, LD. LD yields information about the average orientation of the electronic transition dipole moment in the ground state (g), in terms of the second rank order parameter;
8506 J. Phys. Chem., Vol. 99, No. 21, 1995 Sg = K’/&@)[3 cos2/3
Edman et al.
- llsin /3 d/3
0.4
0.3
Here /3 is the angle between the absorption transition dipole and the optic axis, and fg@) is the normalized orientational distribution function. The order parameter can take values - 1/2 5 S, 5 1, where the limits correspond to a perfect orientation perpendicular (-U2) and parallel (1) to the optic axis. For an isotropic orientational distribution, S, = 0. Fluorescence Anisotropy. The orientational correlation function of excited molecules in a macroscopically isotropic system is given by the time-resolved fluorescence anisotropy,
In eq 2, F d t ) and Fzr(t) denote the time-dependent fluorescence intensity when the excitation light pulses propagate along the laboratory X-axis, while the emission is monitored along the Y-axis. It can be shown that r(t) depends only on the orientational dynamics of the molecules, if the excited state processes are independent of its orientational dynamics (see for example ref 13 and papers cited therein) and provided energy migration is negligible. Within these assumptions one obtains that
0.2
0.1
0.0
0.4
0.3
0.4
.-
0.3
C
a
0.4
0.3
0.2
which is the orientational correlation function written on the basis of second rank irreducible Wigner rotational matrices.I4 The orientation of the molecules at the time of excitation (Le., t = 0) and at a time t = t later is described by the eulerian angles QML. The subscripts indicate that the transformation is from the laboratory fixed (L) frame to a molecule fixed (M) frame. The initial anisotropy r(0) = 2/5, if the absorption and emission transition dipoles are parallel. For rotational correlation times #k of #k >> z, the limiting anisotropy of ro = r(0) gives information about the angle between the absorption and emission transition dipole moments. The fluorescence depolarization from a uniaxial distribution of emitting molecules is also conveniently described by the fluorescence anisotropy given by eq 2. This is achieved by choosing the excitation polarization field parallel to the C, symmetry axis of the uniaxial orientational distribution. The corresponding rotational correlation functions (Le., eq 3) contain information about the second, as well as, the fourth rank order parameters. In particular, at times t t, >> #k
r(tJ = S, = L7’/J33)[3
cos2 /3 - llsin ,8 d/3
(4)
It could be expected that S, = S, for the case of parallel or nearly parallel absorption and emission dipoles. However, if the molecular conformations differ in the ground and excited states, one would expect that S, S, even if the transition dipoles remain mutually parallel.
*
Results and Discussion The chemical structures of MMDPP, MHDPP, and HHDPP differ by the absence or presence of hydrogen (H) and methyl (M) groups, covalently linked to the N-atoms in the pyrrolo[3,4-c]pyrrole skeleton (see Figure 1). Intramolecular hydrogen bonding is feasible between the N-H and the M e 0 groups in MHDPP and HHDPP. The N-Me and M e 0 groups of
0.1
0.0
400
460
600
650
600
650
W a v e l e n g t h (nm) Figure 1. Fluorescence excitation and emission spectra (uncorrected), as well as excitation and emission anisotropies of BDPP, MMDPP, MHDPP, and HHDPP (in the order from top to bottom). The fluorescence anisotropies were obtained at 238 K. The structure formulas are inserted for each set of data.
MMDPP provide steric hindrance for a coplanar arrangement of the aryl moiety and the presumably planar DPP molecule. However, a planar conformation is quite possible in the structurally related BDPP, which is also displayed in Figure 1. Absorption and Fluorescence Spectra. The absorption and fluorescence spectra of the DPP compounds studied in this work are shown in Figure 1. The absorption bands are strong with €(Amax) typically on the order of lo4 dm3 mol-’ cm-I, as can be seen from Table 1. Moreover, the absorption and fluorescence spectra of HHDPP and BDPP are very similar and they indicate mirror symmetry. The Stokes shifts are ca. 10 nm which is a typical value for fluorophores having the same symmetry in the ground and first excited states. The corresponding spectra of MMDPP and MHDPP are significantly different. The vibronic structure is almost lost, and the Stokes shifts are larger, as is evident from Figure 1. These spectral features are not particularly dependent on the polarity of the solvent. A qualitative inspection of the spectra suggests significantly different symmetries or conformations of MMDPP and MHDPP in the ground and first excited states. It is tempting to ascribe these intramolecular changes of conformation to rotations of the aryl groups about the C-C bond formed with the DPP core. The fluorescence and absorption spectra of BDPP and HHDPP are efficiently overlapping, which partly explains the
J. Phys. Chem., Vol. 99, No. 21, 1995 8507
DPPs as a New Class of Fluorescent Molecules
TABLE 1: Molar Absoptivity [e(Amax)],Stokes Shift (AA), Natural Lifetime ( t o ) , Fluorescence Lifetime (t),and Limiting Fluorescence Anisotropy (ro)of the Different DPP Derivatives in DMSO and 1,2-Propanediola (i,,,)/dm3 mol-’ cm-I
E
fluorophore
system
BDPP MMDPP MHDPP HHDPP BDPP MMDPP MHDPP HHDPP
DMSO DMSO DMSO DMSO 1,2-propanediol 1,2-propanediol 1,2-propanediol 1,2-propanediol
a
39 800 (509) 17 200(453) 20 300 (477) 27 600(528)
AIlnm rdns d n s 10 67 59 12
5.8 9.1 7.6 6.3
ro
5.3 6.6 6.5 5.5 5.5 0.381 7.2 7.0 5.5 0.376
The acronyms for the DPP derivatives are 3,6-bis(3,5-di-rerr-
butylphenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (BDPP), 3,6bis(2-methoxyphenyl)-2,5dimethylpyrrolo[3,4-c]pyrrole-1,4dione (MMDPP), 3,6-bis(2-methoxyphenyl)-2-hydro-6-methylpyrrolo[3,4-c]py~ole1,4-dione (MHDPP), and 3,6-bis(2-methoxyphenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione(HHDPP). The M values refer to the difference between the peak maxima of absorption and fluorescence spectra. The radiative lifetimes were calculated from the modified StricklerBerg equation. The fluorescence lifetimes were calculated from deconvolutions of data measured by means of the single photon counting technique. The statistical tests kZ, Durbin-Watson, and null hypothesis) were all accepted for single exponential fluorescence relaxation.
HHDPP
2-0-Me
Figure 2. Schematic illustration of the intramolecular rotation of the aryl group with respect to the DPP core.
relatively large Forster radii of Ro = 46.4 f 1 8, and 48.4 f 0.5 A, respectively. For calculating ROthe fluorescence quantum yields were determined to be @ = 0.92 and 0.88 for BDPP and HHDPP in DMSO. Fluorescence Anisotropy. The fluorescence emission anisotropies of BDDP and HHDPP do not depend on the emission wavelength (see Figure l), which shows that these transitions contain one direction of the electronic transition dipole moment. The limiting anisotropy of ro 0.38, obtained for both compounds in 1,2-propanediol, means that the absorption and emission transition dipoles are nearly parallel. For MMDPP and MHDPP, r(,lem)decreases with increasing wavelength, and this is most pronounced for MMDPP. Such a wavelength dependence is usually observed in viscous systems where the time scales of solvent relaxation and fluorescence relaxation are similar. Under such circumstances the shape of the fluorescence spectrum also depends on the excitation wavelength, and the fluorescence relaxation is multiexponential with positive or negative preexponential factors depending on the emission wavelength. For MMDPP and MHDPP, we observe neither of these characteristics. However, these anisotropy data, as well as the results of the photophysics, are compatible with the presence of intramolecular interactions in the different DDP derivatives, originating from different relative orientations of the planes of the aryl and DPP groups. These planes could be parallel in both HHDPP and BDPP. For HHDPP, a planar configuration can even be stabilized by hydrogen bonding between the functional groups N-H and 0-Me (see Figure 2). Thereby, the resulting overlapping n-systems become larger and qualitatively the energy separation between the SOand S I states should decrease. Indeed, this is what we have observed.
For HHDPP and BDPP the maxima of absorption and fluorescence spectra are typically between 510 and 540 nm. If we take the absorption and fluorescence spectra of the DPP core to be very similar to those of 3,6-dimethyl-2,5-dihydropyrrolo[3,4-c]pyrrole-l,4-dione(3,6MDPP), then the corresponding maxima appear at about 390 and 420 nm, respectively. For MMDPP and MHDPP, steric interactions will hinder the planes of the aryl groups from being parallel with the DPP moiety. The electronic n-overlapping depends on the rotational angle (q)about the C-C bond. Depending on the potential surface for protation, the distribution function K ( 9 ) will have a definite shape, and the electronic transition dipoles, as well as the spectral bandshapes {*A)>, will depend on 9. The potential surface likely contains a number (m) of local minima, each of them representing a particular conformation. For this situation the observed molar absorptivity and fluorescence spectra will be
and
where the subscripts indicate ground (g) and excited (e) states and LZ(A) stands for the spectral bandshape of the fluorescence emission. Provided that there is only one dominating conformation and that Kg K,, one would expect small Stokes shifts, similar shapes of the absorption and fluorescence spectra, as well as equal and ,&independent excitation and emission anisotropies. HHDPP and BDDP respond to these conditions. On the other hand, if there are several conformations and/or the ground and excited state distributions are different (that is, Kg f K,) one should observe different spectral shapes of the absorption and fluorescence spectra and that r(A) is wavelength dependent. For a related compound, 3,6-bis-(2-methylphenyl)2,5-dihydro-2-methylpyrrolo[3,4-c]pyrrolelp-dione the rotational barrier of the aryl groups was found5 to be 74.4 kJ/mol. There are two main conformations of this compound, and this should also be the case for MMDPP. Either the substituents of the aryl groups are on the same side of the chromophore or on the opposite side. This situation is compatible with the results obtained for MMDPP and MHDPP, where both the absorption and fluorescence spectra are blue-shifted, and where the Stokes shifts are significantly larger, as compared to those of HHDPP. The observed blue shift is expected for a less efficient overlapping between the n-systems of the aryl groups and DPP, while the large Stokes shift and the wavelength dependence of r(1) are expected if K g t K,, and/or several conformations are present. DPPs in a Lyotropic Nematic Liquid Crystal. MMDPP, HHDPP, and BDPP were solubilized in a lyotropic nematic phase which aligns in a strong magnetic field. The phase is built up of long rod-shaped micellar aggregates (see Figure 3), and these align macroscopically with their long axis parallel to the applied field, so that a uniaxial liquid crystal is formed.I5 The optical axis is directed parallel to the magnetic field. For HHDPP, we find that the steady-state fluorescence anisotropy, r(Aem), is wavelength independent, while for MMDPP and BDPP r(1) depends on Lem. So far, the results for MMDPP and HHDPP are analogous to those obtained for them in 1,2propanediol (cf. Figure 1). However, for BDPP in the nematic liquid crystalline phase, r(lem)increases with increasing &m as can be seen in Figure 3.
Edman et al.
8508 J. Phys. Chem., Vol. 99, No. 21, 1995
TABLE 2: Ground and Excited State Order Parameters (S, and S,) Obtained from the Reduced Linear Dichroism LD, and the Time-Resolved Fluorescence Anisotropy r(t& Steady-State Fluorescence Anisotropies (rs),as Well as the Fluorescence Lifetimes (z) for Various DPP Derivatives Solubilized in a Macroscopically Aligned Lyotropic Nematic Liquid Crystalline Phasd fluorophore LD, S, dns r, r(t-) = S, BDPP 1.01 i O . 0 7 0.25 i 0.02 6.0 0.399* 0.33 i 0.02 MMDPP 0.63 i 0.04 0.17 i 0.01 7.9 0.163* 0.13 i 0.02 HHDPP Wavelength inmi
I
a
0.16
* 0.01
0.05f 0 . 0 1 6.4 0.099
0.04 A 0.01
The acronyms for the DPP derivatives are 3,6-bis(3,6-di-tert-
butylphenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (BDPP), 3,6bis(2-methoxyphenyl)-2,5-dimethylpyrrolo[3,4-c]pyrrole1,4-dione (MMDPP), and 3,6-bis(2-methoxyphenyl)-2,5-dihydropy~olo[3,4-c]py~o~e-
I’
1,4-dione (HHDPP). The fluorescence relaxation is monoexponential within accepted statistical test values (x2,Durbin-Watson, and null hypothesis). The temperature was 293.7 i 0.5 K.
0
10
20
30
40
50
Time (ns)
Figure 3. The fluorescence spectra (upper graph) of BDPP (A,
-)
and HHDPP (B, - - -) solubilized in a macroscopically aligned lyotropic nematic liquid crystalline (LNLC) phase. The steady-state fluorescence anisotropy of BDPP (A’) and HHDPP (B’) are also shown. The lower graphs illustrate r(t) of BDPP (A”) and HHDPP (B”), as well as the instrumental response function. An aggregate of the LNLC phase is schematically pictured in the upper right corner. The macroscopic orientation of these aggregates, with respect to a laboratory frame X , Y , Z, is also illustrated. The angle /3 denotes the orientation of the electronic transition dipole moment relative to the optical axis (Z).
The time-resolved fluorescence anisotropy, r(t), reports on the local rotational relaxation of the excited fluorophores, as well as their orientation with respect to the optic axis or C, axis. Contributions from reorientational motions of the aggregates on the nanosecond time scale are negligible. In this work the excitation polarization is applied parallel to the C, axis of the aggregates. Hence, the translationaUrotationa1 motions of the fluorophores about the C, axis are not contributing. We find rotational correlation times of I#Q e 1-5 ns, which are typical values for fluorescent probes solubilized in lipid bilayers. More interesting are the residual anisotropy values, r(t,), which report on the orientation of the emission transition dipoIe moment in terms of a second rank order parameter, S, (cf. eq 4). These S, values, all being positive for BDPP, MMDPP, and HHDPP, indicate a preferential orientation of the SI SO transition dipoles parallel with the long axis of the micellar-like aggregates. From LD experiments with the same macroscopically aligned systems, the ground state order parameter, s,, of the SO SI transition dipole moment was determined. These values are reported in Table 2. It is S, significant that S, < Se for MMDPP and BDPP, while S, for HHDPP. These results are compatible with the following interpretation. HHDPP i s rather rigid in the ground and first excited states, due to the intramolecular hydrogen bonding. However, MMDPP may exist in two or more conformations with probabilities of Kg(qm) f Ke(cpm) and with different bandshapes and directions of the transition dipole moments for each conformation. Therefore, it could be expected that r(A) depends on wavelength (see eq 6), and that the second rank
-
-
order parameters of MDPP differ in the ground and excited states. Reconsidering BDPP, why is r(A) constant in liquid solution but wavelength dependent in the liquid crystalline phase? In liquid solutions, BDPP and HHDPP behave in a very similar way, compatible with one conformation where the aryl and DPP planes are essentially parallel. For BDPP in the liquid crystalline phase, the experimental results suggest that the planes of the aryl and DPP moiety are slightly tilted physically originating in the effective local anisotropy of the amphiphile aggregate. A small mutual tilt of the planes is feasible, if the forces which may keep the planes of BDPP parallel are weaker than the intramolecular hydrogen bonding in HHDPP. Photophysics. Time-resolved single photon counting measurements show that the fluorescence relaxation in all systems studied here is single exponential (see Tables 1 and 2 ) . The fluorescence lifetimes of BDDP and HHDPP are slightly shorter than the calculated radiative lifetimes (to)and correspond to fluorescence quantum yields of CP e 0.9, which are in excellent agreement with the measured CP values. The fluorescence lifetimes of MMDPP and MHDPP are significantly longer than those of BDDP and HHDPP, as well as the calculated to values. The latter suggests slightly smaller quantum yields. In this context one must keep in mind that the absence of mirror symmetry of absorption and fluorescence spectra makes the applicability of the modified Strickler-Berg equation questionable. Provided that different conformations of a fluorophore are present, one might expect, contrary to what we observe here, a multiexponential fluorescence relaxation, where each conformation represents a state with a lifetime of .z, However, a single exponential decay could still be observed if the lifetime of each configuration (m) is much shorter than zm. A correlation time on the order of 10 is typical for intramolecular rotation of two aromatic moieties about a C-C bond. The rates of rotation in MMDPP are likely much faster than l/z, Ut, which justifies the our experimental finding of a single fluorescence lifetime.
Concluding Remarks For all DPP derivatives studied in liquids and in a liquid crystalline phase, the fluorescence relaxation is monoexponential. With the exception of HHDPP, the fluorescence anisotropy of the SO S I transitions depends significantly on wavelength. Intramolecular hydrogen bonding of HHDPP would restrict any rotation of the aromatic planes. This conclusion is compatible with the finding of very similar ground and excited state order parameters of HHDPP, while for BDPP and MMDPP, the discrepancies of these values are expected if the molecular
-
DPPs as a New Class of Fluorescent Molecules geometry is different in ground and excited states. In fluorescence depolarization studies it is preferable to work with rigid probes. Therefore in such applications, HHDPP is the best suited probe among the DPP derivatives studied in this work.
Acknowledgment. We are grateful to Mrs. Eva Vikstom for skillful technical assistance. This work was supported by the Swedish Natural Research Council, Deutsche Forschungsgemeinschaft, and Fonds der Chemischen Industrie. References and Notes (1) Farnum, D.G.; Metha, G.; Moore, G . I.; Siegal, F. G. Tetrahedron Lett. 1974, 15, 2549.
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