7754
J. Phys. Chem. A 1998, 102, 7754-7760
Dual Fluorescence and Multiple Charge Transfer Nature in Derivatives of N-Pyrrolobenzonitrile Claudia Cornelissen-Gude and Wolfgang Rettig* W. Nernst Institut fu¨ r Physikalische und Theoretische Chemie, Humboldt-UniVersita¨ t zu Berlin, Bunsenstr. 1, D-10117 Berlin, Germany ReceiVed: February 26, 1998; In Final Form: May 27, 1998
N-Pyrrolobenzonitrile (PBN), its ester derviative PBAEE, and a more twisted model compound DPBN have been compared regarding absorption and fluorescence properties. A long wavelength absorption shoulder in DPBN has been assigned to the charge transfer (CT) state. In polar solvents, single-fluorescence bands with a strong solvatochromic effect establish a CT nature of the emission for all compounds. The long radiative lifetime (ca. 130 ns for PBN, 700 ns for DPBN in CH2Cl2) points to a forbidden emission, and the 5-fold longer value for DPBN indicates a difference in CT nature tentatively assigned to conformations differing in the twist angle. Even in low-temperature polar solvent glasses, the single-fluorescence band of PBN is of CT nature and develops into a dual fluorescence by thermal activation. Also in alkanes, the increased fluorescence rate constants and the temperature effect on spectral structuring indicate emission from an equilibrium involving a CT state with an unstructured spectrum and a less polar locally excited state with structured emission.
1. Introduction
SCHEME 1
Spectroscopic studies of donor/acceptor substituted benzenes often reveal the phenomenon of dual fluorescence or exhibit emission bands with anomalously large Stokes shifts. In general, large Stokes shifts can be produced by molecules possessing excited-state relaxation processes where large amplitude motions are involved. TICT (twisted intramolecular charge transfer) compounds represent a particular class of molecules exhibiting dual emission or anomalous Stokes shifts.1-4 According to the TICT model, the fluorescence originates from the primarily excited “locally excited” (LE) state as well as from a charge transfer (CT) state accessible only by an adiabatic photoreaction from LE, which includes a rotational motion around the bond linking the donor and acceptor moieties. If there is no energy barrier separating the precursor and product species, the excited-state relaxation can occur extremely rapidly. In this case, only long wavelength emission from the CT product state may be observable. This TICT emission process is usually connected with a reduced value of the transition moment due to the small π-overlap of the strongly twisted arrangement of chromophores.1,4,5 Dimethylaminobenzonitrile (DMABN, Scheme 1) represents the prototype TICT compound exhibiting the phenomenon of dual fluorescence.1,6 Furthermore, a number of derivatives show comparable spectroscopic features.3,4,7 In this paper we want to draw the parallels to the closely related pyrrolobenzonitriles, which instead of a saturated dimethylamino group possess the even better donor moiety pyrrol. We want to present a detailed investigation of the fluorescence spectra and corresponding photophysical data at various temperatures of the pyrrolobenzonitriles depicted in Scheme 1b-d. The chemical structure of the compounds investigated (bd) resembles on one hand the TICT system DMABN (a), and on the other hand their donor/acceptor capabilities are comparable with those of PBMes2 (e), a compound which shows an enormous Stokes shift even in nonpolar solvents.8,9 PBMes2
differs from the investigated pyrrolobenzonitriles by the strength of the acceptor unit, which is diminished for the latter compounds.10 The investigation of the spectroscopic behavior of PBN, PBAEE, and DPBN includes a variation of the polarity as well as temperature-dependent effects. Previous studies at room temperature for some of the pyrrolobenzonitriles revealed the connection with donor/acceptor strength and the twist angle.7,11,12 On the other hand, due to the particular electronic properties of pyrrol with a node through N in the highest molecular orbital (HOMO), special features can be expected and large charge separation is even possible for planar geometries.13 2. Experimental Section The compounds were synthesized using the procedures described in refs 11 and 12. The absence of possible traces of
S1089-5639(98)01306-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/12/1998
Dual Fluorescence in N-Pyrrolobenzonitrile Derivatives
J. Phys. Chem. A, Vol. 102, No. 40, 1998 7755
impurities was confirmed by high-performance thin layer chromatography (HPTLC).14 Absorption spectra were measured on a Cary 17 spectrometer, and quantum-corrected fluorescence spectra (concentrations < 10-4 M) were measured on a PerkinElmer 650-60 fluorimeter. Basic Blue 3 15 was used as quantum counter to extend the correction range of the emission up to 700 nm. Fluorescence quantum yields were determined relative to a solution of quinine bisulfate in 0.1 N H2SO4 (φf ) 0.515)16 and corrected for the refractive index of the solvents. Fluorescence spectra, measured in aerated solutions, were independent of concentration (o.d. ≈ 0.05-0.8) and excitation wavelength. The determination of low-temperature fluorescence quantum yields took into account the temperature dependence of the refractive index as well as the increasing solvent contraction (density increase) with decreasing temperature. Fluorescence decay times of the aerated solutions were determined with the time-correlated single-photon counting (SPC) technique17 using equipment described in detail elsewhere.18 Comparative measurements with polarization filters set to the magic angle position were carried out but were not used as standard condition because of the low intensity of the emission. Synchrotron radiation from the Berlin electron storage ring BESSY operating in the single bunch mode was used for excitation.18 The decay times were fitted using the iterative reconvolution procedure (Marquard algorithm), which allowed a time resolution down to 0.1 ns and a relative precision of about 0.1 ns. Solvents for fluorescence spectroscopy were of Merck Uvasol quality, with the exception of butyronitrile, and were used without further purification. Butyronitrile was purified by column chromatography (Al2O3) and repeated vacuum distillation.
Figure 1. Absorption and fluorescence spectra of (a) PBN and (b) PBAEE at room temperature in solvents of different polarity.
3. Results 3.1. Room-Temperature Data. The absorption and emission spectra of PBN and PBAEE are almost identical in the amount of Stokes shift, whereas the fluorescence data of the sterically hindered dimethyl-substituted compound DPBN differ substantially with strongly red-shifted spectra even in nonpolar and weakly polar solvents (Figures 1 and 2). The fluorescence of all three compounds shows significant solvatochromic effects, indicating a high charge transfer character of the emitting state. The absorption bands, however, do not shift significantly in solvents of increasing polarity. Despite the fact that the methyl substituents in ortho position of the pyrrole moiety enhance the donor capability,9,11,12 the main absorption maximum of DPBN is clearly blue-shifted in comparison to the band maxima of PBN and PBAEE. The molecular absorption coefficients were determined for -1 -1 PBAEE and DPBN in n-hexane as (λmax 286 ) ) 22 000 M cm -1cm-1 respectively, which differs by and (λmax ) ) 10 000 M 275 a factor of about 2. The blue shift and the reduction of absorption intensity are typical for twisted compounds. Using Webster’s approach19 we can derive the ground-state twist angle 〈φ〉 by using eq 1.
/Ref ) cos2〈φ〉
(1)
With PBAEE as the reference compound, 〈φ〉 is found to be 48° for DPBN. This value corresponds well to the twist angles of 50° determined by photoelectron spectroscopy.12 In contrast to PBN, a weak shoulder in the red edge of the absorption
Figure 2. Absorption and fluorescence spectra of DPBN at room temperature in solvents of different polarity.
spectra can be observed for DPBN, which can be interpreted as a hint for a hidden transition. The fluorescence decay curves are monoexponential, which allows the evaluation of radiative and nonradiative rate constants according to eqs 2 and 3. In eq 2, ktot nr corresponds to the sum
kf ) φf/τf
(2)
-1 ktot nr ) kf(φf - 1)
(3)
of all nonradiative processes including triplet formation. The measured data and calculated photophysical rates are collected in Table 1a-c. The comparison of spectra and rate constants of PBN and PBAEE exhibits rather similar features except in nonpolar solvents such as n-hexane and in protic media like water and alcohols. Whereas the emission spectrum of PBAEE in n-hexane shows clearly some vibronic structure, that of PBN
7756 J. Phys. Chem. A, Vol. 102, No. 40, 1998
Cornelissen-Gude and Rettig
TABLE 1: Spectral and Photophysical Data at Room Temperature for (a) PBN, (b) PBAEE, and (c) DPBN in Solvents of Different Polarity λem max/nm
solvent
a
∆νSt/cm-1
φf
τf/ns
kf/s-1
knr/s-1
n-hexane diethyl ether dichloromethane acetonitrile
348 404 426 482
6200 10 200 11 500 14 200
(a) PBNa 0.028 0.022 0.022 0.036
2.42 3.80 3.10 8.20
1.16 × 107 0.57 × 107 0.72 × 107 0.43 × 107
4.0 × 108 2.6 × 108 3.2 × 108 1.2 × 108
n-hexane diethyl ether dichloromethane acetonitrile
332 414 434 490
4800 10 800 11 900 14 600
(b) PBAEEa 0.033 0.032 0.036 0.029
1.36 5.60 4.50 5.90
2.41 × 107 0.57 × 107 0.79 × 107 0.49 × 107
7.1 × 108 1.7 × 108 2.1 × 108 1.6 × 108
n-hexane dibutyl ether diethyl ether dichloromethane acetonitrile
420 450 480 530
12 600 14 100 15 500 17 500
(c) DPBNb 0.006 2.9 1.9 × 106 0.010 7.1 1.5 × 106 0.018 12.9 1.4 × 106 0.014 10.7 1.3 × 106 no fluorescence observable at 298 K (φf < 0.001)
3.4 × 108 1.4 × 108 0.8 × 108 0.9 × 108
b exc λexc max ) 286 nm. λmax ) 275 nm.
TABLE 2: Spectral and Photophysical Data of PBN and PBAEE in Alcohols at 298 K solvent
compd
λexc max/nm
φf
τf/ns
knr/108 s-1
HexOH
PBN PBAEE PBN PBAEE PBN PBAEE PBN PBAEE
449 486 468 494 478 496 493 520
0.043 0.011 0.029 0.004 0.022 0.002 0.008 173 K as is shown experimentally by the monoexponential fluorescence decay in this temperature range. For DPBN, the CT state is energetically more favored, due to the stronger donor capability of the ortho methyl substituted pyrrolobenzonitrile compound. As a consequence, the LE state is no longer thermally populated in nonpolar solvents and the vibronic features in hexane are absent. Acknowledgment. Support by the Deutsche Forschungsgemeinschaft (Sfb 337) and by the Bundesministerium fu¨r Forschung und Technologie (Projects 05 414 FAB1 and 05 5KT FAB9) is gratefully acknowledged. References and Notes (1) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, D. J.; Baumann, W. NouV. J. Chim. 1979, 3, 443. (2) Lippert, E.; Rettig, W.; Bonacic-Koutecky´, V.; Heisel, F.; Miehe´, J. A. AdV. Chem. Phys. 1987, 68, 1. (3) Rettig, W. Modern Models of Bonding and Delocalisation; Liebman, J. Greenberg, A., Eds.; VCH: New York, 1988; p 229. (4) Rettig, W. Electron Transfer I. In Topics in Current Chemistry; Mattay, J., Ed.; Springer-Verlag, Berlin, 1994; Vol. 169, p 253. (5) Van der Auweraer, M.; Grabowski, Z. R.; Rettig, W. J. Phys. Chem. 1991, 95, 2083. (6) Lippert, E.; Lu¨der, W.; Boos, H. AdV. Mol. Spectrosc. Proc. Int. Meet. 4th 1959, 1962, 443. (7) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971. (8) Britelli, D. R.; Eaton, D. F. J. Phys. Org. Chem. 1989, 2, 89. (9) Cornelissen-Gude, C.; Rettig, W.; Bonacic-Koutecky´, V. Manuscript in preparation. (10) Schulz, A.; Kaim, W. Chem. Ber. 1989, 122, 1863. Lequan, M.; Lequan, R. M.; Ching, K. C. J. Mater. Chem. 1991, 1, 997. (11) Rettig, W.; Marschner, F. NouV. J. Chim. 1983, 7, 425. (12) Rettig, W.; Marschner, F New J. Chem. 1990, 14, 819. (13) Rettig, W. J. Mol. Struct. 1982, 84, 303. (14) Frey, H.-P.; Zieloff, K. QualitatiVe and quantitatiVe Du¨ nnschichtchromatographie; VCH: Weinheim, 1993. Bauer, K.; Gros, Leo; Sauer, W. Du¨ nnschicht Chromatographie -Einfu¨ hrung; Merck, Fa., Hu¨thig, Dr. A., Eds.;Verlag: Weinheim, 1989. (15) Heinze, J.; Kopf, U. Anal. Chem. 1984, 56, 1931. (16) Meech, S. R.; Phillips, D. J. Photochem. 1983, 23, 193. Velapoldi, R. A.; Epstein, M. S. ACS Symp. Ser. 1989, 383, 98.
7760 J. Phys. Chem. A, Vol. 102, No. 40, 1998 (17) O’Connor, D. V.; Phillips, D. Time Correlated Single Photon Counting; Academic Press: London, 1984. (18) Vogel, M.; Rettig, W. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 1241. (19) Braude, E. A.; Sondheimer, J. J. Chem. Soc. 1955, 3754. Wepster, B. M. Recueil TraV. Chim. Pays-Bas 1957, 76, 335 and 357. (20) Cornelissen, C.; Rettig, W. J. Fluorescence 1994, 4, 71. (21) Lippert, E. Z. Naturforschung 1955, 10a, 541. Mataga, N.; Kaifu, Y.; Kazumi, M. Bull. Chem. Soc. Jpn. 1955, 28, 690; 1956, 29, 465. (22) Ro¨sch, N.; Zerner, M. C. J. Phys. Chem. 1994, 98, 5817. (23) Lumbroso, H.; Bertin, D. M.; Marschner, F. J. Mol. Struct. 1988, 178, 187.
Cornelissen-Gude and Rettig (24) Al-Hassan, K. A.; Rettig, W. Chem. Phys. Lett. 1986, 126, 273. Al-Hassan, K. A.; Azumi, T.; Rettig, W. Chem. Phys. Lett. 1993, 206, 25. (25) Klevens, H. B.; Platt, J. P. J. Am. Chem. Soc. 1949, 71, 1714. (26) Rettig, W.; Wermuth, G.; Lippert, E. Ber. Bunsen-Ges. Phys. Chem. 1979, 83, 692. Wermuth, G. Z. Naturforsch. 1983, 38a, 368. Wermuth, G.; Rettig, W. J. Phys. Chem. 1984, 88, 2729. (27) Suzuki, H. Electronic Absorption Spectra and Geometry of organic Molecules; Academic Press: New York, 1964. (28) Maus, M.; Rettig, W. Chem. Phys. 1997, 218, 151. (29) Swiatkowski, G.; Menzel, R.; Rapp, W. J. Lumin. 1987, 37, 183. (30) Braun, D.; Rettig, W. Chem. Phys. Lett. 1997, 268, 110.
