Photophysical Study of Electron-Transfer and Energy-Hopping

J. Phys. Chem. B 2004, 108, 10721-10731

10721

Photophysical Study of Electron-Transfer and Energy-Hopping Processes in First-Generation Mono- and Multichromophoric Triphenylamine Core Dendrimers Marc Lor,† Lucien Viaene,† Roberto Pilot,† Eduard Fron,† Sven Jordens,† Gerd Schweitzer,† Tanja Weil,‡ Klaus Mu1 llen,‡ Jan W. Verhoeven,† Mark Van der Auweraer,† and Frans C. De Schryver*,† Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200 F, 3001 HeVerlee, Belgium, and Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany ReceiVed: March 3, 2004; In Final Form: May 5, 2004

The combination of transient absorption and fluorescence experiments performed on a femtosecond to nanosecond time scale was used to characterize the electron-transfer process in mono- (N1P1) and multichromophoric (N1P3) triphenylamine core dendrimers carrying one and three peryleneimide electron acceptor chromophores, respectively. Comparison of the monochromophoric N1P1 to the multichromophoric N1P3 allowed us to investigate the influence of the number of chromophores upon the electron transfer kinetics. The solvent effect on the electron transfer process was investigated by comparing the results obtained in the highly polar solvent benzonitrile with those in the less polar solvents as diethyl ether, 2-methyltetrahydrofuran, tetrahydrofuran, and toluene. By means of fluorescence anisotropy and transient absorption anisotropy experiments, the occurrence of energy hopping in the multichromophoric N1P3 has been demonstrated. Furthermore singlet-singlet annihilation, observed earlier in a model compound without an electron donor moiety, was confirmed in the multichromophoric N1P3.

Introduction Because of the wide range of possible applications of dendrimers in several fields,1-10 the investigation of the excitedstate properties of these macromolecules has drawn a lot of attention. Recently the occurrence and characterization of photoinduced charge separation and charge recombination processes in a monochromophoric peryleneimide-substituted (PI) polyphenylene dendrimer with a triphenylamine (TPA) electron donor core (N1P1) in solvents of medium polarity has been reported.11,12 To investigate energy hopping and possibly charge hopping in dendrimers with a triphenylamine core, a molecular system decorated with three PI chromophores (N1P3) at the rim was investigated and compared with a similar structure where the nitrogen core was replaced by an sp3 carbon core (Chart 1). Experimentally, the nature of the excited-state interactions between the PI chromophores can be investigated by fluorescence and transient absorption depolarization measurements. Application of anisotropy experiments to multichromophoric systems13-19 allows us to observe intramolecular energy hopping and charge hopping because these processes are accompanied by the reorientation of the transition dipole, resulting in depolarization of the emission (and transient absorption). The photoinduced electron-transfer process in the mono(N1P1) and multichromophoric (N1P3) dendrimers in the solvent benzonitrile is characterized by steady-state and nanosecond to femtosecond time-resolved spectroscopy. A comparison is made with results obtained in toluene, diethyl ether, tetrahydrofuran (THF), and 2-methyltetrahydrofuran (MeTHF). To attribute the different kinetic components and spectral features, the data reported previously on the compounds m-C1P1 * Corresponding author: e-mail [email protected]. † Katholieke Universiteit Leuven. ‡ Max-Planck-Institut fu ¨ r Polymerforschung.

and m-C1P3, in which no photoinduced electron transfer can occur, were compared to the data on triphenylamine core dendrimers. Results and Discussion Stationary Measurements. The normalized steady-state absorption and emission spectra of N1Px (x ) 1, 3) and m-C1Px (x ) 1, 3) in benzonitrile are displayed in Figure 1. The quantum yield of fluorescence (ΦF) drops significantly in benzonitrile compared to less polar solvents, to 0.03 and 0.07 for the triphenylamine core dendrimers N1P1 and N1P3, respectively, indicating the occurrence of an efficient quenching process in this medium. Also, for the corresponding model compounds a modest drop in fluorescence quantum yield is observed in benzonitrile (ΦF ) 0.89 for m-C1P1 and ΦF ) 0.82 for m-C1P3), in contrast to the fluorescence quantum yield of 1 observed for the model compounds in other solvents.11,12 The normalized steady-state absorption spectra of the triphenylamine core dendrimers are independent of the number of chromophores. In the emission spectra of the model compounds a broadening is observed in the multichromophoric m-C1P3 compared to the monochromophoric m-C1P1 due to a small fraction of excimer-like emitting species.17 Low-Temperature (77 K) Fluorescence Measurements. To allow a comparison with N1P1,11 nanosecond fluorescence experiments were performed on N1P3 in MeTHF at 77 K. The result is shown in Figure 2. Under those conditions, one can observe two distinct emitting species. At delay times below 100 ns, one observes fluorescence with the characteristics of PI, while at longer (microsecond) times the spectrum is converted to a structureless band with a maximum around 620 nm. This behavior was also observed for N1P1 in MeTHF at 77 K.11 The observed emission at micro-

10.1021/jp0490352 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/17/2004

10722 J. Phys. Chem. B, Vol. 108, No. 30, 2004

Lor et al.

CHART 1: Molecular Structures of N1Px (x ) 1, 3) and the Corresponding Model Compounds m-C1Px (x ) 1, 3)

Figure 1. Normalized steady-state absorption and emission spectra for N1P1 (s), N1P3 (9), m-C1P1 (2), and m-C1P3 (0) in benzonitrile. The emission spectra were excited at a wavelength of 495 nm.

Figure 2. Emission spectra of N1P3 in MeTHF at 77 K for different delay times after excitation: delay ) 0 ns, gate ) 10 ns (s); delay ) 100 ns, gate ) 100 ns (2); delay ) 1 µs, gate ) 1 µs (+); and delay ) 5 µs, gate ) 5 µs (9).

second delay times can, as in the case of N1P1,11 be attributed to radiative charge recombination luminescence emerging from the long-lived ion-pair state in MeTHF. Although the long distance makes this radiative recombination [which requires the overlap between the highest occupied molecular orbital (HOMO)

of the triphenylamine moiety and the lowest unoccupied molecular orbital (LUMO) of PI] slow, it slows down at the same time nonradiative decay to S0, which is furthermore hindered by unfavorable Franck-Condon factors. SPT Measurements: Monochromophoric Dendrimers N1P1 and m-C1P1. The fluorescence decays of N1P1 in benzonitrile were determined by single photon timing (SPT) at 488 nm excitation wavelength. To capture the different kinetic components as precisely as possible, the measurements were performed in a long (25 ns) and a short (8 ns) time window. The short time window corresponds to 2 ps/channel and the long time window corresponds to 6 ps/channel-1. While at 580 and 620 nm the fluorescence of the model compound m-C1P1 decays monoexponentially (4.2 ns), the corresponding fluorescence decay of N1P1 in benzonitrile requires a three-exponential decay. The decay times (partial amplitudes) obtained were 80 ps (0.47), 170 ps (0.52), and 3.5 ns (0.005). The two subnanosecond components are the prompt fluorescence components, indicating a charge-transfer process of the two constitutional isomers of N1P1, as also observed in solvents of lower polarity.11,12 However, a delayed fluorescence component is not observed in the solvent benzonitrile, in contrast to the solvents of lower polarity.11,12 The reason for the lack of delayed fluorescence becomes clear when the free enthalpy of the charge transfer state is estimated by use of the Weller approximation.20 An estimate of the free enthalpy of an ion-pair state can be obtained by applying eq 1,20 where the enthalpy is estimated for an ionpair state ∆GD+A- (in kilocalories per mole). The ion-pair state is considered as a solvent-separated ion pair with a center-tocenter distance R (in angstroms) and effective radii r (in angstroms) equal for donor (D) and acceptor (A), submerged in a dielectric continuum with relative permittivity s. s′ is the dielectric constant of the solvent in which Eox (D) and Ered (A) are measured:

∆GD+A- ) 23Eox (D) - 23Ered (A) 331.2/R + (331.2/r)(1/s′ - 1/s) (1) where Eox (D ) TPA) ) +0.98 V21 and Ered (A ) PI) ) -1.1 V.22 For the calculation, an average value of 5 Å for the ionic

Photophysical Study of Triphenylamine Core Dendrimers

J. Phys. Chem. B, Vol. 108, No. 30, 2004 10723

TABLE 1: Overview of Different Spectroscopic Techniques and Measured Decay Times with the Corresponding Processes polarization compound

solvent

magic angle

anisotropy

process

Single Photon Timing benzonitrile toluene benzonitrile toluene

80 ps/170 psa

N1P1

THF

N1P3

THF

4 ps 80 ps/186 ps 4 ps 86 ps/255 ps

N1P1

benzonitrile

N1P1 N1P3

benzonitrile N1P3

benzonitrile benzonitrile

N1P1 N1P3 a

MeTHF MeTHF

840 ps 90 ps/180 psb 104 ps 1.08 ns

quenched fluorescence rotation of overall molecule quenched fluorescence incoherent energy hopping rotation of overall molecule

Femtosecond Fluorescence Upconversion VR quenched fluorescence VR + S1-S1 annihilation quenched fluorescence

Femtosecond Transient Absorption VR formation of PI radical anion decay of PI radical anion to GS 32 ps formation of TPA cation nanosecond component rotation of overall molecule 3 ps/15 ps VR + S1-S1 annihilation 100 ps formation of PI radical anion nanosecond component decay of PI radical anion to GS 60 ps/250 ps incoherent energy hopping and/or formation of TPA cation nanosecond component rotation of overall molecule

4 ps/25 ps 100 ps 700 ps

37 ns 53 ns

Nanosecond Transient Absorption delayed fluorescence delayed fluorescence

3.5 ns impurity fluorescence. b 3.9 ns impurity fluorescence.

radius r has been used. The center-to-center distance of the two constitutional isomers of N1P1 equals 15 and 17 Å for the para,meta,meta isomer and para,para,meta isomer, respectively, as reported earlier.1 In this framework the free enthalpy for the charge separation reaction can be obtained from20

∆GCS ) ∆GD+A- - E00 (A)

(2)

where E00 (A) denotes the energy of the locally excited acceptor state [E00 (PI) ) 58 kcal/mol]. By use of eqs 1 and 2, ∆GCS for N1P1 in benzonitrile was estimated as -10 kcal/mol, which is 9 kcal/mol larger than the value found in diethyl ether. This means that the rate constant for the reverse electron transfer must be 2 × 107 times smaller than that for the forward transfer. As the latter is about 1 × 1010 s-1, the rate constant for the reverse electron transfer will be close to 500 s-1, suggesting that it is impossible to observe delayed fluorescence in benzonitrile. The nanosecond component observed for N1P1 in benzonitrile can possibly be attributed to a fraction of the molecules of N1P1 that underwent an oxidation of the amine, which is hence no longer able to quench the peryleneimide fluorescence. Furthermore, the very minor component of 4 ns corresponds in no way with the 700 ps decay time of the transient absorption, attributed to of the radical ion pair (vide infra). Multichromophoric Dendrimers N1P3 and m-C1P3. By comparison of N1P1 with the multichromophoric compound N1P3, the influence of the number of chromophores on the electron-transfer kinetics can be investigated. The fluorescence decays of N1P3 in benzonitrile were also measured in a long (25 ns) and a short (8 ns) time window. The short time window corresponds to 2 ps ch-1 and the long time window corresponds to 6 ps ch-1. While the fluorescence of the model compound m-C1P3 in benzonitrile decays monoexponentially with a decay time of 4.2 ns, a three-exponential decay is also required for the analysis of the fluorescence decays

of N1P3 in benzonitrile at 580 and 620 nm. The decay times (partial amplitudes) obtained were 90 ps (0.55), 180 ps (0.40), and 3.9 ns (0.04). In analogy to N1P1 (vide supra), the two fast fluorescence decay components can be attributed to electron abstraction from the amine core. An overview of the decay times has been compiled in Table 1. As far as the fluorescence decays measured by SPT, similar data are obtained for N1P1 and N1P3. SPT data indicate, as well for the monochromophoric as the multichromophoric amine core dendrimers, two forward rate constants, 1.3 × 1010 s-1 (1.1 × 1010 s-1) and 5.9 × 109 s-1 (5.6 × 109 s-1) for N1P1 (N1P3), of electron transfer related to the previously presumed two constitutional isomers due to the synthesis.11,12 Femtosecond Fluorescence Upconversion Measurements. To reveal possible ultrafast processes such as vibrational relaxation and singlet-singlet annihilation, occurring on a time scale less than 20 ps, and to validate the presence of two prompt fluorescence components, femtosecond fluorescence upconversion experiments were performed. Triphenylamine Core Dendrimers N1P1 and N1P3. Also in THF, two fluorescence decay times smaller than that of the model compound are retrieved for N1P1 and N1P3, as shown by femtosecond fluorescence upconversion experiments performed in THF under magic-angle polarization. By global analysis, four decay components were retrieved for both N1P3 and N1P1 in THF. The wavelength-dependent amplitudes of the different decay components are depicted in Figure 3. The 86 and 255 ps components of N1P3 have similar amplitude and wavelength dependence as the 80 and 186 ps components observed for N1P1 in THF. They are both attributed to the decay of direct fluorescence of the isomers of N1P3 quenched by intramolecular electron transfer. Two similar quenched fluorescence decay times (80 and 186 ps) are also obtained for the monochromophoric compound N1P1. From the shape and the positive/negative amplitude behavior, the 4 ps component of N1P1 can be attributed to vibrational and/or


Figure 3. Wavelength dependence of the partial amplitudes of the decay times of N1P1 and N1P3 in THF determined by femtosecond fluorescence upconversion: 4 ps components of N1P1 (9) and N1P3 (0), 80 ps component of N1P1 (b), 86 ps component of N1P3 (O), 186 ps component of N1P1 (2), and 255 ps component of N1P3 (4).

Figure 4. Dependence of the partial amplitudes of the different kinetic components of N1P3 in THF, determined by femtosecond fluorescence upconversion from the laser excitation power at detection wavelengths 590 nm [4 ps component (0), 86 ps component (O), and 255 ps component (4)] and 630 nm [4 ps component (9), 86 ps component (b), and 255 ps component (2)].

solvent relaxation in the electronically excited state of the PI chromophore.23-25 In the multichromophoric compound N1P3, the same decay component of 4 ps is retrieved. However, as shown in Figure 3, a positive offset of the partial amplitude curve of the 4 ps decay component of N1P3 compared to that of N1P1 is observed. This offset indicates that at least one other process leading to the decay of the S1 population contributes to the 4 ps decay component of the multichromophoric compound N1P3 as opposed to the monochromophoric compound N1P1. Results obtained for the model compound m-C1P3 suggested that this process could be singlet-singlet annihilation. To be able to separate these two processes, a series of excitation powerdependent measurements were performed at two different fluorescence detection wavelengths. As shown in Figure 4, for the 4 ps component of the multichromophoric N1P3, the typical intensity dependence of an annihilation process is observed,24,26 while the amplitudes of the 86 and 255 ps components show no dependence on the excitation power. Therefore, the two channels contributing to the 4 ps decay component of N1P3 can be attributed to a intramolecular relaxation process (vide supra) and an intramolecular singletsinglet annihilation process. The fourth decay component revealed by the analysis varies between 500 fs at short fluorescence detection wavelengths and 2 ps at longer fluorescence detection wavelengths. This ultrashort componentsnot displayed in Figure 3swas also observed in the mono- and

Lor et al. multichromophoric model compounds24 and was attributed to an intramolecular vibrational redistribution process in the electronically excited state of the chromophore.27 Furthermore, the nanosecond component as mentioned in the SPT results is observed here also, yet with a very small amplitude. The femtosecond fluorescence upconversion results confirm the rate constants of electron transfer obtained by SPT. Moreover, these data suggest by the behavior of the 4 ps component the presence of singlet-singlet annihilation in N1P3. Transient Absorption Measurements Performed under Magic-Angle Polarization Conditions. To give direct proof for the formation of a radical ion pair, transient absorption experiments were performed. Monochromophoric Dendrimers N1P1 and m-C1P1. In the picosecond transient absorption spectrum of the model compound m-C1P1 in benzonitrile (Figure 5A), two different bands can be observed. Both bands can be seen immediately after excitation and decay on a nanosecond time scale. The negative band can be attributed to ground-state depletion and stimulated emission. The positive band is the S1-Sn absorption band of the peryleneimide chromophore (PI). In m-C1P1 a very small hypsochromic shift from 645 to 640 nm is observed (see Figure 5B) in the first few picoseconds. This small shift in the S1-Sn absorption band of the model compound, where no electron transfer is possible, can be attributed to vibrational and mainly solvent relaxation (vide infra). The much smaller hypsochromic shift observed in toluene28 suggests that the solvent relaxation is the major process. Performing nanosecond transient absorption experiments on m-C1P1 revealed no formation of a triplet or other nano- or microsecond transient. Also in the picosecond transient spectra of N1P1, a small hypsochromic shift (see Figure 6A) due to vibrational and/or solvent relaxation processes in the electronically excited state of the peryleneimide chromophore23,29-32 has been observed in the first few picoseconds. However, in the transient absorption spectra of N1P1, changes also occur on a 100 ps time scale where nothing (no shift) occurs in the model compound m-C1P1. These findings are shown in Figure 6B. On a 100 ps time scale a decrease of the S1-Sn absorption band and the S1-S0 induced emission band of the peryleneimide chromophore and an increase of a new absorption band situated hypsochromic of the S1-Sn absorption band has been observed in this transient absorption spectrum of N1P1. These observations are very similar to the ones observed in previous measurements on N1P1 in the solvents THF and MeTHF.12 Hence this new band can unambiguously be attributed to the radical anion absorption band of the peryleneimide. The maximum of this newly formed band is situated at 620 nm. As shown in Figure 6C, this PI radical anion band decreases on a much faster time scale in benzonitrile compared to solvents of lower polarity.12 The value of the PI radical anion band can be calculated from the concentration of excited states. The concentration of the initially excited state (Franck-Condon state) itself can be calculated from the known value ( ) 38 000 L mol-1 cm-1)33 of the ground state absorption, by making the reasonable assumption that at 520 nm only ground-state bleaching is contributing to the transient signal. This results in a value of at least 33570 L mol-1 cm-1 at 620 nm, the maximum of the PI radical anion absorption band. To obtain the different kinetic components, all monochromatic transient absorption traces were globally analyzed over the three



Figure 5. Three-dimensional display of the transient absorption spectra of m-C1P1 recorded in benzonitrile in 1400 ps (A) and 50 ps (B) time windows.

Figure 6. Three-dimensional display of the transient absorption spectra of N1P1 recorded in benzonitrile in 50 ps (A), 420 ps (B), and 1400 ps (C) time windows.

time windows (50, 420, and 1400 ps; Figure 7). Global analysis was done for decays obtained at 43 wavelengths in each time window, resulting in a global data set consisting of 129 decay traces for each dendrimer under investigation. In this way three kinetic components (4 ps, 25 ps, and a nanosecond component) were retrieved in the model compound m-C1P1. In N1P1, besides the 4 and 25 ps, also a 100 ps and a 700 ps component are found, as shown in Figures7 and 8. The amplitude of this extra component is negative over the wavelength region between 545 and 635 nm. This indicates that this component must be attributed to stimulated emission of the primary excited species (S1) or to transient absorption of a species formed with a 100 ps time constant from the primary excited species. Furthermore in this wavelength range the molar extinction coefficient of this species must be larger than that of the S1-Sn absorption. At longer wavelengths the difference in

molar extinction coefficient gets smaller, leading to a less negative amplitude of the 100 ps time constant. The resulting amplitude spectra of N1P1 and m-C1P1 are depicted in Figures8 and 9. The 100 ps component, which is not present in m-C1P1, is therefore attributed to the formation of the PI radical anion absorption band. The 100 ps component is the average rise time over the two constitutional isomers of N1P1 as indicated by SPT and femtosecond fluorescence upconversion experiments.11,12 More important, a previously not observed 700 ps component is found that can be attributed to the decay of the radical anion absorption band of the PI. As the ground-state depletion band and the PI radical anion band decay with the same rate constant (see Figures 6C and 8), it can be concluded that the PI radical anion band decays mainly to the ground state and not to the triplet state.


Lor et al.

Figure 9. Wavelength dependence of the partial amplitudes of the short decay times of N1P1 and m-C1P1 in benzonitrile determined by femtosecond transient absorption: 4 ps components of N1P1 (b) and m-C1P1 (O) and 25 ps components of N1P1 (9) and m-C1P1 (0).

Figure 7. Time-resolved monochromatic transient absorption traces of N1P1 and m-C1P1 at 620 nm probe wavelength in three different time windows of 1400 ps (A), 420 ps (B), and 50 ps (C).

Figure 8. Wavelength dependence of the partial amplitudes of the long decay times of N1P1, N1P3, and m-C1P1 in benzonitrile determined by femtosecond transient absorption: 100 ps components of N1P1 (0) and N1P3 (9), 700 ps component of N1P1 (O), 1500 ( 500 ps component of N1P3 (b), and nanosecond component (2) of m-C1P1.

The short picosecond components (4 and 25 ps) retrieved from the global analysis, observed both in the model compound (m-C1P1) and in N1P1, can be attributed to a distribution of vibrational and/or solvation relaxation times. The shape and the sign of the amplitude spectra (Figure 9) are characteristic for such relaxation processes.23,24 To verify whether singlet-singlet annihilation occurs, transient absorption experiments are performed on the multichromophoric N1P3 and the corresponding model compound m-C1P3.

Multichromophoric Dendrimers N1P3 and m-C1P3. In the picosecond transient absorption spectrum of the model compound m-C1P3 in benzonitrile, the same features can be observed as for its monochromophoric counterpart m-C1P1. However, the picosecond transient absorption signal of m-C1P3 is lower in intensity than that of m-C1P1, as also observed in solvents of lower polarity.28 This suggests the occurrence of an ultrafast singlet-singlet annihilation process or dimer formation in the multichromophoric compound. The further decrease in signal is due to a singlet-singlet annihilation process, occurring on a 10 ps time scale in multichromophoric systems as already elaborated extensively in previous publications23,28 and discussed above. These singletsinglet annihilation processes are also observed as an additional decay signal in the transient absorption measurements of m-C1P3 compared to m-C1P1. As can be seen in Figure 10A, in the first few picoseconds a small hypsochromic shift from 645 to 640 nm can be observed due to vibrational and/or solvent relaxation processes in the electronically excited state of the PI chromophore. In the transient absorption spectrum of N1P3 in benzonitrile recorded in a 50 ps time window we also observe, in analogy to the multichromophoric model compound m-C1P3, a smaller transient absorption signal compared to N1P1. This is attributed to the occurrence of singlet-singlet annihilation in N1P3, which occurs on a time scale of 10 ps or less. This confirms the results obtained by fluorescence upconversion. As the singlet-singlet annihilation occurs on a much faster time scale than the electrontransfer process, which happens on a 100-200 ps time scale, it will also reduce the yield of the latter process. In the transient spectrum of N1P3 in benzonitrile we observe, in analogy to N1P1 (see Figure 10B), the formation of a new band, attributed to the radical anion absorption band of the peryleneimide, situated hypsochromic of the S1-Sn absorption band. Remarkably, however, despite having the same ion-pair formation time as N1P1, the PI radical anion band of N1P3 decreases on a 1500 ( 500 ps time scale compared to the 700 ps decay time of the PI radical anion band of N1P1. When the results of N1P1 are compared with those of N1P3, the same increase in decay time going from the monochromophoric N1P1 to the multichromophoric N1P3 is observed, namely, the decay time increases from 5 ns or 37 ns, depending on the constitutional isomer12 (N1P1), to 53 ns (N1P3, vide infra) in MeTHF and from 700 ps (N1P1) to 1500 ( 500 ps (N1P3) in benzonitrile. Although this phenomenon is no artifact, it is difficult to rationalize. The analysis of the monochromatic transient absorption traces of N1P3 required a four-exponential decay, as for the mono-



Figure 10. Three-dimensional display of the transient absorption spectra of N1P3 recorded in benzonitrile in 50 ps (A) and 1400 ps (B) time windows.

Figure 11. Wavelength dependence of the partial amplitudes of the short decay times of N1P3 and m-C1P3 in benzonitrile determined by femtosecond transient absorption: 3 ps components of N1P3 (b) and m-C1P3 (O), 15 ps component of N1P3 (9), and 11 ps component of m-C1P3 (0).

chromophoric N1P1. The amplitudes of the different kinetic components of N1P3 and m-C1P3 are depicted in Figures8 and 11. In N1P3 a kinetic component is present that is nonexistent in the corresponding model compound m-C1P3. This new component has a time constant of 100 ps and can be attributed to the formation of the PI radical anion absorption band. The amplitude of this new kinetic component is negative over the wavelength region between 545 and 635 nm, as shown in Figure 8. The sudden drop of amplitude of the 100 ps component beyond 635 nm can be explained by the fact that this component arises from the difference between the PI-S1-Sn absorption spectrum and the PI radical anion absorption spectrum (S1 Sn - anion). The absolute value of this difference gets smaller at longer wavelengths, hence leading to a smaller absolute amplitude for the 100 ps component at longer wavelengths. The component of the transient absorption of N1P3 decaying in the 1500 ( 500 ps range can be attributed to the decay of the radical anion absorption band of the PI. The obtained longer decay time for the radical anion absorption band of N1P3 (1500 ( 500 ps) compared to N1P1 (700 ps) can possibly be explained by the formation of an excimerlike entity between two closer-lying PI chromophores, leading to a stabilization of the radical anion of the lone PI chromophore. The obtained 1500 ( 500 ps component can therefore be considered as the average of a 700 ps and a (2-4) ns component belonging to the decay of the anion of the lone PI chromophore and that of the dimeric state, respectively. These

Figure 12. Normalized transient absorption traces of N1P1 (A) and N1P3 (B) recorded at 530 nm probe wavelength and at high and low excitation power.

two components cannot be discriminated between by the fitting routine in a 1400 ps time window for a 0-80% contribution of this dimeric state. However, it can also not be excluded that the obtained 1500 ( 500 ps component is actually a combination of a 700 ps component, due to decay of the radical ion pair, and a 4 ns component, due to a fraction of the molecules that underwent an oxidation of the amine. The 3 and 10 ps components retrieved from the global analysis, observed both in the model compound m-C1P3 and in N1P3, have contributions of a vibrational/solvent relaxation process and a singlet-singlet annihilation process. Both processes occur with the same time constant.23 To confirm the occurrence of intramolecular singlet-singlet annihilation processes in the multichromophoric compound N1P3, monochromatic transient absorption experiments were performed in which the laser power impinging on the sample was varied between two extreme values: low excitation power (80 µW) and high excitation power (530 µW). As shown in Figure 12, for the multichromophoric N1P3 the amplitude of the 10 ps component increases at high excitation power, proving the contribution of a singlet-singlet annihilation process, leading to an extra population decay, in this multichromophoric compound. To cross-check these findings, the same experiment was performed for the monochromophoric N1P1. As expected, no difference (Figure 12) is observed between high and low excitation powers for N1P1 and m-C1P1.


Lor et al.

Figure 13. Nanosecond transient absorption traces of aerated (A) and deoxygenated (B) solutions of N1P3 in MeTHF as a function of different delay times after excitation: (A) 0, 50, 200, and 400 ns; (B) 0, 10, 30, 50, and 100 ns.

The presence of the PI radical anion band is also demonstrated by means of nanosecond transient absorption experiments performed on N1P3 in MeTHF. In the more polar solvents, THF and benzonitrile, the PI radical anion band decays on a time scale faster than the temporal resolution of the experimental nanosecond setup and can hence no longer be observed with this setup. As the presence of oxygen modified the kinetics of the different photoinduced processes occurring on a nanosecond time scale,11 leading to an increase of the decay rates of the delayed fluorescence components upon deoxygenation, the measurements in the solvent MeTHF were performed on both deoxygenated and aerated solutions of N1P3. In the nanosecond transient absorption spectra of N1P3 in MeTHF, three bands are observed. In the aerated solution, Figure 13A, the positive bands observed between 10 and 400 ns after excitation can be attributed to the triplet-triplet absorption spectra of PI on the basis of literature data.34 The negative band can be attributed to ground-state depletion due to the correspondence with the ground-state absorption spectrum. Clearly different nanosecond transient absorption spectra are obtained for the deoxygenated solution of N1P3 in MeTHF (see Figure 13B). The negative band can again be attributed to the groundstate depletion. The positive absorption band observed between 10 and 100 ns at 620 nm can be attributed to the radical anion of the PI.11,35 From the spectra a rate constant of 1.9 × 107 s-1 (corresponding to a decay time of 53 ns) is derived for the decay of the PI radical anion absorption band. The bands observed at 400 nm and at the red edge of the spectrum can be assigned to the radical cation of the donor (TPA), which has its absorption maxima at 400, 700, and 770 nm.36 The role of oxygen on the photophysical kinetics has already been discussed in a previous publication of N1P1 where the same observations were made11,12 and is not in the focus of this publication. When the results of N1P1 and N1P3 obtained in different solvents at room temperature with nanosecond and femtosecond transient absorption experiments are combined, a remarkable trend is observed. The PI radical anion band of N1P1 in benzonitrile decays with a time constant of 700 ps. In the less polar solvent MeTHF the PI radical anion band decays with a time constant of 5 or 37 ns, depending on the constitutional isomer,12 while in diethyl ether the decay is even slower.11 This drop in decay time from MeTHF to benzonitrile by a factor of 50 can be explained by the difference in solvation energy. When these results of N1P1 are compared with those of N1P3, the same decrease in decay time going from MeTHF (53 ns) to benzonitrile (1500 ( 500 ps) is observed. This decrease is due to a combination of a less exothermic back electron transfer to the ground state with a larger reorganization energy in more polar solvents. Both effects will increase an electron transfer rate

TABLE 2: Fit Parameters of Fluorescence Anisotropy Decaysa compound

τ1 (ns)

a1 (%)

Θ1 (ns)

β1

Θ2 (ps)

β2

r0

N1P1 N1P3

4.1 4.1

100 100

0.84 1.08

0.36 0.12

104

0.25

0.36 0.37

a Measured for N1Px (x ) 1, 3) in toluene with λexc ) 488 nm and λfluo ) 560 nm.

constant in the inverted region, which is clearly the case here for electron transfer to the ground state.37 On the other hand, both effects will decrease the rate of the slightly uphill back electron transfer to the locally excited state of the peryleneimide. The 700 ps component of N1P1 and the 1500 ( 500 ps component of N1P3 obtained in benzonitrile have a different photophysical meaning from the long nanosecond components of N1P1 and N1P3 obtained in the less polar solvents diethyl ether and MeTHF. The long nanosecond components of N1P1 and N1P3 in diethyl ether are the delayed fluorescence components, repopulating the S1 state from the ion-pair state. In the highly polar solvent benzonitrile, the ion-pair state of N1P1 and N1P3 decays directly to the ground state. In the solvent MeTHF of medium polarity, a competitive back electron transfer to S1 (k12) and S0 (k02) occurs.12 However, even when the long decay time is corrected for k12, a systematic increase of k02 from diethyl ether over MeTHF and THF to benzonitrile is observed for both N1P1 and N1P3. Anisotropy Transient Absorption and Fluorescence Measurements. Anisotropy Fluorescence Results of N1Px (x ) 1, 3). Energy hopping can be revealed by time-resolved anisotropy experiments.16,17 Although more complex methods of analysis are possible,38,39 this is beyond the scope of our paper. Fluorescence anisotropy experiments have been performed on N1P1 and N1P3 in toluene. The results are summarized in Table 2. The anisotropy decay time of 840 ps observed for N1P1 can be attributed to rotational diffusion and would lead to a molecular radius of 11 Å by use of the Stokes-Einstein-Debye equation. In contrast to N1P1, the fluorescence anisotropy decay of N1P3 has both a 1.08 ns and a 104 ps component. While the former can be attributed to rotational diffusion, the 104 ps component must rather be attributed to an incoherent excitation energy migration process between the PI chromophores.16,17 The long depolarization time of N1P3 is similar to that obtained for N1P1. The slight increase of the magnitude of this long decay time increases with the number of chromophores and can be attributed to a slower rotational diffusion time of the (slightly larger) entire molecule. The amplitude of the slow-decaying anisotropy component (0.12) corresponds within experimental error to the value (0.10) expected for three chromophores of



Figure 14. Calculated transient absorption anisotropy traces of model compounds m-C1Px (x ) 1, 3) and triphenylamine core dendrimers N1Px (x ) 1, 3) in benzonitrile at 620 nm probe wavelength. Inset: Transient absorption traces of N1P3 measured at parallel and perpendicular polarization angles with respect to the probe beam.

Figure 15. Wavelength dependence of the partial amplitudes of the different anisotropy decay times of m-C1P3 (A), N1P1 (B), and N1P3 (C) obtained by femtosecond transient absorption anisotropy: (A) 45 ps component (9), 330 ps component (b), and nanosecond component (2); (B) 32 ps component (9) and nanosecond component (2); (C) 60 ps component (9), 250 ps component (b), and nanosecond component (2).

which the coplanar transition dipoles make an angle of 120°. For the multichromophoric model compounds, an initial anisotropy (r0 ) 0.28 for m-C1P3 and r0 ) 0.37 for N1P3) smaller than 0.37 was obtained, as reported earlier.17 Anisotropy Transient Absorption Results of N1Px and m-C1Px (x ) 1, 3). Anisotropy transient absorption experiments have been performed on N1P1 and N1P3 in benzonitrile. These measurements have been done at five different detection wavelengths, more specifically in the maximum (530 nm) of the ground-state depletion band, around the maximum (620 nm) of the PI radical anion absorption band, and around the maximum of the PI-S1-Sn absorption band (645 nm). As shown in Figure 14, also in the anisotropy transient absorption traces of the multichromophoric compounds m-C1P3 and N1P3 an extra kinetic component is observed compared to the corresponding monochromophoric compounds m-C1P1 and N1P1. Furthermore, for the multichromophoric compounds m-C1P3 and N1P3 a significantly lower initial anisotropy is observed compared to the corresponding monochromophoric compounds m-C1P1 and N1P1. This loss in initial anisotropy can be explained by the occurrence of ultrafast singlet-singlet annihilation or dimer formation within the temporal resolution of the setup, also observed in the transient absorption spectra (vide supra). The results of the global analysis are displayed in Figure 15. The anisotropy of the transient absorption of the monochromophoric m-C1P1 decays monoexponentially on a nanosecond time scale (Figure 15A). This nanosecond anisotropy component can be attributed to the rotational diffusion time of the entire molecule. For m-C1P3 (Figure 15A) a three-exponential fit was necessary to properly fit the data. The 45 and 330 ps anisotropy

components, not present in m-C1P1 can be assigned to two energy-hopping time constants that are related to the presence of different constitutional isomers of m-C1P3, each characterized by different interchromophoric distances. The two energyhopping rate constants obtained should be considered as the two extreme values of a distribution of energy-hopping time constants. The existence of the different constitutional isomers of m-C1P3, each displaying different photophysical behavior, has been demonstrated by single-molecule spectroscopy.34 The incoherent energy hopping of m-C1P3 in benzonitrile occurs on a longer time scaleskhopp ∼ R06 ∼ n-440scompared to toluene17 because of the larger refractive index of benzonitrile. The long nanosecond anisotropy component can again be attributed to the rotational diffusion time of the entire molecule. For N1P1 (Figure 15B) a two-exponential fit with picosecond and nanosecond decay components was necessary to properly fit the data. The picosecond component, which has a small partial amplitude that is at all wavelengths below 0.04, can possibly be attributed to the formation of the TPA radical cation by the electron-transfer process. The small amplitude and wavelength independence of this component suggest that the transition dipoles of the PI-S1 and the PI anion can be assumed to be parallel. If both transition dipoles would be perpendicular, one would expect this component to have a larger amplitude and to change sign at the wavelength where the S1-Sn extinction coefficient of PI would become larger than that of the D0-Dn transition of the radical anion. A three-exponential fit was necessary to fit the anisotropy traces of N1P3 (see Figure 15C). The long nanosecond anisotropy decay component can again be attributed to the rotational diffusion time of the entire molecule. The 60 and 250 ps

10730 J. Phys. Chem. B, Vol. 108, No. 30, 2004 anisotropy components can possibly be attributed to two energyhopping time constants and/or the formation of the TPA cation. Conclusions By use of fluorescence (SPT) anisotropy and transient absorption spectroscopy, incoherent energy-hopping processes in m-C1P3 and N1P3 are detected. The time scale of these energy-hopping processes is longer in benzonitrile compared to toluene as expected in the framework of Fo¨rster theory. Femtosecond fluorescence upconversion and transient absorption experiments revealed the occurrence of singlet-singlet annihilation processes in the multichromophoric N1P3. Furthermore, these experiments demonstrated the presence of a photoinduced intramolecular electron-transfer process in the amine core dendrimers N1P1 and N1P3, leading to a short-lived ion-pair state in benzonitrile, as opposed to the long-lived ion-pair state observed in solvents of lower polarity. In benzonitrile no delayed fluorescence process occurs as the ion-pair state decays directly to the ground state. The ion-pair state is formed with the same time constant in N1P1 and N1P3. However, the decay time of this ion-pair state is longer in N1P3 than in N1P1. Moreover, a similar increase in decay time going from N1P1 to N1P3 is also observed in less polar solvents. Experimental Section The synthesis of the amine core dendrimers (N1P1 and N1P3) and the model compounds (m-C1P1 and m-C1P3) bearing one and three peryleneimide (PI) chromophores, respectively, at the rim has been reported elsewhere.2,41 Steady-state absorption and corrected fluorescence spectra were recorded on Lambda 40 (Perkin-Elmer) and Fluorolog spectrophotometers, respectively. The absorbance of the solutions obtained by dissolving the dendrimers in benzonitrile (spectroscopic grade) was always smaller than 0.1 at the absorbance maximum in a 1 cm cuvette, which corresponds to a concentration of about 2.5 × 10-6 M. The fluorescence quantum yields have been determined by use of Rhodamine 101 and Cresyl Violet Perchlorate as references.42 The integrity of all samples during pulsed excitation experiments was checked by taking steady-state absorption and emission spectra before and after each set of measurements. No photodegradation was observed. The fluorescence decay times of these amine core dendrimers (N1P1 and N1P3) in benzonitrile have been determined by the single-photon-timing method (SPT) described in detail previously.12,43 All SPT measurements were performed in 1 cm optical path length cuvettes on solutions of N1P1 and N1P3 in benzonitrile at an absorbance of ca. 0.1 at the excitation wavelength 488 nm, which is very close to the absorbance maximum. The fluorescence magic angle and anisotropy decays were recorded with 10 000 counts in the peak channel in two different time windows and analyzed globally with a timeresolved fluorescence analysis (TRFA) software.44,45 A Berek compensator and a polarizer allowed the recording of the fluorescence decays at three different orientations of the emission polarizer (magic angle, parallel, and perpendicular) relative to the polarization of the excitation light. The full width at half-maximum (fwhm) of the instrumental response function (IRF) was typically in the order of 40 ps. This was determined by recording a histogram of the light scattered from a 1 cm cuvette containing a Ludox suspension. The quality of the fits has been judged by the fit parameters χ2 (

Photophysical Study of Electron-Transfer and Energy-Hopping

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