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J. Phys. Chem. C 2007, 111, 622-630
Photophysical Characterization of a Light-Harvesting Tetra Naphthalene Imide/Perylene Bisimide Array Lucia Flamigni,*,† Barbara Ventura,† Chang-Cheng You,‡ Catharina Hippius,‡ and Frank Wu1 rthner*,‡ Istituto ISOF-CNR, 40129 Bologna, Italy, and UniVersita¨t Wu¨rzburg, Institut fu¨r Organische Chemie, 97074 Wu¨rzburg, Germany ReceiVed: August 21, 2006; In Final Form: October 13, 2006
Photoinduced processes in a light-harvesting array PBI-NI4, made of a central perylene bisimide (PBI) with four appended naphthalene imide (NI) units, have been studied in toluene by several spectroscopic techniques, and the steps of energy dissipation have been identified. The singlet excited states 1PBI (λ max ) 960 nm, ) 40 000 M-1 cm-1) and 1NI (λ max ) 810 nm, ca. 4 000 M-1 cm-1) and the charge-separated state PBI--NI4+ (λ max ) 775 nm, ) 80 000 M-1 cm-1) were identified as reaction intermediates. In the array PBI-NI4, energy transfer from the peripheral 1NI to the PBI core was shown to compete efficiently (90%) with electron transfer from the NI unit to the PBI unit. Subsequently, 1PBI displays a slow electron abstraction (k ) 4.6 × 108 s-1) from the NI unit to form the charge-separated state PBI--NI4+, which competes with emission of light energy from the PBI core. The CS state PBI--NI4+ decays faster (k ) 2.6 × 109 s-1) than it is populated, preventing any back reaction to re-form the singlet excited state localized on PBI.
Introduction The issue of light-energy collection is of great relevance to the field of light-energy conversion into any usable form of energy (chemical or electrical) and to the application in molecular logic, molecular electronic, or molecular photonic technologies. In most cases, following the natural photosynthetic model, light collection in artificial devices is achieved by multichromophoric dye architectures (antennae) where an energy gradient provides the driving force to funnel the electronic energy into a specific low-energy site. Several reports on efficient, often even quantitative energytransfer processes within such molecular architectures have been reported;1-11 however, in the majority of cases competing processes in the excited dye manifold cause undesirable quenching of the collected photon energy and an overall reduction of the light-harvesting efficiency. In many cases, ground- or excited-state interactions between dyes in close proximity decrease the electronic energy available for transfer, by dimer or excimer formation or by annihilation processes, or in other cases competing electron transfer dissipates electronic energy in undesired processes. This was also the case in a recently reported light-harvesting molecular square composed of perylene bisimide walls bearing appended pyrene chromophores. Energetically, this assembly was constructed for efficient absorption of light in the wavelength range between 200 and 600 nm and showed energy-transfer characteristics (90%) that give proof of efficient light collection.12 However, if the fluorescence intensity is taken as the desirable final outcome, then this system was not very successful; the fluorescence quantum yield dropped down to only 5% because of competing photoinduced electron-transfer reactions from the appended electron-rich pyrene chromophore to the electron-poor * Corresponding authors. E-mail:
[email protected] and wuerthner@ chemie.uni-wuerzburg.de. † Istituto ISOF-CNR. ‡ Universita ¨ t Wu¨rzburg, Institut fu¨r Organische Chemie.
perylene bisimide wall occurring from the excited states of both perylene bisimide and pyrene. More recently, to improve the performances, a new molecular square has been designed and studied bearing appended naphthalene imide units.13 Like in the former assembly, efficient light collection from the antennae dyes was observed but now the absorbed photons remained available for a longer time on the PBI square manifold as revealed by a significantly higher fluorescence quantum yield of 38% of this multichromophoric assembly in dichloromethane. However, it turned out that the efficiency of this light-harvesting system was strongly dependent on the solvent and the fluorescence quantum yield dropped considerably in the less-polar solvent toluene (19%). Therefore, in this paper we want to perform a study by time-resolved spectroscopy of the elementary steps following photoexcitation of the array in order to shed light on the mechanisms of light dissipation and to derive useful hints for the construction of more efficient, perylene bisimide based, antenna systems. To make our task easier, we selected the simplest conditions: (i) we studied the process in the model PBI-NI4, a single wall of the molecular square, Scheme 1 and (ii) we used toluene as solvent because the final luminescence output turned out to be lower in this solvent than in other solvents, suggesting a larger contribution by the competing processes object of our study. Experimental Section Spectroscopy and Photophysics. Spectrophotometric-grade toluene at 295 K and at 77 K was used without further purification. Standard 10 mm fluorescence cells were used at 295 K, whereas experiments at 77 K made use of capillary tubes in a homemade quartz Dewar filled with liquid nitrogen. Oxygen-free solutions were obtained by bubbling for 10 min with a stream of argon in home-modified 10 mm fluorescence cells. A Perkin-Elmer Lambda 5 UV/vis spectrophotometer and a Spex Fluorolog II spectrofluorimeter were used to acquire absorption and emission spectra. Emission quantum yields were
10.1021/jp065394g CCC: $37.00 © 2007 American Chemical Society Published on Web 12/09/2006
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J. Phys. Chem. C, Vol. 111, No. 2, 2007 623
SCHEME 1
Figure 1. Absorption spectra of PBI (thin solid), NI (dash), and PBINI4 (dot) in toluene. The superposition of the molar absorption coefficients of PBI + 4 NI (thick solid) is also reported for comparison purposes.
TABLE 1: Molar Absorption Coefficients of the Models and Array in Toluene NI PBI
determined after correction for the photomultiplier response, with reference to air-equilibrated CH2Cl2 solutions of PBI with a Φem ) 0.88.12 Phosphorescence spectra were detected by a Spex Fluorolog II spectrofluorimeter equipped with a phosphorimeter accessory (1934D Spex) set to a delay of 70 µs and a time window of 30 ms; the toluene solution was added with a few drops of CH2I2. Luminescence lifetimes in the nanosecond range were obtained with an IBH single photon counter with excitation at 373 nm or 560 nm from pulsed diode sources (resolution 0.3 ns). For the picosecond range, an apparatus based on a Nd:YAG laser (35 ps pulse duration, 355 nm, 1.5 mJ) and a Streak Camera with overall resolution of 20 ps was used.14 Transient absorption spectra and lifetimes in the picosecond range were determined by a pump-probe apparatus with 30 ps resolution based on a Nd:YAG laser (355 nm or 532 nm, 3 mJ, 35 ps, 10 Hz).15 A solution of benzophenone in acetonitrile (A355 nm ) 0.33) was the actinometer for the determination of the molar absorption coefficient of 1PBI, and the following parameters were used for the triplet state of benzophenone: 520 nm ) 6500 M-1 cm-1 and φisc ) 1.16 Nanosecond laser flash photolysis experiments were performed by a system based on a Nd:YAG laser (JK Lasers, 355 nm, 4 mJ, 18 ns pulse) described previously, using a right-angle analysis of the excited sample.17 Experimental uncertainties are estimated to be within 10% for lifetime determination involving simple exponential, 20% for lifetime determinations involving multiple exponentials or more complex kinetics, 15% for quantum yields, 20% for molar absorption coefficients, and 3 nm for emission and absorption peaks. The temperature of operation was 295 K except where stated otherwise. Molecular distances and radii were determined by Hyperchem (Version Professional 7.5) after geometry optimization was performed with a MM+ force field.18 Results and Discussion Steady-State Spectroscopy. The absorption spectra of the array PBI-NI4 and of the respective model chromophores PBI and NI (Scheme 1) in toluene are reported in Figure 1, and the related data are summarized in Table 1. The superposition of the spectra of the components, PBI + 4 NI, is also shown for comparison purposes in Figure 1. The use of four NI units is
PBI-NI4
λmax/nm
/ 104 M-1cm-1
405 450 538 579 411 534 572
1.07 1.67 2.91 4.90 4.89 2.78 4.31
beneficial in that it enhances the light-energy collection ability in the 350-480 nm range. In PBI-NI4 there is a slight blue shift (4-7 nm) of the perylene bisimide bands, and a ca. 10% decrease in the molar absorption coefficient with respect to the model PBI. Overall, there is a remarkably good agreement between the absorption of the models and of the array, which indicates a very modest electronic coupling between components. Therefore, a localized description of the individual subunits can be used with a high level of confidence to predict the photophysical properties of the array. The components in the array can be excited with a good selectivity; at 405 nm 88% of the photons are absorbed by the four naphthalene imide components, whereas above 500 nm the perylene bisimide absorbs exclusively. The luminescence spectra of the models and array in toluene, excited at 405 and 535 nm, are reported in Figure 2. The concentration of the solutions is adjusted to provide absorption of the same number of photons in the reference model and in the same component in PBI-NI4. To calculate the photons absorbed by the unit in the array, we made use of the molar absorption coefficients of Figure 1. Upon excitation at 405 nm, the luminescence of the NI component in PBI-NI4 is quenched almost completely (99.5%), whereas the PBI component luminescence is enhanced with respect to the model by a factor of ca. 3.5. This is consistent with an energytransfer process from the NI to the PBI component, but the final luminescence output is lower than expected. In fact, because the reference PBI model absorbs 12% of the incident light as the PBI component in the array whereas the remaining 88% is absorbed by the NI component, an enhancement of the luminescence on the order of 7-8 times would have been expected following a quantitative sensitization. The absence of a unity luminescence output is in agreement with the observations after selective excitation of the PBI component at 535 nm (Figure 2): the luminescence of the perylene bisimide in PBINI4 is reduced by ca. 4.5 times with respect to the model PBI, indicating a substantial quenching process of the PBI unit in the multichromophoric array. Excitation spectra of PBI-NI4 determined at 620 nm, that is, on the fluorescence band of the PBI component, agrees quite well with the absorbance of the
624 J. Phys. Chem. C, Vol. 111, No. 2, 2007
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Figure 4. Normalized luminescence of PBI (thin solid), NI (dash), and PBI-NI4 (dot) in toluene at 77 K.
Figure 2. Luminescence in toluene solutions. Upper panel: excitation at 405 nm, NI (dash) A405 nm ) 0.104, PBI (thin solid) A405 nm ) 0.013, and PBI-NI4 (dot) A405 nm ) 0.121 at 295 K. In the inset is the phosphorescence from NI at 77 K, in toluene with a few drops of CH2I2, excitation at 405 nm. Lower panel: excitation at 535 nm, PBI (thin solid), A ) 0.07, and PBI-NI4 (dot), A ) 0.07 at 295 K.
Figure 3. Excitation spectra of PBI-NI4 (solid) measured at λem ) 620 nm and normalized absorption spectrum of PBI-NI4 (dash-dot).
full array (Figure 3) confirming a nearly quantitative energy transfer from the NI to the PBI component. The luminescence properties were also probed at 77 K in a rigid toluene matrix; the normalized luminescence spectra of the various components are reported in Figure 4, and the pertinent data is reported in Table 2. Though quantitative considerations on absolute quenching are difficult in these experimental conditions (see experimental part for details), by comparison with the luminescence of PBI-NI4 at room temperature, Figure 2, a reduction in PBI-NI4 of the luminescence localized on PBI can also be detected at 77 K. Excitation spectra of PBI-NI4 determined at 620 nm at 77 K was also superimposable to the absorption spectrum, indicating an efficient energy transfer from NI to PBI (data not shown). The phosphorescence spectra, probed with a phosphorimeter up to 800 nm, shows a luminescence band for NI with a maximum at 607 nm (inset of Figure 2), ascribable to 3NI. The wavelength is lower than the phosphorescence emission of other similar naphthalene imi-
des,19,20 but the charge-transfer character arising form the paraamino substituent is expected to alter the energy of the excited states. Time-Resolved Luminescence. The luminescence lifetimes in toluene at room temperature of the models and array are determined by a single photon counting apparatus with 0.3 ns resolution upon excitation at 373 and 560 nm. The luminescence lifetime of NI is 7 ns and that of PBI is 5.8 ns, both identified as fluorescence from the lowest singlet excited state. For a similar amino-substituted naphthalene imide, ANI, Wasielewski and co-workers reported a fluorescence λmax in toluene at 497 nm and a lifetime of 8.5 ns, similar to the present values of 488 nm and 7 ns, respectively.21 For the ANI chromophore, a lowest singlet excited state with a large (70%) charge transfer (CT) character was determined, and given the similarity of the systems we can assume a similar characteristic for NI. In PBI-NI4 the fluorescence from PBI unit (λ ) 605 nm) is reduced to a lifetime of 1.6 ns, and the fluorescence of the NI component (λ ) 488 nm) is undetectable on the nanosecond time scale (Table 2). An experiment conducted with a picosecond laser (λ exc ) 355 nm) and a streak camera allowed us to determine a lifetime of 50 ps for the decay of the NI band at 490 nm and a similar risetime was detected on the 605 nm band of the PBI (Figure 5). The latter rise was followed by a decay with lifetime of 1.5 ns, in good agreement with the 1.6 ns determined by the nanosecond fluorescence experiment. The time evolution of the spectra reported in the inset of Figure 5 clearly shows that an energy transfer within PBI-NI4 from the NI (λmax ) 490 nm) to the PBI (λmax ) 600 nm) unit occurs; however, we are unable, so far, to exclude other parallel reactions depleting the excited-state PBI-1NI4. Accordingly, both fluorophores are quenched in the array; although the quenching of the NI unit can be assigned at least in part to an efficient energy transfer to the perylene bisimide unit (τ ) 50 ps), the quenching of the PBI unit (τ ) 1.6 ns) needs further investigations (see below). At 77 K in rigid matrix, the lifetimes of the two model chromophores are substantially unaffected, 5.5 ns for PBI and 7 ns for NI. In PBI-NI4 the fluorescence at 490 nm (1NI) is still remarkably quenched to a lifetime lower than 0.3 ns,22 whereas the fluorescence of the 600 nm band (1PBI) is essentially unaltered, with a lifetime of 5.2 ns. The different effect of the rigidification of the solvent on the quenching of the two bands is compatible with a different nature, at least in part, of the two processes. Energy transfer is, in general, not very dependent on the temperature and physical state of the medium, whereas electron transfer is strongly affected by the rigidification of the medium, which, preventing the stabilization of the resulting charge-separated state by solvent reorganization, can inhibit the reaction. The effect of temperature on the luminescence lifetime
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TABLE 2: Luminescence Properties of the PBI-NI4 Array and Model Chromophores 295 K state NI PBI PBI-NI4
d
1
NI 3 NI 1 PBI PBI-1NI4 1 PBI-NI4
λem/nm
Φa
77 K τ/ns
488
0.82b
7.0d
603, 653 (sh) 488 600, 645 (sh)
0.89c 0.0047b 0.19c
6.0e 0.050f 1.6e,f
a Emission quantum yield; the standard used is PBI in CH2Cl2 with a quantum yield of 0.88.12 Excitation at 373 nm. e Excitation at 560 nm. f Excitation at 355 nm.
Figure 5. Luminescence decay profiles of NI (490 nm) and PBI (605 nm) with the fitted lifetimes, excitation at 355 nm (1.5 mJ, 35 ps pulse). In the inset are the time-resolved spectra in the time windows 0-50 ps (thick solid) and 100-150 ps (thin solid) after the laser pulse.
SCHEME 2
of 1PBI-NI4 points to a quenching at room temperature ascribable to electron transfer. The luminescence data at 295 and 77 K are summarized in Table 2. Energy Level Diagram. Scheme 2 represents an approximate energy level diagram of the states involved in the photoinduced processes in PBI-NI4. The excited-state energy can be derived from the spectroscopic data of Table 2 and from a literature value reporting the energy of the lowest excited triplet state of a similar perylene bisimide at 1.19 eV.23 The energy of the charge-separated (CS) state can be calculated with an approximate approach by making use of the electrochemical potentials. For NI the first oxidation potential in dichloromethane was measured at about E1/2 ) +0.75 V versus Fc/Fc+ where
λem/nm 481 607 604, 656 481 598, 644 b
τ/ns 7.0d 5.5e
