Intermolecular Interactions Determine Exciton Lifetimes in Neat Films

Subscriber access provided by Washington University | Libraries

Article

Intermolecular Interactions Determine Exciton Lifetimes in Neat Films and Solid State Solutions of Metal-Free Phthalocyanine Benjamin W Caplins, Tyler K. Mullenbach, Russell J. Holmes, and David A Blank J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09817 • Publication Date (Web): 11 Nov 2015 Downloaded from http://pubs.acs.org on November 12, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Intermolecular Interactions Determine Exciton Lifetimes in Neat Films and Solid State Solutions of Metal-Free Phthalocyanine Benjamin W. Caplins,† Tyler K. Mullenbach,‡ Russell J. Holmes,‡ and David A. Blank∗,† †Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, USA ‡Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA E-mail: [email protected]

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Thin films of vapor deposited metal-free phthalocyanine (H2 Pc) were studied using ultrafast transient absorption spectroscopy in the visible region. Following photoexcitation an excited state absorption feature located near 532 nm was observed which served as a probe of the excited state. For exciton densities larger than 5 × 1018 excitons/cm3 the time-dependent measurements of the excited state absorption included the presence of non-exponential decay kinetics attributed to exciton-exciton annihilation. At exciton densities less than 5 × 1018 excitons/cm3 annihilation was negligible and the decay kinetics appeared single exponential within the signal-to-noise. The fitted time constant, 239 ± 24 ps, was attributed to the lifetime decay of the singlet excitons. When the H2 Pc was diluted into a wide energy gap host via vapor deposition, the observed lifetime was significantly reduced, reaching 87 ± 9 ps for a concentration of 25% H2 Pc. The decreased exciton lifetime with dilution was remarkable since it has been commonly reported that excited state lifetimes decrease as the chromophore concentration is increased. The reduced lifetime was correlated to the loss of α-phase ordering as indicated in the UV/Vis spectra of the films. Within the context of photovoltaic applications this highlights the importance of both molecular level ordering and chromophore concentration when trying to engineer fundamental material properties such as exciton diffusion length.

Introduction Understanding and controlling energy transport is one of the most important challenges in the physical sciences. In the context of organic photovoltatics, this challenge is manifest in the transformation of energy from light to electricity. In organic photovoltaic materials, a photon initially creates an excited electronic state commonly referred to as an exciton. 1,2 In order for these excitons to generate an electric current, they must diffuse to a heterojunction which supplies the energetic driving force necessary to drive charge separation. Since the photon absorption length is frequently longer than the exciton diffusion length a portion of 2


Page 2 of 26

Page 3 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the absorbed photons are not able to contribute to the electric current and thus limit the efficiency of planar heterojunction organic solar cells. 3 Understanding the mechanism and timescales by which the energy moves through a system is a necessary first step towards realizing improved efficiency and material design. In the incoherent limit, electronic energy transfer in organic systems is typically described by two different mechanisms: Förster and Dexter transfer. 3 Förster resonance energy transfer (FRET) occurs when two chromophores separated by a distance, r, interact with one another through their transition dipole moments. The rate of FRET is proportional to (R0 /r)6 where R0 is the Förster radius, which is a function of the photophysical parameters of the chromophores. Whereas the Förster mechanism occurs through transition dipole coupling, Dexter energy transfer occurs through an electron exchange process, which relies on molecular orbital overlap. Since electronic orbitals decay exponentially in space, this leads to an exponential falloff for the Dexter transfer process with distance. Thus Dexter energy transfer is typically limited to molecules in direct contact with one another. In contrast to FRET, Dexter transfer is operative for all states, including those with small transition dipole moments (‘dark’ states) such as triplets. Both the Förster and Dexter energy transfer only occur at relatively short length scales. For Dexter the length scale is set by the size of the molecule and for Förster the length scale is set by R0 , which is typically less than 10 nm. 4 Because of the short length scale of energy transfer, a high number density of chromophores are needed to facilitate exciton transport. It has been noted that FRET based transport requires chromophore concentrations greater than 1018 /cm3 to achieve diffusion lengths on the order of 1-10 nanometers. 5 As molecular densities for neat organic semiconductors are typically on the order of 1021 /cm3 , concentrations of chromophores on the order of 1% by volume are necessary. At this density, intermolecular interactions often cause the the photophysical parameters of the molecules to change. 6 For example, non-radiative decay rates typically increase substantially when two chromophores √ come into contact. Since the diffusion length is given by LD = D × τ where D is the 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

diffusion constant and τ is the exciton lifetime, the increase in non-radiative decay rate is deleterious to the distance an exciton can travel. Although high chromophore densities are necessary to achieve long range energy transport, the relationship between chromophore density and exciton diffusion length can be complicated by density-induced changes in the photophysical parameters of the system. Finding the optimal chromophore density can be important for maximizing the exciton diffusion length in materials. A recent study found that the diffusion length of boron subphthalocyanine chloride (SubPc) was maximized when it was diluted to ca. 25% in a wide energy gap material which served as an inert matrix. 7 The exciton diffusion in the dilute films was well-described by a Förster mechanism. Competition between reduced intermolecular interactions and chromophore density resulted in a Förster radius that increased substantially as the SubPc density was decreased. The authors found that the exciton lifetime increased monotonically as the SubPc was diluted, which was attributed to a decrease in the non-radiative decay rate. In contrast, recent work on diluting C60 in wide energy gap materials resulted in substantially arrested energy transport despite maintaining efficient charge transport pathways. 8,9 This opposing behavior might reflect either a nanoscale phase segregation 9 or a Dexter type energy transfer process. 8 The increase in the non-radiative decay rate and the associated decrease of the exciton lifetime with the reduction of the average intermolecular distance in SubPc are in line with expectations from other studies. For a series of perylene derivatives doped into a polymer matrix the lifetimes decreased monotonically from more than a nanosecond to ca. 100 ps going from 1 mM to neat films. 5 This decrease in lifetime was attributed to aggregate formation that increased the non-radiative decay rates. In order to combat this process, efforts have been made to functionalize chromophores with bulky sidechains that inhibit aggregate formation. 10,11 In the present work, we seek to build upon these previous studies by investigating the effect of diluting a model chromophore, metal-free phthalocyanine (H2 Pc) (Figure 1), in a

4


Page 4 of 26

Page 5 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N N

HN

NH

N

N

N N

H2Pc

H2Pc dimer

α-phase H2Pc

Figure 1: (left) the molecular structure of H2 Pc, (center) the face-on-face H2 Pc dimer viewed edge-on, (right) the herringbone structure of α-phase H2 Pc viewed edge-on wide energy gap material, p-bis(triphenylsilyl)benzene (UGH2). Steady state and transient absorption spectroscopy was used to determine the effect of the matrix on the structure and lifetimes of the H2 Pc chromophores. The results show that, counter to common assumptions, the exciton lifetime in the films decreases as the H2 Pc is diluted. We rationalize this behavior in terms of changes in chromophore aggregate structure. These results highlight that the nominal chromophore density is not necessarily the dominating factor in determining the exciton lifetimes; instead, the specific intermolecular packing and interactions can play the decisive role.

Experimental The pump-probe measurements were performed using a home-built Ti:sapphire laser system described elsewhere in more detail. 12 For some experiments the sample was excited with a small portion of the fundamental of the regenerative amplifier (λ = 820 nm, ∆λfwhm = 26 nm, ∆tfwhm =75 fs). Experiments with visible excitation used the output of a nonlinear optical parametric amplifier tunable throughout the visible (∆λfwhm = 10 − 15 nm, ∆tfwhm = 50 fs). In both cases the pump beam was chopped at either the second (500 Hz) or third (333 Hz) subharmonic of the amplifier. The pump power was measured with a silicon power meter and adjusted with a waveplate followed by a polarizer. The probe consisted of a white light continuum generated by focusing 820 nm pulses into a 2 mm sapphire window. The residual fundamental light was removed prior to the sample with a Calflex X optical filter (Optics Balzers). 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The pump and probe beams were focused with a parabolic mirror and intersected at the sample. Typically the pump beam was ca. 225 µm fwhm and the probe was ca. 50 µm fwhm as measured by rastering a pinhole across the sample plane. Excitation densities were calculated according to ρexciton = α (λpump ) × I0 (λpump ) which is valid for low optical densities at the pump wavelength. Here α is the absorption coefficient (units of 1/cm) as obtained with ellipsometry of the 100% H2 Pc film, and I0 is the incident photon flux (units of photons/cm2 /pulse) calculated using the fwhm of the pump beam at the sample plane. As the focus of this work is not the annihilation itself, no great effort was made to quantify the excitation density more precisely, and we estimate a potential systematic error on the order of 50% could be present due to uncertainties in the spatial profile of the beams, uncertainties in the absorption coefficient, the unknown amount of scattered pump light, the excitation profile in the propagation direction of the beam, the calibration of the power meter, and the pump probe overlap. This systematic uncertainty does not affect the results or conclusions. The probe beam transmitted the sample, was collimated, and directed through an interference filter centered at 532 nm (∆λfwhm = 10 nm) and focused on a photodiode which was amplified and digitized by a lock-in amplifier (SRS SR830). Thin films of neat H2 Pc and H2 Pc diluted in UGH2 were deposited by vacuum thermal sublimation. The H2 Pc (98% pure) was purchased from Sigma-Aldrich and the UGH2 (sublimed, >99% pure) was obtained from Lumtec. Both materials were used as purchased without further purification. Prior to deposition, the quartz substrates were cleaned by sonicating in tergitol and acetone before being boiled in isopropyl alcohol. All organic materials were deposited at pressures below 10−6 Torr with a total deposition rate (as measured by a calibrated quartz crystal monitor) of 0.2 nm/s. Dilution was achieved by simultaneously depositing H2 Pc and UGH2 from separate sources. The film thicknesses were measured by spectroscopic ellipsometry to be 260 nm, 455 nm, 465 nm, and 350 nm thick for the 100%, 75%, 50%, and 25% samples of H2 Pc in UGH2 respectively. For the intensity dependence data in Figure 3 the samples were stationary during mea-

6


Page 6 of 26

Page 7 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surements. For the data that was used to compare the annihilation free dynamics (Figure 4a) the samples were rotated at a rate that replaced the sample volume between each excitation pulse. In the present system there was no significant difference in the kinetics of the stationary and rotating films. Experiments were performed at room temperature in air. Temporally stretching the pump pulse by a factor of three resulted in no detectable changes to the observed kinetics which indicates that multiphoton processes were not important. All fitting was performed using an iterative least squares approach with the following functional form, f (t; a, τ, b) = a × exp(−t/τ ) + b. Uncertainties are reported at the 95% confidence interval level.

Results and Discussion UV/Vis Absorption Spectra Representative UV/Vis spectra for the samples under study are shown in Figure 2. The H2 Pc molecule in dilute toluene solution shown at the bottom of Figure 2a has electronic absorption bands in two regions known as the Q band in the visible, and the B band in the near ultraviolet. 13 For photovoltaic applications the intense absorptions (ca. 2 × 105 /M/cm) of the Q band are the bands of interest since they blue shift and broaden in thin films (vide infra), covering a substantial fraction of the solar spectrum. For H2 Pc the Q band consists of two vibronic progressions denoted the Qx and Qy bands which are split by the presence of hydrogen atoms on the inner ring. For comparison, metallated phthalocyanines with D4H symmetry show a single (doubly degenerate) transition in this region with a very similar vibronic progression to higher energy. Upon forming a neat film via vapor deposition it can be seen that the absorption of the region associated with the Q band changes substantially. 14–19 The band maxima shift to ca. 614 nm with a second peak of substantial intensity near 700 nm. In agreement with previous work we observe a shoulder near 570 nm which for CuPc films has been attributed 7



a)

B-band

absorbance (a.u.)

Q-band

100% H2Pc 75% H2Pc 50% H2Pc 25% H2Pc H2Pc/toluene

300

400

500 600 700 wavelength (nm)

800

900

400

500 600 700 wavelength (nm)

800

900

b) absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

300

Figure 2: a) UV/Vis spectra of H2 Pc thin film samples normalized to have equal height and offset for visualization. The spectrum in dilute toluene solution is shown at the bottom for comparison. b) The same spectra as in part a), except they have not been offset which highlights the changes in the band shapes. to transitions with charge transfer character from electroabsorption experiments. 20 Previous studies have shown that the fluorescence quantum yield for H2 Pc thin films are quite low, on the order of Φ ≈ 10−4 , with broad bands in the neighborhood of 800-900 nm. 17,21–23 As the H2 Pc is diluted in UGH2 the absorption features change systematically, Figure 2b. The prominent shoulder near 700 nm is seen to decrease, and the weak shoulder around 570 nm also disappears. A shoulder on the B band near 370 nm also decreases in intensity as the H2 Pc concentration is decreased. The Q band maximum blue-shifts from 614 nm to 604 nm between the 100% H2 Pc and 25% H2 Pc samples. By comparison to the toluene solution spectrum, the weak structure seen in the 25% concentration is attributed to a low concentration of monomers. 8


Page 9 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The spectra for the 100% vapor deposited H2 Pc are consistent with what is commonly referred to as α-phase H2 Pc in the literature, which has a herringbone crystal structure. 16,18 The spectrum is dominated by a broadening and blue shift of the Q band absorption relative to the toluene solution spectrum. In this system, changes to the Q band can be qualitatively explained by the formation of H-aggregates. 6 As the H2 Pc molecules condense into the thin film, they pack closely, which causes the Q band states to mix. The spatial orientation and spacing of these dipoles dictates whether the absorption spectrum will blue shift or red shift. For the case of face-on-face stacking with transition dipoles in the molecular plane (i.e. for the Q and B bands of H2 Pc) this results in the concentration of oscillator strength into the upper (blue-shifted) exciton branch.1 Changes to the spectra as the H2 Pc is diluted in UGH2 are consistent with the loss of α-phase ordering and the formation of smaller aggregates. Blue shifting of the main Q band peak, though somewhat subtle, occurs simultaneously with the loss of the shoulders at 370 nm, 570 nm, and 700 nm. The blue shift might be attributed to a larger exciton coupling and thus better face on face stacking, however, we cannot eliminate the role of solid state solvation effects 24 which may also contribute to the blue shift. Support for the assignment of the absorption spectra of the lower weight % samples to face-to-face aggregates comes from previous studies on stoichiometric crown ether H2 Pc dimers, 25 statistical assemblies of alkoxy H2 Pc dimers/trimers, 26 and H2 Pc liquid crystals, 27,28 all of which found similar band shapes and locations.

Transient Absorption Spectroscopy Neat H2 Pc Initial experiments excited the sample on resonance (610 nm) with a photon fluence of 1.2 × 1015 photons/cm2 /pulse, and probed the sample with white light dispersed on a spectrograph 1

Theoretical studies for α-phase H2 Pc that quantitatively account for Davydov, charge transfer, and vibronic effects have not yet been performed to the authors knowledge, which precludes a more detailed analysis of the band shapes.

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(see SI for details). These data showed a broad excited state absorption feature near 532 nm that was consistent with previous studies on both α- and β-phase H2 Pc thin films, 19,29–31 in addition to several metallated phthalocyanines. 32,33 Though this wavelength resolved data showed that 532 nm was a viable place to probe the excited state population, the decays measured in this manner were highly non-exponential, which suggested either inhomogeneous or bi-molecular effects. For these experiments, excitation densities were on the order of 10% of the molecular density. Under these conditions excitons are only a few lattice sites apart and can annihilate resulting in the net loss of an exciton. This process results in a decay rate term that scales quadratically with exciton density and yields a time dependent decay rate analogous to a bimolecular chemical reaction. 34 In the limit of small exciton densities the bimolecular annihilation term becomes negligible and the lifetime decay term becomes dominant assuming no other processes are at work. When pumping on resonance (λmax ) the photon fluence must be several orders of magnitude lower than 1.2 × 1015 photons/cm2 /pulse to remove annihilation effects. The low excitation densities required to avoid annihilation effects makes the experiments difficult due to small transient signals. The high exciton densities result from the high effective concentrations of the films and the high oscillator strength of the chromophore. This yields a naperian absorption coefficients at λmax of ca. 2 × 105 /cm, thus the excited states are exponentially distributed in the film with a decay length of only 50 nm. In order to attain lower densities of excitations while maintaining acceptable signal to noise, the samples were excited off resonance where the absorption lengths are much longer and more closely matched to the absorption length at the probe wavelength. This makes it possible to use thicker samples where the excited states are distributed over a larger probe volume. One potential problem with exciting the sample off resonance is that the pump may selectively excite a minority population within an inhomogeneously broadened distribution of states. To determine the role of inhomogeneity on the measurements, experiments were performed exciting at both the red and blue tails of the main absorption peak. For these

10


Page 10 of 26

Page 11 of 26

experiments changing the pump wavelength was found to change the fitted decay time by less than the measurement uncertainty. a)

intensity (a.u.)

6.3×1017/cm3 4.7×1018/cm3 3.1×1019/cm3 4.7×1019/cm3 7.9×1019/cm3 220 ps 223 ps 118 ps 93 ps 71 ps 0

200

400 600 800 time delay (ps)

1000

b) 300 λpump

250 lifetime from fit (ps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


820 nm 610 nm 532 nm 510 nm

200 150 100 50 0 17 10

1018

1019

1020

estimated exciton density (cm-3)

Figure 3: a) Decays of the transient signal at 532 nm when exciting at 820 nm for various excitation densities. The lifetimes became longer and the fit quality improved as the density was decreased. b) A plot of the lifetime extracted from a fit of the data to a single exponential versus excitation density. Data were collected using several different pump wavelengths as indicated in the legend. Overlaid is the expectation for the 1D diffusion model. There are two distinct methods to extract the exciton lifetime from time-resolved data when annihilation is present. Data collected at relatively high exciton densities can be fitted to exciton annihilation models that assume a particular functional form for the exciton annihilation rate coefficient. 35 This method can be prone to error because it will necessarily yield a lifetime that depends on the assumed model. Alternatively, the excitation density can be lowered until the decay becomes single exponential and independent of exciton density. 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

For a single series of data collected using a 820 nm excitation and 532 nm probe the data are shown in Figure 3a. At an excitation density near 1020 /cm3 the decay is clearly not singleexponential with a decay time of less than 100 ps. At an excitation density of 4.7 × 1018 /cm3 the decay becomes single-exponential within the signal to noise and has a decay time of greater than 200 ps that does not change as the excitation fluence is lowered by almost another factor of ten. Data at many different powers and several different pump wavelengths was collected and fitted to a single exponential decay. The lifetimes are plotted as a function of exciton density in Figure 3b. It can be seen that the fitted time constants increase from ca. 75 ps at 1020 /cm3 to over 200 ps at 1018 /cm3 . Below about 5 × 1018 /cm3 the measured time constant remains unchanged down to the lowest measured densities. These data show power dependent population dynamics that become faster as the exciton density increases. Such behavior is commonly interpreted as an indication of exciton annihilation and several models are known for such processes including the diffusion limited and static annihilation conditions with various dimensionality. 36,37 With the signal to noise of the data these models could not be conclusively differentiated. The 1D diffusion limited annihilation model that has been widely used on phthalocyanine thin films in previous studies fit the present data well. 32,35,38,39 The results show that we are able to achieve exciton densities low enough to avoid bimolecular quenching, which is critical for understanding the dynamics that are relevant in solar materials under solar illumination. The lifetimes reported here are an order of magnitude longer than a previous transient absorption study. 29 Though it is likely that there will be some amount of variation in the measured lifetime for films grown under different conditions, this order of magnitude difference is attributed to exciton annihilation. The present measurements agree remarkably well with a phase-shift fluorescence measurement that reported a lifetime of 350 ± 100 ps. 22 This last experiment is circumstantial evidence that the weak fluorescence from the thin films (Φ ≈ 10−4 ) corresponds to the same excited state as that probed in our experiments using a 532 nm probe wavelength. The correspon-

12


Page 12 of 26

Page 13 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dence between these measurements should not be taken for granted. For materials with low fluorescence quantum yield there is no reason to expect that the emissive excitons probed with fluorescence techniques and the ‘dark’ excitons probed with absorption techniques are one and the same. Diluted H2 Pc Having identified the excitation density at which annihilation becomes negligible we turn to the measurements of the intrinsic excited state lifetimes for the UGH2 diluted films. Experiments at low excitation densities (ca. 1.1 × 1018 /cm3 ) were performed on the 75%, 50%, and 25% H2 Pc samples in a back-to-back manner where only the laser power was adjusted. These dilutions were chosen as this was approximately the same range as used in previous dilution studies which observed notable changes in the photophysical properties of organic photovoltaic materials. 7–9 The dynamics with a 510 nm pump and 532 nm probe are presented in Figure 4a. In all cases the data fit well to a single exponential function. The lifetimes are found to be 239 ± 24 ps, 226 ± 29 ps, 147 ± 24 ps, and 87 ± 9 ps going from the 100% to the 25% film. Data taken on different samples and on different days with different pump wavelengths was consistent with the data shown in Figure 4a. For comparison, Figure 4b shows the time-correlated single photon counting data for a dilute solution of H2 Pc in toluene, which yields a 5.6 ± 0.4 ns lifetime in agreement with previous studies. The lifetime versus concentration data are summarized in Figure 4c. Previous studies have found that higher concentrations of chromophores leads to shorter lifetimes. For example, in PMMA films doped with Perylene red the lifetime decreased from 260 ps to 170 ps as the concentration increased from 30 mM to 90 mM. 40 The lifetimes of a series of different dyes was found to decrease from the nanosecond timescale to the 100 ps timescale going from 100 mM to neat films. 5 Films of SubPc ranging over a similar range of densities used in our work were found to have fluorescence lifetimes that decreased monotonically as the concentration was increased. 7 Similar trends were found for high concentrations

13



a)

239 ps

ΔOD

100% H2Pc

226 ps

75% H2Pc

147 ps

50% H2Pc

87 ps

25% H2Pc 0

200

400 600 time delay (ps)

800

1000

counts

b) 5.6 ± 0.4 ns

0

c)

5

10 15 20 time delay (ns)

25

6000 250 lifetime (ps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

200 150 100 50 0

0

20 40 60 weight % H2Pc

80

100

Figure 4: Decays of the transient absorption signal for different concentrations of H2 Pc in UGH2. The pump wavelength was 510 nm and the probe was 532 nm. The exciton density was maintained at ca. 1.1 × 1018 /cm3 which is well below the annihilation regime. b) Fluorescence decay trace of dilute H2 Pc in toluene collected using time-correlated singlephoton counting (excited at 375 nm, detected at 700 nm). c) The lifetimes of the films and toluene solution plotted versus concentration. of BODIPY in a PMMA matrix; a monotonic lifetime decrease was observed going from 23 mM to 1.2 M concentrations. 41 Shortening of the lifetime is generally attributed to aggregates with short intermolecular 14


Page 15 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


distances that lead to larger non-radiative decay rates. Studies on neat films of mesotetraphenyl porphyrin molecules where the steric bulk of the meso substituents was varied to control the intermolecular separation confirmed this to be the case. The lifetime decreased from 4.4 ns to 260 ps as the substituents were changed from trimethylphenyl to phenyl. 10 From a simple perspective, if the lifetime is 239 ps in the neat film and 5.6 ns in a dilute solution then the lifetime might be expected to monotonically increase as the chromophores are diluted. However, we find that the lifetime increases as the nominal concentration increases, Figure 4c. Understanding this unusual behavior starts by inspecting the steady state absorption spectra. The 100% and 75% films have a pronounced peak near 700 nm, which is indicative of α-phase structure. In the 50% and 25% films this feature is notably less obvious. The change in structure is correlated with the change in lifetime. Recent theoretical work showed that for two perylene based materials, small changes in the molecular arrangement can strongly influence the efficiency of non-radiative decay pathways. 42 Computational studies suggest that when the dimer symmetry is broken, such as occurs at heterojunctions, donor/donor charge transfer states may play an important role in excited state deactivation. 43,44 Thus the data suggest that the lifetime is reduced when the α-phase structure is lost. Transient absorption experiments have shown that a crown ether H2 Pc dimer with a welldefined face-to-face geometry has an excited state lifetime of 113 ps in ethanol solution. 25 It is known that in the α-phase the H2 Pc molecules adopt a slipped herringbone type configuration that yields a system with mixed H- and J- aggregate character such as been discussed for polymers. 45 As the molecules are diluted the longer range α-phase structure is broken up. However, the UV/Vis spectra do not show the presence of a higher monomer concentration. The strong blue shift from the monomer absorption in combination with previous studies on phthalocyanine aggregates suggest that while the long range α-phase order has been lost, in some sense the local face-to-face stacking has not been reduced. The ca. 10 nm blue shift of the main UV/Vis band as the H2 Pc is diluted in UGH2 suggests an enhanced face-to-

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

face stacking. Studies on the crystalline and liquid-crystalline phases of alkoxy substituted phthalocyanines support of this assignment, showing the disappearance of a band at ca. 700 nm and a blue shift of the higher energy portion of the Q band as the slipping angle between molecules is decreased at the liquid crystal phase transition. 27,28 Thus it appears that the diluted films consist of clusters of H2 Pc molecules which are stacked in a face-to-face manner, which is correlated with a reduced lifetime with respect to the neat film. Long-lived Transients For all the collected decays it was found that there was an offset at long time delays that was included in our fitting function. For our data, the offset is ca. 10% of the maximum signal. Initially this signal was assumed to be caused by thermal changes of the ground state spectra due to small temperature increases in the film as has been observed in previous studies on phthalocyanine thin films. 31,32 This hypothesis was tested via comparing the transient spectra collected at 900 ps with the difference spectra that results from subtracting the steady state spectra at room temperature and ca. 40 Celsius (see SI). There was no thermally induced absorption in the vicinity of 532 nm making the offset difficult to reconcile with a thermal effect. An alternative explanation might be absorption by free carriers generated via annihilation processes, however, the magnitude of the offset relative to the maximum signal was not found to change substantially as a function of excitation fluence. It is possible that the offset could arise from excitons trapped at domain boundaries or defect sites which have extended lifetimes; a recent study of diindenoperylene which found a similar 10% offset invoked this explanation. 46 Though we cannot conclusively rule the influence of trap states out, it would be expected that the number of such states would dramatically change in the different concentrations of films, however, no large variation in the offset was found for the different weight % films. The most likely explanation is that the offset is due to intersystem crossing to triplet states that have lifetimes much greater than the timescale of our experiments. For many related

16


Page 16 of 26

Page 17 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


phthalocyanine systems including monomers in dilute solution, 25,47 oligomers in solution, 25,48 and liquid crystals, 27,49 the triplet states were found to have excited state absorptions in the vicinity of 500 nm which strongly overlaps with the singlet excited state absorption in good agreement with our transient spectra. The intersystem crossing rate is known to dramatically increase during dimer formation in solution, potentially keeping triplet formation competitive with internal conversion. 25 Though the absorption cross section for the triplet states in neat films has not been quantitatively determined, previous work using the z-scan technique on ZnPc monomers showed that at 532 nm the triplet cross section was nearly identical to the first excited singlet state cross section. 50 Since both transitions are ligand centered this is not expected to change dramatically for H2 Pc. Assuming the absorption cross sections are equal, this suggests that the phthalocyanine films investigated herein may have triplet yields on the order of 10% which has been previously assumed for H2 Pc liquid crystal assemblies. 27,49 Given the strong excitonic coupling, the energy gap between the lower singlet exciton branch and the triplet state is substantially reduced contributing to an increased intersystem crossing rate. Implications For Organic Photovoltaic Materials These data have implications for the design and understanding of organic photovoltaic materials. First, the monomer is not the relevant building block at the chromophore densities that are necessary for long range exciton transport in H2 Pc. In line with a previous study on perylene derivatives, 42 the dimer is a more accurate conceptual starting point when discussing the optical and exciton transporting properties of concentrated phthalocyanine thin films. Larger aggregates are almost certainly present in the concentrated films as well, but seem to have photophysical properties far more in-line with those of the dimer than the monomer. The dimer has a larger Stokes shift, a shorter lifetime, and substantially lower fluorescence quantum yield than the monomer. The dramatically different photophysical parameters of the dimer imply that the rate of Förster energy transport in the thin films

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

will be orders of magnitude lower than expected based on the monomer properties. Given the photophysical parameters of the dimer, it is not inconceivable that Dexter transport dominates in these materials, though further characterization is necessary to illuminate this point. If this were found to be true then it would demonstrate that an important class of organic semiconductor materials did not follow the conventional logic that singlet excited states transfer via a Förster transfer mechanism. The trend in lifetimes is notable as well. Previous studies have typically found that the exciton lifetime increases as the concentration of chromophores is decreased. Our data clearly shows that for these H2 Pc samples the opposite is true. The lifetime decreases by a factor of three as the film is diluted to 25%. Though understanding the precise cause of this trend will require more work, the implications for tailoring exciton diffusion are relatively clear. In contrast to previous work on SubPc/UGH2 mixtures, the shortened lifetime of H2 Pc will clearly be detrimental contribution to the exciton transport as the H2 Pc is diluted.

Conclusions The excited state lifetime was measured in a series of H2 Pc/UGH2 films. Under annihilation free conditions, the lifetime of the excited state in the 100% H2 Pc film was found to be 239 ± 24 ps, whereas for H2 Pc film diluted to 25% by weight in an UGH2 matrix the exciton lifetime decreases to 87 ± 9 ps. The decrease with dilution contrasts the majority of reports for similar systems. The decreased lifetime is correlated with a loss of the α-phase order which suggests molecular level structure and organization becomes more important than nominal chromophore concentrations. The UV/Vis spectra suggest a possible explanation, that increased face-to-face stacking may be important. Further theoretical and experimental investigations will be needed to address this question. More broadly, as aggregation is ubiquitous in planar polycyclic aromatic hydrocarbons routinely used in organic electronic materials, these results indicate that further work is needed to understand the nanoscale

18


Page 18 of 26

Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


structuring of these diluted films that hold promise for engineering properties such as exciton diffusion length.

Supporting Information Available Wavelength resolved transient absorption data, temperature dependent UV/Vis spectra, and fits to an annihilation model are given in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgement The authors are grateful to Dr. Tom Pundsack for initiating this work and collecting early data, and Philip Goff for technical assistance in setting up the experiments. This work was supported primarily by the National Science Foundation (NSF) Program in Solid State and Materials Chemistry (DMR-1307066).

References (1) Scholes, G. D.; Rumbles, G. Excitons in Nanoscale Systems. Nat. Mater. 2006, 5, 683–696. (2) Bardeen, C. J. The Structure and Dynamics of Molecular Excitons. Annu. Rev. Phys. Chem. 2014, 65, 127–148. (3) Menke, S. M.; Holmes, R. J. Exciton Diffusion in Organic Photovoltaic Cells. Energ. Environ. Sci. 2014, 7, 499–512. (4) Krainer, G.; Hartmann, A.; Schlierf, M. farFRET: Extending the Range in SingleMolecule FRET Experiments Beyond 10 nm. Nano Lett. 2015, 15, 5826–5829.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(5) Al-Kaysi, R. O.; Sang Ahn, T.; Muller, A. M.; Bardeen, C. J. The Photophysical Properties of Chromophores at High (100 mM and above) Concentrations in Polymers and as Neat Solids. Phys. Chem. Chem. Phys. 2006, 8, 3453–3459. (6) Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi, M. The Exciton Model in Molecular Spectroscopy. Pure Appl. Chem. 1965, 11, 371–392. (7) Menke, S. M.; Luhman, W. A.; Holmes, R. J. Tailored Exciton Diffusion in Organic Photovoltaic Cells for Enhanced Power Conversion Efficiency. Nat. Mater. 2013, 12, 152–157. (8) Menke, S. M.; Lindsay, C. D.; Holmes, R. J. Optical Spacing Effect in Organic Photovoltaic Cells Incorporating a Dilute Acceptor Layer. Appl. Phys. Lett. 2014, 104, 243302. (9) Bergemann, K. J.; Amonoo, J. A.; Song, B.; Green, P. F.; Forrest, S. R. Surprisingly High Conductivity and Efficient Exciton Blocking in Fullerene/Wide-Energy-Gap Small Molecule Mixtures. Nano Lett. 2015, 15, 3994–3999. (10) Huijser, A.; Savenije, T. J.; Siebbeles, L. D. A. Effect of the Structure of Substituents on Charge Separation in Meso-Tetraphenylporphyrin/TiO2 Bilayers. Thin Solid Films 2006, 511-512, 208–213. (11) Mullenbach, T. K.; McGarry, K. A.; Luhman, W. A.; Douglas, C. J.; Holmes, R. J. Connecting Molecular Structure and Exciton Diffusion Length in Rubrene Derivatives. Adv. Mater. 2013, 25, 3689–3693. (12) Huss, A. S.; Pappenfus, T.; Bohnsack, J.; Burand, M.; Mann, K. R.; Blank, D. A. The Influence of Internal Charge Transfer on Nonradiative Decay in Substituted Terthiophenes. J. Phys. Chem. A 2009, 113, 10202–10210. (13) Gouterman, M. Spectra of Porphyrins. J. Mol. Spectrosc. 1961, 6, 138–163. 20


Page 20 of 26

Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Lucia, E. A.; Verderame, F. D. Spectra of Polycrystalline Phthalocyanines in the Visible Region. J. Chem. Phys. 1968, 48, 2674–2681. (15) Vincett, P.; Popovic, Z.; McIntyre, L. A Novel Structural Singularity in VacuumDeposited Thin Films: The Mechanism of Critical Optimization of Thin Film Properties. Thin Solid Films 1981, 82, 357–376. (16) Bayliss, S. M.; Heutz, S.; Rumbles, G.; Jones, T. S. Thin Film Properties and Surface Morphology of Metal Free Phthalocyanine Films Grown by Organic Molecular Beam Deposition. Phys. Chem. Chem. Phys.. 1999, 1, 3673–3676. (17) Sakakibara, Y.; Bera, R. N.; Mizutani, T.; Ishida, K.; Tokumoto, M.; Tani, T. Photoluminescence Properties of Magnesium, Chloroaluminum, Bromoaluminum, and MetalFree Phthalocyanine Solid Films. J. Phys. Chem. B 2001, 105, 1547–1553. (18) Yim, S.; Heutz, S.; Jones, T. S. Model for the α-β Phase Transition in Phthalocyanine Thin Films. J. Appl. Phys. 2002, 91, 3632–3636. (19) Gadalla, A.; Crégut, O.; Gallart, M.; Hönerlage, B.; Beaufrand, J. B.; Bowen, M.; Boukari, S.; Beaurepaire, E.; Gilliot, P. Ultrafast Optical Dynamics of Metal-Free and Cobalt Phthalocyanine Thin Films. J. Phys. Chem. C 2010, 114, 4086–4092. (20) Yoshida, H.; Tokura, Y.; Koda, T. Charge-transfer Excitation Bands in Electroabsorption Spectra of Metal (Co, Ni, Cu, Zn)-phthalocyanine Films. Chem. Phys. 1986, 109, 375–382. (21) Yoshino, K.; Hikida, M.; Tatsuno, K.; Kaneto, K.; Inuishi, Y. Emission Spectra of Phthalocyanine Crystals. J. Phys. Soc. Jpn 1973, 34, 441–445. (22) Menzel, E.; Jordan, K. J. Fluorescence of Solid Metal-free Phthalocyanine. Chem. Phys. 1978, 32, 223–229.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(23) Pakhomov, G. L.; Gaponova, D. M.; Lukyanov, A. Y.; Leonov, E. S. Luminescence of Phthalocyanine Thin Films. Phys. Solid State 2005, 47, 170–173. (24) Madigan, C. F.; Bulović, V. Solid State Solvation in Amorphous Organic Thin Films. Phys. Rev. Lett. 2003, 91, 247403. (25) Nikolaitchik, A. V.; Korth, O.; Rodgers, M. A. J. Crown Ether Substituted Monomeric and Cofacial Dimeric Metallophthalocyanines. 1. Photophysical Studies of the Free Base, Zinc(II), and Copper(II) Variants. J. Phys. Chem. A 1999, 103, 7587–7596. (26) Schutte, W. J.; Sluyters-Rehbach, M.; Sluyters, J. H. Aggregation of an Octasubstituted Phthalocyanine in Dodecane Solution. J. Phys. Chem. 1993, 97, 6069–6073. (27) Markovitsi, D.; Lecuyer, I.; Simon, J. One-dimensional Triplet Energy Migration in Columnar Liquid Crystals of Octasubstituted Phthalocyanines. J. Phys. Chem. 1991, 95, 3620–3626. (28) Duzhko, V.; Singer, K. D. Self-assembled Fibers of a Discotic Phthalocyanine Derivative: Internal Structure, Tailoring of Geometry, and Alignment by a Direct Current Electric Field. J. Phys. Chem. C 2007, 111, 27–31. (29) Gadalla, A.; Beaufrand, J.-B.; Bowen, M.; Boukari, S.; Beaurepaire, E.; Crégut, O.; Gallart, M.; Hönerlage, B.; Gilliot, P. Ultrafast Optical Dynamics of Metal-free and Cobalt Phthalocyanine Thin Films II : Study of Excited-state Dynamics. J. Phys. Chem. C 2010, 114, 17854–17863. (30) Greene, B. I.; Millard, R. R. Singlet-exciton Fusion in Molecular Solids: a Direct Subpicosecond Determination of Time-dependent Annihilation Rates. Phys. Rev. Lett. 1985, 55, 1331–1334. (31) Greene, B. I.; Millard, R. R. Subpicosecond Spectroscopic Studies of Singlet Exciton Fusion in Molecular Solids. J. Phys. Colloq. 1985, 46, 371–376. 22


Page 22 of 26

Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(32) Gulbinas, V.; Chachisvilis, M.; Valkunas, L.; Sundström, V. Excited State Dynamics of Phthalocyanine Films. J. Phys. Chem. 1996, 100, 2213–2219. (33) Gulbinas, V. Transient Absorption of Photoexcited Titanylphthalocyanine in Various Molecular Arrangements. Chem. Phys. 2000, 261, 469–479. (34) Steinfeld, J. I.; Francisco, J. S.; Hase, W. L. Chemical Kinetics and Dynamics; Prentice Hall, 1999. (35) Ma, Y. Z.; Xiao, K.; Shaw, R. W. Exciton-exciton Annihilation in Copperphthalocyanine Single-crystal Nanowires. J. Phys. Chem. C 2012, 116, 21588–21593. (36) Engel, E.; Leo, K.; Hoffmann, M. Ultrafast Relaxation and Exciton-exciton Annihilation in PTCDA Thin Films at High Excitation Densities. Chem. Phys. 2006, 325, 170–177. (37) Marciniak, H.; Li, X. Q.; W¨ urthner, F.; Lochbrunner, S. One-dimensional Exciton Diffusion in Perylene Bisimide Aggregates. J. Phys. Chem. A 2011, 115, 648–654. (38) Gulbinas, V.; Chachisvilis, M.; Persson, A.; Sundström, V.; Svanberg, S. Ultrafast Excitation Relaxation in Colloidal Particles of Chloroaluminum Phthalocyanine: Onedimensional Exciton-exciton Annihilation. J. Phys. Chem. 1994, 98, 8118–8123. (39) Zhou, J.; Mi, J.; Zhu, R.; Li, B.; Qian, S. Ultrafast Excitation Relaxation in Titanylphthalocyanine Thin Film. Opt. Mater. 2004, 27, 377–382. (40) Schlosser, M.; Lochbrunner, S. Exciton Migration by Ultrafast Forster Transfer in Highly Doped Matrixes. J. Phys. Chem. B 2006, 110, 6001–6009. (41) Vu, T. T.; Dvorko, M.; Schmidt, E. Y.; Audibert, J. F.; Retailleau, P.; Trofimov, B. A.; Pansu, R. B.; Clavier, G.; Méallet-Renault, R. Understanding the Spectroscopic Properties and Aggregation Process of a New Emitting Boron Dipyrromethene (BODIPY). J. Phys. Chem. C 2013, 117, 5373–5385. 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(42) Settels, V.; Schubert, A.; Tafipolski, M.; Liu, W.; Stehr, V.; Topczak, A. K.; Pflaum, J.; Deibel, C.; Fink, R. F.; Engel, V. et al. Identification of Ultrafast Relaxation Processes as a Major Reason for Inefficient Exciton Diffusion in Perylene-based Organic Semiconductors. J. Am. Chem. Soc. 2014, 136, 9327–9337. (43) Jailaubekov, A.; Willard, A.; Tritsch, J.; Chan, W.-L.; Sai, N.; Gearba, R.; Kaake, L.; Williams, K.; Leung, K.; Rossky, P. et al. Hot Charge-transfer Excitons Set the Time Limit for Charge Separation at Donor/acceptor Interfaces in Organic Photovoltaics. Nat. Mater. 2013, 12, 66–73. (44) Lee, M. H.; Dunietz, B. D.; Geva, E. Donor-to-donor vs Donor-to-acceptor Interfacial Charge Transfer States in the Phthalocyaninefullerene Organic Photovoltaic System. J. Phys. Chem. Lett. 2014, 5, 3810–3816. (45) Spano, F. C.; Silva, C. H- and J-aggregate Behavior in Polymeric Semiconductors. Annu. Rev. Phys. Chem. 2014, 65, 477–500. (46) Nichols, V. M.; Broch, K.; Schreiber, F.; Bardeen, C. J. Excited-state Dynamics of Diindenoperylene in Liquid Solution and in Solid Films. J. Phys. Chem. C 2015, 119, 12856–12864. (47) McVie, J.; Sinclair, R. S.; Truscott, T. G. Triplet States of Copper and Metal-free Phthalocyanines. J. Chem. Soc. Farad. T. 2 1978, 74, 1870–1879. (48) Gunaratne, T.; Kennedy, V. O.; Kenney, M. E.; Rodgers, M. A. J. Synthesis and Excited State Dynamics of µ-oxo Group IV Metal Phthalocyanine Oligomers: Trimers and Tetramers. J. Phys. Chem. A 2004, 108, 2576–2582. (49) Markovitsi, D.; Thu Hoa Tran Thi, .; Briois, V.; Simon, J.; Ohta, K. Laser Induced Triplet Excitons in the Columnar Phases of an Octasubstituted Metal Free Phthalocyanine. J. Am. Chem. Soc. 1988, 110, 2001–2002.

24


Page 24 of 26

Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(50) De Boni, L.; Piovesan, E.; Gaffo, L.; Mendon¸ca, C. R. Resonant Nonlinear Absorption in Zn-phthalocyanines. J. Phys. Chem. A 2008, 112, 6803–6807.

25



Graphical TOC Entry monomer (5.6 ns) α-H2Pc film (239 ps)

H2Pc N N

HN

NH

N

N

N

N

lifetime

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

H-aggregate (

Intermolecular Interactions Determine Exciton Lifetimes in Neat Films

Recommend Documents