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(c) In polymer HJ-aggregates, Coulombic interchain coupling is positive ( Jinter > 0), whereas the effective
intrachain coupling between adjacent repeat units is negative ( Jintra < 0) owing to through-bond interactions
in 1D direct band-gap semiconductors. Also shown is the energy dispersion, E(k), corresponding to the lowest
vibronic band in each aggregate (higher bands are omitted for clarity). The band curvature at k = 0 is positive
(negative) in J- (H-)aggregates. The red dot indicates the (k = 0) exciton that is optically allowed from the
ground state, |G> (black dot). The energies of the one- and two-phonon states within the electronic ground
state are also indicated. The dispersionless (Einstein) phonons of wave vector q derive from the intramolecular
vibrations with frequency ω0. Arrows indicate emission pathways at low temperatures, such that emission
originates primarily from the lowest-energy exciton. In J-aggregates, 0-0 emission is strongly allowed,
leading to superradiance. In contrast, in H-aggregates, rapid intraband relaxation subsequent to absorption
populates the lowest-energy k = π exciton, which cannot radiatively couple to |G>, thereby preventing
0-0 emission (assuming no disorder). In the HJ-dimer, the J-like intrachain band in each polymer is split
into symmetric (+) and antisymmetric (−) bands by interchain interactions. Owing to selection rules, only
the k = 0 symmetric state can radiatively couple to |G>. Hence, in HJ-aggregates, as well as H-aggregates,
0-0 emission is thermally activated. 0-0 emission is also made allowed by symmetry-breaking disorder
(((((((((((((((
A π-stack of such J-like polymer chains presents an interesting photophysical dilemma—does
the stack behave like an H-aggregate, as dictated by the side-by-side arrangement of chains, or
like a J-aggregate, as dictated by the intrachain head-to-tail arrangement of repeat units within
a chain? As we have found, the π-stack displays a unique set of hybrid photophysical properties
depending mainly on the competition between intrachain electronic coupling, which favors J-like
behavior, and interchain coupling, which favors H-like behavior. Hence, we refer to a polymer
π-stack as an HJ-aggregate (47). An HJ-dimer is depicted in Figure 1. The delicate interplay
of interactions, as manifest in changes in the absorption and PL lineshapes, is evident in many
luminescent conjugated polymers. For example, spin-cast films of P3HT (with chloroform as the
solvent) behave remarkably like H-aggregates, likely because of the predominance of aggregates
comprising chains with relatively short conjugation lengths, owing to the high level of disorder
produced by spin casting from a low-boiling-point solvent (24–26). Several groups have determined that the excitonic interaction between two neighboring polymer chains diminishes with
chain length (48–52); hence, short conjugation lengths lead to stronger interchain interactions
and H-aggregate behavior (24–27, 34). In such P3HT films, the 0-0 peak in the steady-state PL
spectrum is substantially attenuated compared with solution, as expected for H-aggregates. Also
consistent with H-aggregation is the increase in the relative 0-0/0-1 vibronic peak ratio in the PL
spectrum with increasing temperature (25, 27) and the decrease of the PL ratio following impulsive
excitation higher into the exciton band (25, 53–55). All these observations can be accounted for
with the H-aggregate model (24–27), which derives directly from Kasha’s model: Each polymer
(or polymer segment) in the π-stack is treated as a single chromophore coupled via Coulombic
interactions to neighboring chromophores within the stack. The model is 1D, as it ignores exciton
motion along the polymer chain.
To account for the H- and J-like photophysical properties exhibited by many emissive conjugated polymers, we developed the HJ-aggregate model (47), which accounts for both inter- and
intramolecular degrees of freedom, incorporating the H-like behavior induced by interchain coupling and the J-like behavior induced by intrachain coupling. Such a model can potentially reconcile
the diverse range of photophysical behaviors exhibited by thiophene-, phenylene vinylene-, and
fluorene-based conjugated polymers. The inter- versus intrachain competition is quite sensitive
to the nature and magnitude of disorder and can lead to exotic photophysics, such as disorderinduced crossover from H- to J-aggregate behavior. Moreover, one can generally deduce the 2D
extent of the exciton coherence in π-stacks from the structure of the PL spectral lineshape. Exciton
coherence is strongly dependent on morphology (72–76) and has recently been analyzed in detail
in MEH-PPV solutions and nanoparticles using ultrafast polarization decay (73). Recent work on
P3HT suggests that exciton coherence in P3HT π-stacks is spread approximately isotropically
along the polymer backbone and along the stacking axis in a manner that depends on molecular
weight (76).
Below we review the experimental evidence for H- and J-aggregate behavior in several emissive conjugated polymers, beginning with P3HT spin-cast films under the H-aggregate model in
Section 2, followed by the seminal work by Schott and coworkers on single PDA chains described
within the J-aggregate model in Section 3. We next consider P3HT whiskers and MEH-PPV
red-phase aggregates within the HJ-aggregate model in Section 4. Interestingly, the PPV-based
polymers are more J-like than is P3HT, suggesting an increased intrachain bandwidth or a decreased interchain bandwidth. Both J- and H-aggregate influences are readily apparent in the
2D exciton coherence function considered in Section 5. Section 6 summarizes our findings and
outlines important future directions.
4. HJ-AGGREGATES IN P3HT AND M EH-PPV FILMS
In a polymer π-stack, typical of the packing present in P3HT films, there exists competition
between the interchain (H-favoring) interactions considered in the H-aggregate model and intrachain ( J-favoring) interactions. As discussed in Section 2, the H-aggregate model well describes the
photophysical properties of P3HT π-stacks in spin-cast films. However, in a recent investigation,
P3HT whiskers formed by slowly cooling a P3HT/toluene solution showed dominant J-aggregate
photophysical behavior (56). As shown in Figure 2 c, the absorption and PL spectra of such whiskers
are dominated by the 0-0 vibronic component, unlike the case for the spin-cast H-aggregates
shown in Figure 2 a. The ability of one polymer to assume both H- and J-aggregate forms most
likely results from differing morphologies (see Figure 2 b): In whiskers, superior ordering along
the chains promotes stronger intrachain interactions and weaker interchain interactions (see the
discussion following Equation 1). The mostly H-like behavior occurring in P3HT films cast from
the lowest-boiling-point solvents, such as chloroform, arises from greater disorder from rapid
solvent evaporation and hence shorter conjugation lengths (and greater interchain interactions).
The HJ-aggregate model outlined in detail in Reference 47 considers excitons delocalized both
along and across polymer chains within a π-stack and is therefore able to unravel the competitive
effects of intrachain ( J-favoring) versus interchain (H-favoring) interactions and their impact on
the photophysical response. In Reference 47, a pair of cofacial polymer chains was considered with
Coulombic interactions between adjacent repeat units on neighboring chains. When disorder is
ignored, as may occur between two PDA chains prepared in situ from the monomer crystal, the
interchain interaction leads to a symmetric and antisymmetric version of each intrachain exciton
of wave vector k, with a splitting, E, independent of k, as depicted in Figure 1. Here the
symmetry refers to a reflection plane bisecting the dimer pair. Interestingly, only the k = 0,
symmetric exciton, which is of higher energy, can provide 0-0 emission; in other words, it is the
only state that couples radiatively to the vibrationless ground state. Hence, 0-0 emission must be
thermally activated, as in an H-aggregate. Under the parabolic band approximation and within the
thermodynamic limit, the complete temperature dependence of the PL ratio takes the form (47)
When the splitting vanishes, as in two noninteracting chains, Equation 6 reduces to the singlechain result in Equation 5. As demonstrated in Reference 47, when kbT is approximately E,
the PL ratio peaks and thereafter decreases with increasing temperature, just like a J-aggregate.
The maximum PL ratio scales as √ωc/ E, directly demonstrating the competitive influences
of intrachain (ωc ) and interchain ( E) interactions. The overall temperature dependence shows
that, with respect to the PL, the dimer displays an H-to-J transition upon increasing temperature.
The dependence of the PL ratio as a function of disorder has a similar form (N.J. Hestand,
H. Yamagata & F.C. Spano, unpublished data). For example, for diagonal energetic disorder,
in which each repeat unit has a randomly chosen excitation energy taken from a Gaussian
distribution of width, σ, one can show that at low temperatures (kbT E), the PL ratio initially
increases with σ, like an H-aggregate; peaks when σ is of the order of the splitting E; and
then decreases with further increases in σ, as is characteristic of a J-aggregate. (This assumes a
constant interchain interaction throughout.)
http://en.wikipedia.org/wiki/Book_of_Soyga
http://wh40klib.ru/codex/
http://www.wdl.org/en/item/3042/
https://www.google.ru/#newwindow=1&q=codex+seraphinianus+pdf
https://docs.google.com/file/d/0B7LgTk_W9V8PbmliZ0RZVTlIMTg/edit
http://mysteriousuniverse.org/2010/02/gift-of-the-gods-download-jungs-alchemical-red-book/
http://vk.com/doc53690634_143270371?hash=7775825b92acff1eba&dl=22cf3be38f0a1c0052