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One of the parameters to determine the carrier hopping rate is the
reorganization energy (l), which comes from the contributions of external
reorganization energy and internal reorganization energy. The later includes the molecular geometry modifications when an
electron is added or removed from a molecule and the former
is the modifications in the surrounding medium due to polarization
effects. Norton and Brédas 46 reported that the external reorganization energy is significantly
lower than that of the inner part based on a polarized force field calculation. It is also fundamental to calculate the ionization potential (IP)
and electron affinity (EA), which characterize the reduction and oxidation
properties 73. These descriptors can be used to evaluate the energy
barriers of holes and electron injection including charge mobility and balanced
charge. The calculated vertical and adiabatic IPs (VIP and AIP), the vertical
and adiabatic EAs (VEA and AEA), as well as the  reorganization energies for holes (lhole) and electrons (lelec)  for the compounds under
probe at B3LYP/6-31G(d) are presented in Table 4. First
inspection of Table
4 shows that the R01 P9 compound has the lowest IP (5.395 eV) and
medium EA (0.267 eV).  However, the
parent compound R01 has the highest EA (0.462 eV), but its IP is also high
(5.751 eV) in comparison with other compounds.  

For
reorganization energy, as is seen in Table
4, the lholes values of the investigated molecues are ranged from  0.152 eV to 0.664 eV, while
the  lelecs values are in the
interval of 0.338 – 0.517 eV. As shown, the
first important remark to be noted is that the investigated molecules exhibit lelecs values larger than the lholes values, except of R01 P1 and R01 P7, implying that the electron
mobility is larger than the hole mobility, which is reflected by the khole
and kelec values  (Table 5). We find that the
lhole of the parent
molecule R01 is only 0.0.297 eV, while its lelec reaches as high as 0.368 eV; the latter is higher than the former.
Hence, R01 should be suit to transporting holes than electrons. These results indicate, from the reorganization
energy point of view, that R01 is p-channel rather than n-channel. In addition, the lhole of R01 is approximately equal to
that of a typical hole transport material N,N’-diphenyl-N,N’-bis(3-methylphenyl)-(1,1
0-biphenyl)-4,4′-diamine (TPD) with lhole  = 0.29 eV. This also means that its hole transfer rate, (khole:
1.9
x 1015 s-1),
Table 5, should be approximately equal that of TPD, thus compound R01
could be good hole transfer materials. Unlike the others, R01 P7 and R01
P9 displayed considerable hole reorganization energies (0.664 eV and 0.460 eV),
which can be ascribed to their large structural distortions when the ionization
process takes place (Table 1). In
comparison with the parent molecule (R01), most R01 derivatives exhibit larger lholes and lelecs
values, but R01 P3, R01 P6 and R01 P10 exhibit smaller lhole value and
only R01 P1 exhibits smaller lelec value. The
calculated lholes for R01 P3, R01 P6 and R01 P10 are 0.28, 0.152 and 0.264 eV,
respectively, which are clearly smaller that the value of R01 (0.297 eV). These results indicate that
R01 P3, R01 P6 and R01 P10 may possess higher hole
mobilities, which can be easily noticed
from the khole values Table 5. 

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