Communication
the same wavelength range as the steady-state fluorescence,
and therefore corresponded with stimulated emission (SE)
from the excited state. Both features were observed to decay
within hundreds of picoseconds; the 450 nm transient exhibits
a lifetime of 119 ps, as was determined with a single-exponen-
tial fit (inset of Figure 3).
and package used to compute their IR spectra for Figure 1. Cal-
culated Raman spectra for the neutral and reduced model
structures are plotted with the data in Figure 4.
Although our calculated Raman spectra did not account for
resonance enhancement, comparisons between experiment
and calculations showed reasonable agreement. The most in-
tense excited-state features observed correspond with C=C
stretching (1516 cmÀ1) and ring stretching (1568 cmÀ1) in the
reduced cyanovinyl arene moiety. A weaker C=O stretching
band appeared at 1630 cmÀ1, whereas three features in the
region between 1000 and 1400 cmÀ1 arise from vibrational
modes associated with the arene ring of the negatively
charged acceptor (SiÀC stretch, CÀH bend, and CÀH rock). The
ground-state feature at 1592 cmÀ1 is consistent with the C=C
stretching mode predicted for the neutral cyanovinyl. We note
that because the C=C and C=O stretches are both IR and
Greater structural insight was obtained by applying a vibra-
tionally sensitive probe of the excited state.[10] Herein, we have
used femtosecond stimulated Raman spectroscopy (FSRS) to
interrogate the properties of photoexcited 1a–c. FSRS uses the
combination of a narrowband (ca. 20 cmÀ1) Raman excitation
and ultrafast (<100 fs) broadband probe to stimulate Raman
transitions of multiple vibrational modes coherently.[11] In our
measurements, resonantly enhanced excited-state FSRS spectra
were obtained by using a Raman-excitation wavelength of
480 nm. The photoselectivity on resonance enables us to char-
acterize charge-separated character of the electronic state that
underlies the transient absorbance peaked at 450 nm.[12]
The excited-state FSRS (ER) spectra of 1a–c and the ground-
state FSRS (GR) spectrum of 1c are plotted in Figure 4. The
ground-state spectrum exhibited a feature discernible from
noise near 1600 cmÀ1; the weak intensity of this spectrum can
be attributed to a lack of resonant enhancement with the
ground-state transition in the near UV. In contrast, the Raman
spectra of the excited oligomers exhibited a series of intense
CÀ
Raman active the predicted vibrational spectra of 3b and b
are consistent. Thus, Figures 1 and 4 in combination illustrate
that charge separation occurred in the excited, but not ground
states of these compounds.
Figure 4 also reveals a correlation between the C=C stretch-
ing frequency and silane chain length: For 1b and c, the C=C
and ring stretching modes are separated by 54 and 60 cmÀ1,
respectively, with partial overlap of these features in the spec-
trum of the former. For compound 1a, the C=C and ring
stretching modes merge entirely. This correlation implies that
the degree of photoinduced charge transfer is related to the
electron-donating strength of the silane chain,[4,13] a general
property of charge transfer complexes.[8]
resonantly enhanced features between 1000 and 1650 cmÀ1
.
Time-resolved measurements showed that all of these features
decay simultaneously on a timescale commensurate with the
decay of (Figure 3) the transient electronic absorption spec-
trum and therefore can be assigned to vibrations of the corre-
sponding photoprepared electronic state.
To establish the relevance of the optical charge-transfer
mechanism in the solid state, we explored the photoresponse
of 1d drop-cast as a film on a glass slide. We have previously
shown that replacement of the methyl ester with an n-hexyl
ester resulted in a more continuous film morphology without
perturbing solution phase electronic properties.[2] Figure 3
compares the TA spectra of 1d in film (black line) with 1b in
dichloromethane (red line), both probed 1 ps after 360 nm ex-
citation. Both excited samples exhibited the absorption band
near 450 nm attributed to the reduced cyanovinyl arene
moiety based on the results of our resonant FSRS measure-
ments. This similarity reveals that charge transfer within the oli-
gosilane compounds can be induced by photoillumination in
the solid state as well. The most significant difference between
the two TA spectra is the lack of stimulated emission from the
film, which may reflect intermolecular charge separation; fur-
ther characterization of photoinduced electron transfer and
transport in the solid-state is underway.
The Raman features apparent in Figure 4 arise largely from
cyanovinyl vibrational modes. We calculated Raman spectra of
the neutral and reduced forms of 3b in their optimized
ground-state geometries with the same computational level
In conclusion, we have characterized the photophysics of
a series of s–p hybrid materials by using a combination of
steady-state and transient spectroscopies. These compounds
undergo Franck–Condon (optical) intramolecular electron
transfer from the silane to cyanovinyl moieties upon excitation.
This is illustrated most clearly with excited-state FSRS spectra
that showed reduction of the cyanovinyl arene moiety in the
photoprepared excited state. Additionally, the frequency of the
C=C stretch mode is correlated with the length of the central
silane chain and is related to the degree of charge transfer.
Figure 4. Excited-state Raman (ER) of 1a–c and ground-state Raman (GR) of
1c in dichloromethane with l=360 nm actinic and 480 nm Raman excita-
tions. The time delay between actinic and Raman pulses is 10 ps. Calculated
Raman spectra of neutral and reduced forms of 3b are also plotted for com-
parison. Residual solvent signals at 1139 and 1415 cmÀ1 are marked with as-
terisks.
Chem. Eur. J. 2016, 22, 6204 – 6207
6206
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