gradual growth of cross peaks, followed by a subsequent decay due to vibrational
relaxation [157]. Taking into account the dependence of the energy transfer rate
[77, 145, 182, 183] on l
4 r
À6 (Eq. 5), two possible reasons have been proposed,
which may explain the missing cross peaks. First, the azide functional groups
exhibit a comparatively low extinction coefficient (* 550 M
-1 cm
-1
), thus keeping
the energy transfer rate intrinsically low. Second, the even stronger dependence on
the inter-particle distance further reduces the transfer rate, thus possibly making the
process even more unlikely. Moreover, the azide group has a comparatively short
vibrational lifetime (\ 2 ps) and any energy transfer significantly exceeding the
lifetime will be just too difficult to be observed experimentally. This interpretation
is consistent with the observation that vibrational coupling and intermolecular
energy transfer have been resolved in aggregated or structurally more rigid samples
with much stronger absorption coefficients such as metal-carbonyl samples
[156, 158].
Further investigations of 2D IR at interfaces concerned the orientation of
molecules and in particular orientation of functional groups with respect to the
interface. The interfacial orientation of functional groups has important consequences for specific applications, e.g. for coordination-chemistry compounds, for
molecular recognition or chemical reactivity of catalysts. In addition to simple
considerations regarding chemical accessibility, interfacial orientation can also have
an impact on substrate-adsorbate interactions. Xiong et al. have demonstrated how
to exploit the properties of the fourth-order nonlinear susceptibility of a sample from
immobilized molecules in 2D SFG spectroscopy to obtain insight into molecular
orientation and substrate-adsorbate interactions [184–186]. Being based on the
even-order susceptibility terms, SFG methods (stationary as well as time-resolved)
can beneficially determine the average orientation of functional groups by the sign
of signal [15, 187]. This advantage is based on the fact that the susceptibilities are in
general complex-valued quantities with real and imaginary parts, which may have
both positive and negative contributions [188–191]. Note that in contrast to evenorder methods, odd-order methods (e.g. 2D IR) result in the same signal signs for
parallel and anti-parallel orientation of transition dipole moments with respect to the
surface normal and thus cannot differentiate the two cases of orientation. Figure 13
(a) shows an exemplary heterodyne-detected (HD) 2D SFG spectrum of a model
compound, Re(diCN-bpy)(CO) 3 Cl, on an Au surface. Such tricarbonyl complexes
exhibit three strongly IR-active normal modes in the spectral range of investigation
[A
0 1 ([ 2000 cm
-1 ) as well as A
0 2 and A
00 (\ 2000 cm
-1 ), respective directions of
the vibrational modes are indicated by the blue sticks in Fig. 13c]. Figure 13b
shows for comparison a third-order 2D IR spectrum of the same complex dissolved
in bulk solution (DMSO). The 2D SFG spectrum shows that upon immobilization,
the A
0 1 and A
0 2/A
00 modes experience extensive line broadening as compared to the
bulk solution sample and furthermore exhibit opposite signs in the ground state
bleach and excited state absorption signals (yellow/red vs. blue/green, respectively).
The opposite signs indicate opposite orientations of the modes with respect to the
surface. However, this information was concluded to be insufficient to determine the
absolute orientation of the modes. To gain detailed insight into absolute
Top Curr Chem (Z) (2017) 375:86
123
148
Reprinted from the journal
relaxation [157]. Taking into account the dependence of the energy transfer rate
[77, 145, 182, 183] on l
4 r
À6 (Eq. 5), two possible reasons have been proposed,
which may explain the missing cross peaks. First, the azide functional groups
exhibit a comparatively low extinction coefficient (* 550 M
-1 cm
-1
), thus keeping
the energy transfer rate intrinsically low. Second, the even stronger dependence on
the inter-particle distance further reduces the transfer rate, thus possibly making the
process even more unlikely. Moreover, the azide group has a comparatively short
vibrational lifetime (\ 2 ps) and any energy transfer significantly exceeding the
lifetime will be just too difficult to be observed experimentally. This interpretation
is consistent with the observation that vibrational coupling and intermolecular
energy transfer have been resolved in aggregated or structurally more rigid samples
with much stronger absorption coefficients such as metal-carbonyl samples
[156, 158].
Further investigations of 2D IR at interfaces concerned the orientation of
molecules and in particular orientation of functional groups with respect to the
interface. The interfacial orientation of functional groups has important consequences for specific applications, e.g. for coordination-chemistry compounds, for
molecular recognition or chemical reactivity of catalysts. In addition to simple
considerations regarding chemical accessibility, interfacial orientation can also have
an impact on substrate-adsorbate interactions. Xiong et al. have demonstrated how
to exploit the properties of the fourth-order nonlinear susceptibility of a sample from
immobilized molecules in 2D SFG spectroscopy to obtain insight into molecular
orientation and substrate-adsorbate interactions [184–186]. Being based on the
even-order susceptibility terms, SFG methods (stationary as well as time-resolved)
can beneficially determine the average orientation of functional groups by the sign
of signal [15, 187]. This advantage is based on the fact that the susceptibilities are in
general complex-valued quantities with real and imaginary parts, which may have
both positive and negative contributions [188–191]. Note that in contrast to evenorder methods, odd-order methods (e.g. 2D IR) result in the same signal signs for
parallel and anti-parallel orientation of transition dipole moments with respect to the
surface normal and thus cannot differentiate the two cases of orientation. Figure 13
(a) shows an exemplary heterodyne-detected (HD) 2D SFG spectrum of a model
compound, Re(diCN-bpy)(CO) 3 Cl, on an Au surface. Such tricarbonyl complexes
exhibit three strongly IR-active normal modes in the spectral range of investigation
[A
0 1 ([ 2000 cm
-1 ) as well as A
0 2 and A
00 (\ 2000 cm
-1 ), respective directions of
the vibrational modes are indicated by the blue sticks in Fig. 13c]. Figure 13b
shows for comparison a third-order 2D IR spectrum of the same complex dissolved
in bulk solution (DMSO). The 2D SFG spectrum shows that upon immobilization,
the A
0 1 and A
0 2/A
00 modes experience extensive line broadening as compared to the
bulk solution sample and furthermore exhibit opposite signs in the ground state
bleach and excited state absorption signals (yellow/red vs. blue/green, respectively).
The opposite signs indicate opposite orientations of the modes with respect to the
surface. However, this information was concluded to be insufficient to determine the
absolute orientation of the modes. To gain detailed insight into absolute
Top Curr Chem (Z) (2017) 375:86
123
148
Reprinted from the journal