A Series on TAS - Application in Molecular Systems

In the previous article, we thoroughly examined the basic process and detailed principles of transient absorption spectroscopy detection. In this article, we will further explore the application of ultrafast transient absorption spectroscopy in molecular systems, focusing on both ultrafast transient absorption and ultrafast transient reflection spectroscopy.

01 Molecular Excited State (Ground State Bleaching)

Let’s begin with a simple example using an organic molecule C and its most basic excited-state process S₁, to illustrate the principle behind the generation of transient absorption spectra.

Figure 1a shows the steady-state absorption spectrum of molecule C in its ground state (S₀). When molecule C is excited (pumped) by a pulsed laser (transient absorption spectroscopy requires a pulsed light source for excitation), a portion of the molecules absorb photons and undergo an S₀ → S₁ electronic transition, forming excited-state molecules C* (Figure 1b). At this point, the sample consists of a mix of unexcited ground-state molecules (C) and excited-state molecules (C*). The proportion of C* depends on the pump wavelength and the intensity of the excitation light source.

If we then probe the sample’s instantaneous absorption spectrum at a specific delay time (before C* decays back to the ground state), we will observe that the excited sample’s absorption spectrum AC+C* is weaker than that of the same concentration of ground-state molecules AC (as shown in Figure 1a). This is because the excited-state molecules C* no longer exhibit the same absorption characteristics as ground-state molecules C—i.e., AC ≠ AC*. This change in absorption before and after excitation forms the basis of transient absorption spectroscopy.

Thus, the observed decrease in absorbance in Figure 1a after excitation is due to the presence of excited-state molecules C*. As shown in Figure 1c, by subtracting AC from AC+C*, we obtain the transient absorption spectrum (∆A) of molecule C at a specific time point after excitation. This differential spectrum ∆A is typically expressed as: ∆A = Apump – Aunpump that is, the change in absorption before and after excitation. Based on this principle, a negative ∆A signal that aligns with the steady-state absorption peak indicates a ground-state bleach (GSB).

Essentially, any process in which light excitation causes a molecule to leave the ground state results in a ground-state bleaching signal. By adjusting the time delay between pump and probe pulses using appropriate techniques, we can obtain the time-resolved transient absorption spectra ∆A(t) of molecule C (as shown in Figure 1d). Since excitation occurs on a sub-femtosecond timescale, the corresponding ∆A signal is generated instantaneously after excitation. In practice, however, the generation of the ∆A signal is constrained by the instrument response function (IRF).

According to the photophysical processes shown in Figure 1b, this series of spectra reflects the decay of excited-state molecule C* back to the ground state through radiative (Kr) or non-radiative (Knr) pathways. Correspondingly, the GSB signal appears immediately upon excitation and gradually diminishes over time.

From the ∆A(t) spectra, we can extract the transient kinetic curve at a specific wavelength (Figure 1d). The fast-rising edge reflects the excitation process, while the recovery to zero indicates the decay of the excited state. By fitting this curve, we can obtain kinetic parameters such as the excited-state decay rate.

Figure 1. The process of generating a transient absorption spectrum using molecule C as an example.

(a) A comparison between the steady-state absorption spectrum of the molecules and the absorption spectrum at a specific time after excitation. (b) The fundamental electronic transition process after the excitation of molecules C. After molecules C are excited, a portion of molecules C are excited and transformed into excited state molecules (C*). The kr and knr are the radiative and nonradiative decay rates of the excited state molecules C*, respectively. (c) The transient absorption spectrum ∆A at a specific time after the excitation of molecules C, which is obtained by subtracting the ground state absorption spectrum (A_C) from the absorption spectrum (A_{C + C*}) at a specific time after excitation. (d) By collecting transient absorption spectra (∆A) at various times (t) after the excitation of molecules C, the transient evolution process of the excited state of molecules C and the relaxation kinetics at a specific wavelength can be determined.

Through the above explanation, we have learned the basic principles behind the generation and detection of transient absorption spectra for molecule C. However, in practical applications, in addition to the previously discussed ground-state bleach (GSB) signal, transient absorption spectra also contain other characteristic signals that reflect the dynamics of the excited state C*.

For example, the excited-state molecule C* can undergo new photon absorption processes, known as excited-state absorption (ESA).

As shown in Figure 2a, the S₁ state can absorb additional photon energy to transition to a higher excited state (Sₙ). Therefore, after the sample is excited, the probe light can detect new excited-state absorption signals. These ESA signals appear as positive signals in transient absorption spectra (Figure 2b). Since the energy levels of excited-state transitions are often distributed across a wide range, the resulting ESA signal typically spans a broad wavelength range (Figure 2b).

It is important to note that the spectral range and intensity of ESA can vary greatly among different molecules. If the ESA overlaps with the GSB signal, the resulting transient absorption spectrum may appear as shown in Figure 2c. The excited-state decay process illustrated in Figure 1b corresponds to the temporal evolution of the transient spectra shown in Figure 2d, where both the ground-state bleach and the excited-state absorption signals decrease over time.

Since both GSB and ESA signals originate from the same excited state, we may observe an isosbestic point in the transient spectrum of Figure 2d—a wavelength at which the absorbance remains constant over time. This indicates that the changes in absorption on either side of this point are due to the same photophysical process—namely, the decay of the excited state.

Figure 2. (a) Excited state absorption process of molecules C. (b) Transient absorption spectroscopy signal resulting from the excited state absorption of molecules C. (c) Transient absorption spectroscopy signal after mixing the excited state absorption signal with the ground state bleach signal. (d) The time evolution of the transient absorption spectrum incorporating excited state absorption and ground state bleach.

02 Molecular Triplet State

Transient absorption spectroscopy can also be used to observe the intersystem crossing (ISC) process between a molecule’s singlet state and triplet state.

Figure 3a illustrates the ISC process, where K₀(S) and K₀(T) represent the decay rate constants of the singlet excited state and the triplet state, respectively. These include all decay pathways—radiative and non-radiative. As long as the molecule has not returned to the ground state, the ground-state bleach (GSB) signal will persist, regardless of whether the molecule is in the singlet or triplet state. Therefore, GSB alone cannot provide information about transitions between excited states.

However, we can extract triplet-state kinetic information via excited-state absorption (ESA) of the triplet state, which follows the same principle as singlet ESA. To use transient absorption spectroscopy to study the triplet state, two conditions must be met:

1. The ISC rate from singlet to triplet must be sufficiently high—comparable to or faster than the decay rate of the singlet state, K₀(S)—otherwise, the triplet state will not form.

2. The ESA signal of the triplet state must fall within the detectable spectral range.

Figure 3b presents a typical transient absorption spectrum for the singlet-to-triplet ISC process. The ESA signal of the triplet state partially overlaps with the GSB signal, and it is assumed that K_ISC ≫ K₀(S).

As the pump-probe delay time (t) increases, we observe the triplet ESA signal forming and gradually intensifying, which reflects the kinetics of the ISC process. If K_ISC ≫ K₀(S), then the GSB signal should remain unchanged until the triplet state decays back to the ground state. However, in Figure 3b, we observe a slight decay and spectral shift in the GSB signal. This is not due to an actual change in the ground-state bleach, but rather caused by the growth of the overlapping positive ESA signal from the triplet state.

This type of signal interference is quite common in transient absorption experiments and must be carefully accounted for during data analysis. Techniques like global fitting or singular value decomposition (SVD) are commonly used to separate and interpret overlapping signals.

As the delay time t continues to increase, both the triplet ESA signal and the GSB signal eventually decay to zero, reflecting the triplet-state decay kinetics, K₀(T). Due to the spin-forbidden nature of triplet decay, this process often occurs on a relatively long timescale.

From the ∆A(t) spectrum, a transient kinetics curve can be extracted at a specific wavelength within the triplet ESA region (see Figure 3b), and kinetic parameters like K_ISC and K₀(T) can be obtained through curve fitting.

Figure 3. (a) Dynamic processes such as intersystem crossing (ISC) and decay between the singlet and triplet states of a molecule. In transient absorption spectroscopy, the dynamics of the singlet and triplet states can be captured through the ground state bleaching and the triplet state's excited state absorption signals. (b) Temporal evolution of the transient absorption spectrum, illustrating the ISC process and the kinetic curve of the triplet state.

03 Photoinduced Electron Transfer

Photoinduced electron transfer is a critically important kinetic process in photoconversion systems and represents a core mechanism in devices such as solar cells, photocatalysts, and photodetectors.

Transient absorption spectroscopy is regarded as one of the most effective techniques for probing photoinduced electron transfer processes, whether occurring between different materials or within a single material.

In this article, we begin by illustrating how transient absorption spectroscopy can be used to study electron transfer processes between different molecular species. Electron or charge transfer processes in other systems—such as semiconductors or semiconductor/molecule hybrid systems—will be discussed in later sections.

Figure 4. The photoinduced electron transfer reaction process between molecules C and D, along with the electronic transitions between the corresponding molecular orbitals. K0(C) represents the sum of radiative and non-radiative decay pathways.

Figure 4 illustrates the process of photoinduced electron transfer between two molecules, C and D. Molecule C serves as the electron donor, and molecule D as the electron acceptor. Upon photoexcitation, molecule C enters an excited state and transfers an electron to molecule D. After the transfer, C and D become the oxidized radical C⁺ and the reduced radical D⁻, respectively.

If no further reactions are triggered, the transferred electron will eventually return to molecule C via back electron transfer (BET), restoring the system to its initial state.

Back electron transfer typically occurs much more slowly than the forward electron transfer, which is desirable in photocatalysis and solar cells. A slower BET allows C⁺ and D⁻ to exist longer, enabling them to participate in other catalytic reactions (as in photocatalysis) or facilitate charge extraction and output (as in solar cells).

In transient absorption spectroscopy, it is important to understand that from a molecular standpoint, C⁺ and D⁻ are distinct species with absorption characteristics different from C and D. Therefore, in molecular systems, the absorption spectra of the donor and acceptor change after electron transfer—unlike in semiconductors (such as quantum dots), where the spectra before and after charge transfer may not significantly differ. The absorption spectra of the C⁺ and D⁻ radicals can be determined by electrochemical methods combined with steady-state absorption or via transient absorption spectroscopy in systems where charge transfer occurs.

We illustrate two scenarios to show how transient absorption spectroscopy detects photoinduced electron transfer:

1) C and D’s steady-state absorption spectra are known and within detection range, but C⁺ and D⁻ spectra are unknown or outside detection range

Figure 5a shows the steady-state absorption spectra of C and D, and the absorption spectrum of the sample at a specific time after C is excited and undergoes electron transfer. After electron transfer, the absorption intensities of C and D decrease—similar to the situation in Figure 1a—because part of the molecules have become C⁺ and D⁻, which have different absorption characteristics.

If the electron transfer rate (K_ET) is much faster than the intrinsic excited-state decay rate of C (K₀(C)), then C* primarily decays through electron transfer. In this case, the temporal evolution of the transient absorption spectrum would appear as in Figure 5b. With increasing pump-probe delay time, the ground-state bleach (GSB) signal of D gradually appears (as D⁻ forms, reducing D population), reflecting the electron transfer kinetics from C to D.

At the same time, the GSB of C appears immediately after excitation but remains unchanged over time (under K_ET ≫ K₀(C)), since C* transitions to C⁺ without returning to the ground state. If C’s excited-state absorption (ESA) is also visible, this signal initially appears after excitation and then decays as C* transitions to C⁺. Thus, the decay of the ESA signal can also be used to track the electron transfer process.

Note: If K_ET is comparable to K₀(C), the GSB of C will decay before BET occurs, and the ESA of C will reflect both electron transfer and intrinsic decay.

As delay time increases further, the back electron transfer process begins to dominate. The GSB signals of both C and D begin to decay and eventually vanish, returning the system to its initial state (Figure 5c).

Figure 5d presents kinetic curves extracted from various spectral features in the transient absorption spectrum (tracking both forward and backward electron transfer).

• Under K_ET ≫ K₀(C), C’s ESA decay reflects K_ET (since ESA disappears as C becomes C⁺).

• D’s GSB kinetics reflect both K_ET and K_BET.

• C’s GSB recovery reflects K_BET.

2) C’s steady-state absorption spectrum is known and within detection range, but C⁺’s spectrum is unknown or outside range; D’s spectrum is unknown or undetectable, but D⁻’s spectrum is known and within detection range

Figure 6a shows the absorption spectra of C and D⁻. Figure 6b shows the time-resolved transient absorption spectra.

After excitation of C, the D⁻ absorption signal appears and increases with delay time, indicating electron transfer from C to D. Meanwhile, C’s GSB does not decay with time under K_ET ≫ K₀(C), but C’s ESA decays due to conversion to C⁺.

As delay time continues to increase, the BET process begins to dominate. Signals from C and D⁻ decay, and the system returns to its ground state.

Figure 5. (a) Steady-state absorption spectra of molecules C and D, alongside the transient absorption spectrum of molecule C at a certain moment following the electron transfer from C to D upon excitation. (b) Transient absorption spectra at various delay times, demonstrating the photoinduced electron transfer process between C and D. (c) Transient absorption spectra at various delay times, showing the reverse electron transfer process. (d) Kinetic curves extracted at different positions of the transient absorption characteristic spectra.

Figure 6. (a) Steady-state absorption spectra of molecules C and D⁻. (b) The corresponding transient absorption time-evolution spectra reveal the photoinduced electron transfer process from molecules C to molecules D.

Similarly, if the absorption spectrum of the C⁺ molecule is known, we can also observe the growth of its absorption signal in the transient absorption spectrum as photoinduced electron transfer occurs.

3) In the third scenario, only the steady-state absorption peak of the electron donor molecule C is known, while the spectral information of C⁺, D, and D⁻ is unknown and outside the spectral detection range

In this case, we can determine whether electron transfer has occurred by comparing the excited-state absorption (ESA) signal of molecule C under conditions with and without electron transfer.

As shown in Figure 7, under electron transfer conditions, the decay of C's ESA signal becomes faster. This is because the excited-state decay process now includes not only the intrinsic decay rate K₀(C), but also an additional decay pathway from electron transfer (K_ET).

Figure 7b shows the kinetic curves of C's ESA signal under both conditions. By comparing these two decay profiles, the rate constant of electron transfer (KET) can be calculated.

Figure 7. (a) The photoinduced electron transfer accelerates the decay of the excited state absorption signal of the donor molecule C. (b) Kinetic curves of the excited state absorption signal, with and without electron transfer. The kinetic rate of electron transfer can be obtained by comparing their kinetic parameters.

Figure 8. (a) The steady-state absorption spectra of the energy donor molecules C and the energy acceptor molecules D in the photoinduced energy transfer, along with the energy transfer reaction process. (b) Time-evolution spectra of the transient absorption of molecules C and D during the energy transfer process.

04 Photoinduced Energy Transfer

Transient absorption spectroscopy can also be used to effectively detect photoinduced energy transfer processes between molecules.

For example, Figure 8a shows the steady-state absorption spectra of the donor molecule C and the acceptor molecule D. After photoexcitation, molecule C enters an excited state. The excitation energy is then transferred to molecule D via energy transfer, causing D to be excited to an excited state, while molecule C returns to the ground state.

This energy transfer process is reflected in the transient absorption spectrum as shown in Figure 8b:

• The ground-state bleach (GSB) signal of molecule C decays after excitation,

• While the GSB signal of molecule D gradually appears, indicating that D has become excited.

By extracting kinetic curves from the transient spectra, the energy transfer rate constant can be determined.

As we can see, unlike photoinduced electron transfer, the GSB signal of molecule C decays during the energy transfer process, because the excited state of C returns to the ground state as a result of energy transfer.

Note that in Figure 8b, the excited-state absorption (ESA) signals of C and D are not shown. If observable, their kinetic behavior during energy transfer would mirror the evolution of their corresponding GSB signals.

05 Stimulated Emission

Transient absorption spectroscopy can also be used to detect the stimulated emission (SE) process of molecules.

The SE process arises from coherence between molecular fluorescence emission and the probe light within a resonant spectral range (as illustrated in Figure 10a). Specifically, when the probe light arrives, some molecules in the S₁ excited state interact with the probe photons and emit stimulated light.

The spectral position of the SE signal matches that of the sample’s spontaneous fluorescence, and thus SE typically appears on the red edge of the ground-state bleach (GSB) signal. In many cases, the SE and GSB signals are located so closely that they partially overlap, as shown in Figure 9b. 

Figure 9. The stimulated emission process of molecules and the corresponding transient absorption spectra

The stimulated emission (SE) signal in transient absorption spectroscopy appears as a bleach-like (negative) signal, because after SE occurs at the fluorescence emission wavelengths, the intensity of the probe beam reaching the detector increases due to the addition of stimulated emission:

I_1-pump + ISE > I_1-unpump

As a result, the change in absorbance is calculated as:

This yields a negative ∆A value, which resembles a ground-state bleach (GSB) signal.

However, it is important to note that the SE signal reflects the population of molecules in the S₁ excited state, whereas GSB reflects the depletion of ground-state molecules. For example, during an S₁-state electron transfer process, the SE signal rapidly decays (as the S₁ population decreases), while the GSB signal remains unchanged (since ground-state molecules have not yet recovered).

06 Conclusion

A summary of the major excited-state kinetic processes in molecular systems and their corresponding transient spectral signals is shown in Figure 10. As illustrated, transient absorption spectroscopy is capable of detecting most key excited-state processes in molecular systems. However, these various kinetic signals often overlap and intertwine, which presents challenges in interpreting both the transient spectra and kinetic data.

In actual experiments, careful analysis is required. Techniques such as global fitting, kinetic modeling, and control experiments can be employed to assign and confirm the origins of different transient signals.

Figure 10. The main excited-state dynamic processes and their corresponding spectral characteristics that can be detected by transient absorption spectroscopy of molecular systems. The arrow in the right figure indicates the possible dynamic process of the spectral signal.

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