3.1. Rheology
Rheology measurements of four fluorescent liquid derivatives were performed by steady shear mode at
(see
Table 1). The measured viscosities show a clear correlation with the length of two alkyl side chains attached to the tetrazine core. The compound with the shortest side chains
turns out to be the least viscous of the studied derivatives with
. Note this value is lower than the reported value in a previous study (
) [
11]. For the longest side chains
, we obtained the highest viscosity, with
. Thus, an increase in the length of alkyl side chains led to an increase in the viscosity, possibly due to an increase in the probability of entanglement. When the symmetry of the arms is broken, i.e., for
or
, the viscosities were found to stand in between symmetric derivatives and not very different from each other,
and
, respectively. The effect of chain length on viscosity was found to be more pronounced when the molecule had symmetry in its arms. A previous study reports a shear thickening effect for low shear rate for the
derivative [
11]. In the present work, we addressed only the Newtonian regime and no information is obtained about a possible shear-thickening effect of fluorescent liquid tetrazines for lower shear rates (Indeed, the measured torque is too low (<40
) for lower shear rates (
).).
3.3. Nanosecond Transient Absorption
Global fitting the transient absorption matrix using three components is enough to obtain an adequate fit (
Table 3,
Figure 2). The spectrum associated with each of the components can be found in the
SI (Figures S8–S11), and the absorption maxima of each component is given in
Table 4.
The faster decay is consistent with the fluorescence lifetime, so this component is assigned to the state. For the dialkoxy series, this lifetime is of 30 ns, and it is of 124 ns for . The longer fluorescence lifetime of is due to its slower rate of intersystem crossing in the chloroalkoxy compounds, which explains their higher fluorescence quantum yield.
The lifetime of the second component is dramatically reduced when oxygen is allowed to enter the solution (
Figure S12, Table S1), so we assign this component to the
state. Moreover, the spectrum is similar to that recently assigned to the triplet state in amino-substituted tetrazines [
29]. This lifetime is affected by the excitation power
Figure S12, Table S1), indicating that this species decays via a triplet-triplet quenching pathway.
The spectrum of the third component, which appears to be formed in small quantities, has an absorption consistent with the absorption spectrum of the tetrazine radical anion, which has been previously experimentally measured for
[
30]. The formation of the tetrazine cation from two triplet molecules would be highly energetically unfavorable (
= 4 eV for
at the
B97XD/cc-pVTZ level of theory), so the source of the electron remains unclear.
3.4. Picosecond Transient Absorption
Ultrafast transient absorption spectra in a time window between 0.1 ps and 3 ns show a single absorption band in the 500 nm to 600 nm region, which is ascribed to the state.
By monitoring the anisotropy decay of the transient absorption (
Figure 3), we obtain insight into the rotational dynamics of the molecule in solution. In the di-alkoxy neat liquid we observe a decay of the anisotropy faster than what one would expect for the rotational decorrelation in the viscous environment that the molecules will experience as a neat liquid (Page 9,
Table 1 and
Table 5).
To obtain a consistent estimate of the molecular volumes of the compounds, we used the CPK volumes calculated using Spartan18 for the optimized geometries. For
, this volume is calculated to be 383 Å
, which agrees well with the molecular volume from the density of the neat liquid (377 Å
) [
11]. The volumes were used to calculate the rotational diffusion time constants predicted by the Stokes–Einstein equation (Equation (
2)) for a sphere of the same volume [
31].
The rotational decorrelation time constants for a perfectly spherical molecule in a solvent with the viscosity of DCM calculated using the Einstein–Stokes equation are 1.8 to 3 times as large as the measured values (
Table 5). As the measured molecules are not spherical, a deviation from the Einstein–Stokes value is expected. The order of magnitude of these rates is, however, what we would expect to see if the main anisotropy decay pathway is the randomization of the orientations via rotational diffusion.
The decay of the anisotropy in the neat films occurs with a similar order of magnitude as the decay in DCM (
Table 5). This is surprising because in the neat liquid the molecules will experience an environment with the viscosity of the neat liquid (
Table 1), and rotational decorrelation is expected to be much slower, as calculated using the Einstein–Stokes equation (
Table 5). An explanation is proposed in the Discussion (
Section 4).
3.5. Computation
In order to explore the electronic structural differences between the dialkoxy and chloro/alkoxy compounds, we investigated the electronic structure of 3,6-dimethoxy s-tetrazine (
) and 3-chloro-6-methoxy s-tetrazine (
) in vacuum. We begin with the results of geometry optimizations and TD-DFT calculations at the
B97XD/cc-pVTZ level of theory. The vertical transition energies at the different minima can be found in the SI (
Tables S2 and S5, and the geometric parameters are tabulated in
Tables S8 and S9).
The coordinate system is defined such that the s-tetrazine core lies in the yz plane, with the CO bond lying along the z-axis. Both model compounds are predicted to have an n-type HOMO and a type LUMO. The first transition corresponds to the state, which is calculated to be at 520 nm for and 530 nm for in agreement with experiment. The transition is only weakly allowed for both compounds because of a small electronic transition dipole moment (ETDM) along the x-axis (f = 0.005).
The strong absorption band near 340 nm corresponds to the band. This is calculated to be the transition of (calc: 303 nm, exp: 350) and the transition for (calc: 284 nm, exp: 330). The ETDM of this transition lies in the ZY plane and has a moderate oscillator strength (f = 0.056).
The ground-state TDDFT vertical excitation calculations are in good agreement with the steady state absorption for the transition, but overestimate the vertical energy of the state.
The vertical transition from the first excited state at the minimum is calculated to be 604 nm (exp: 590) for and 595 nm (exp: 566) for , which corresponds to the measured emission.
The time-resolved absorption measured in our pump-probe experiments can be correlated to the vertical transition energies at the sufficiently long-lived intermediates explored by the molecule during its excited state dynamics. In addition, the time-resolved anisotropy measurements allow us to determine the relative angle between the electronic transition dipole moments of the pumped transition and the probed transitions.
At the TD-DFT level of theory, the transition at the minimum is calculated to occur at 661 nm for and 543 nm for . We do not observe such a large difference in the absorption maxima of the two groups of compounds in our transient absorption spectra. The nature of this transition is , which at the MS-CASPT2 level (see later discussion) is shown to have a low oscillator strength and an ETDM parallel to the ETDM for . The transition is predicted at 488 nm for and at 520 nm for .
The
state is of
character at the
,
, and
minima. At the
minimum, the energy difference between the
and
states is 1.47 eV (TDDFT)/1.37 (CASPT2) for
, a bit smaller than the reported phosphorescence energy [
17]. The vertical transition
is calculated to occur at 716 nm for
and at 930 nm for
. The
state has
character, and the
transition is forbidden for both compounds (f = 0.000). The
transition is predicted at 530 nm (f = 0.051) for
and 558 nm (f = 0.029) for
. The strong excited state absorption that we see in the ns time-resolved experiments is likely due to this transition, observed around 500 nm.
The results at the MS-CASPT2 level paint a similar overall picture (
Tables S3 and S6), but the
state is moved up in energy by about 1 eV. This higher energy breaks the degeneracy at the
crossing optimized at the CASSCF level. The CASPT2 wave functions can also be used to compute the oscillator states and ETDM between the different excited states by using the RAS State Interaction method (RASSI) (
Tables S4 and S7). According to these results, the lowest-energy transition that should result in an anisotropy value of −0.2 are the
for
(at 479 nm) and the
for
(464 nm). In both cases, the state corresponds to a
state, where the
LUMO orbital becomes doubly occupied, and the transition dipole moment is along the z-axis.
To explain why the rate of ISC is greater for the dialkoxy than for the chloro/alkoxy compounds, we first calculated the spin–orbit coupling matrix elements (SOC). The between and is calculated to be 0.2 cm at the minimum for both compounds. On the other hand, we observe a moderate SOC (9.7 cm for , 7.2 cm for ) between the and the state. This is consistent with the El Sayed rules, which predict a strong SOC when the transition involves an electron moving from a to an n orbital, as is the case for the .
Based on the hypothesis that ISC occurs efficiently at geometries near the
/
minimum-energy crossing point (MECP), we located these at the CASSCF(14,10)/ANO-RCC-VDZP level of theory. The energy needed to reach the
crossing is 8.5 kcal/mol for
and 9.7 kcal/mol for
(no solvation model). In addition to the higher energy required for the crossing, we can see that reaching the
MECP requires a expanding and twisting the tetrazine core, while the core of
only has to expand (
Figure 4;
Tables S8 and S9). At the
crossing points there is little difference between the SOC’s for
and
(
Table 6). Therefore, we attribute the more efficient ISC in the dialkoxy compounds compared to the chloro/alkoxy compounds to the relatively higher energy of the crossing point in the latter.