Bright Solvatochromic Fluorophores With Dual Emission Enabled by a Donor‐Engineering Strategy

ABSTRACT

Single‐component dual‐emission fluorophores have recently attracted considerable attention. However, achieving dual emission from two independent emissive states within a simple molecular framework remains challenging. Herein, we report bright dual‐emission fluorophores arising from the coexistence of a locally excited (LE) state and a planar intramolecular charge‐transfer (PICT) state, and propose a donor‐engineering design principle that enables this functionality through straightforward structural modification. Specifically, we screened tertiary amines for installation on the donor unit of highly emissive donor–acceptor‐type solvatochromic fluorophores based on a 9,9‐dimethyl‐2‐phenyl‐9H‐fluorene scaffold, with the aim of enabling two bright emissive states to coexist. As a result, the azetidine derivative [4] and the azepane derivative [7] exhibited clear dual emission with high fluorescence quantum yields (> 0.7) over a wide range of solvents. Notably, only one of the two emission bands displayed pronounced fluorescence solvatochromism, as evidenced by substantial shifts in its peak position and distinct lifetime components. Accordingly, these fluorophores are applicable to ratiometric polarity sensing using the intensity ratio of the two emission bands. Overall, our donor‐engineering strategy provides a promising approach to the molecular design of bright dual‐emission solvatochromic fluorophores.

We screened tertiary amines for installation on the donor unit of highly emissive donor–acceptor‐type solvatochromic fluorophores based on a 9,9‐dimethyl‐2‐phenyl‐9H‐fluorene scaffold, with the aim of enabling two bright emissive states to coexist. As a result, the azetidine derivative [4] and the azepane derivative [7] exhibited clear dual emission with high fluorescence quantum yields (> 0.7) over a wide range of solvents.

1. Introduction

Dual‐emission materials have found applications across a wide range of fields—including ratiometric chemical sensing or bioimaging [1, 2, 3, 4, 5, 6, 7, 8], information recording or storage [9, 10, 11, 12, 13, 14], and white OLEDs [15, 16]—by virtue of their multiple emission signals. In particular, in fluorescence imaging and sensing, ratiometric analyses based on the intensity ratio or peak‐position differences of dual emission have attracted considerable attention because they mitigate the influence of fluctuations in excitation intensity and background signals (e.g., autofluorescence) from the sample, thereby enabling high‐accuracy measurements [1, 2, 3, 4, 5, 6, 7, 8]. However, most conventional organic fluorophores obey Kasha's rule and exhibit only a single emission signal from the lowest excited singlet (S₁) [16, 17]. Consequently, many applications that require multiple emission signals have relied on strategies that combine different emitters. Such multicomponent systems, however, can suffer from differences in the photophysical properties of each unit and their compatibility with the medium, which may distort the emission color and/or imaging readouts and thereby severely compromise the accuracy of fluorescence detection and quantitative analysis [15, 18]. To overcome these limitations, single‐component organic fluorophores that exhibit dual emission have garnered significant attention in recent years [17, 18]. In particular, dual‐emission materials based on a single fluorophore can exhibit more sophisticated functions by tuning the intensity balance and the ratio between the two emission bands, thereby enabling advanced applications such as real‐time emission‐color switching, selective sensing, and display functions [18]. Indeed, such materials have been applied to white OLEDs [19, 20], ratiometric fluorescence sensing or imaging [21, 22, 23, 24], and information encryption or anti‐counterfeiting [25, 26]. Numerous single‐component dual‐emission fluorophores have been reported based on diverse photophysical mechanisms, including monomer‐excimer dual‐emission [27, 28, 29], non‐Kasha emission [30, 31], simultaneous fluorescence and phosphorescence [32, 33, 34, 35, 36, 37, 38], excited‐state conjugation enhancement (ESCE) [39, 40, 41, 42, 43, 44, 45], excited‐state intramolecular proton transfer (ESIPT) [46, 47, 48, 49, 50, 51, 52], and twisted intramolecular charge transfer (TICT) [53, 54, 55, 56, 57, 58], as well as other mechanisms that are not yet clearly classified [59, 60]. In these systems, dual emission generally arises from two distinct emissive species (or states) in the excited‐state manifold. However, when one emissive channel is intrinsically inefficient or when excited‐state energy is efficiently dissipated through thermally activated nonradiative deactivation [35, 44, 61, 62, 63], it becomes difficult to clearly resolve multiple emission signals. Such limitations restrict applications that exploit dual‐emission characteristics, such as high‐accuracy sensing and multicolor imaging. Therefore, there is a strong demand for single‐component dual‐emission fluorophores that maintain high emission efficiency. Donor–acceptor‐type π‐conjugated molecules can undergo twisted intramolecular charge transfer (TICT) in the excited state as a consequence of partial conformational twisting [53, 63, 64]. Because two emissive states can be involved—namely, a locally excited (LE) state, in which the excitation is localized on a specific part of the molecule prior to TICT formation, and a TICT state—TICT‐type fluorophores often exhibit dual emission. However, in the TICT state, the reduced spatial overlap between the HOMO and LUMO lowers the oscillator strength, and fluorescence from the TICT state is therefore typically weak [53, 63, 64]. In recent years, donor‐engineering strategies that modify the tertiary amine group on the donor unit of donor–acceptor fluorophores have attracted considerable attention [65, 66, 67, 68]. This approach is particularly useful because it can improve the fluorescence quantum yield (Φ _f) of TICT‐type fluorophores through straightforward structural modification of the donor substituent. Accordingly, we explored applying this donor‐engineering strategy to the molecular design of bright dual‐emission fluorophores. However, such modification of the tertiary amino group generally enhances Φ _f by suppressing formation of the TICT state (Figure 1a) [67, 68], which is one of the emissive states responsible for dual emission. Therefore, conventional donor engineering is not well suited to the design of dual‐emission fluorophores that intrinsically require an emissive ICT state. Herein, we propose a donor‐engineering strategy for developing functional dual‐emission fluorophores based on emissive planar ICT (PICT)‐type fluorophores through straightforward structural modification of the donor substituents. Notably, a recent review that classified dual‐emission molecules according to their underlying mechanisms did not discuss PICT‐type dual‐emission fluorophores [17, 18]. Considering what was previously reported, this type of fluorophore generally exhibits weak fluorescence [69, 70, 71]; our approach may provide a new framework for the molecular design of dual‐emission systems.

FIGURE 1.

Donor engineering strategies for (a) TICT‐type fluorophores and (b) PICT‐type fluorophores.

To demonstrate this strategy, we selected a donor–acceptor‐type π‐conjugated molecule, FπCM ([2, 2]; Figure 2), built on a p‐phenylfluorene scaffold [72, 73, 74] that possesses a large transition dipole moment along the molecular long axis, as an emissive PICT‐type fluorophore [75]. FπCM exhibits high Φ _f over a wide range of solvent polarities and shows pronounced fluorescence solvatochromism, with a substantial red shift of its emission band upon increasing solvent polarity [75]. These features make FπCM an ideal design platform for constructing bright dual‐emission fluorophores in which both emission bands can be clearly resolved.

FIGURE 2.

Chemical structures of FπCM derivatives. [2,2] and [6] (enclosed in a blue square) have already been reported by reference [75]. The other compounds (enclosed in a red square) were newly synthesized in this paper.

In this work, we systematically screened tertiary amine groups to be introduced into the donor unit of FπCM derivatives so that an LE state and an emissive ICT state—responsible for dual emission—can coexist on the potential energy surface (PES), thereby validating a new donor‐engineering strategy based on PICT‐type fluorophores (Figure 1b).

2. Results and Discussion

We synthesized a series of FπCM derivatives ([1, 1], [4], [4O,4N], [5], and [7]; Figure 2) to investigate the effect of tertiary amine substituents on dual‐emission behavior, and compared their photophysical properties with those of the previously reported compounds [2, 2] and [6] [75]. The key intermediates, 2‐amino‐7‐bromofluorene derivatives bearing tertiary amine substituents, were prepared by Buchwald–Hartwig amination of 2,6‐dibromofluorene derivatives and subsequently converted into the target compounds via Suzuki–Miyaura coupling. The above synthetic schemes follow the procedures reported in our previous report.⁷⁵ (Figures S8–S10) All newly reported compounds were characterized by ¹H‐NMR, ¹³C‐NMR, and mass spectrometry (see the Supporting Information).

2.1. Absorption Spectra

The maximum absorption wavelength (λ _abs) of the FπCM derivatives in dichloromethane (DCM) depended on the donor substituents ([1, 1] 366 nm; [2, 2] 376 nm; [4] 362 nm; [4O,4N] 357 nm; [5] 374 nm; [6] 357 nm; [7] 376 nm; Figure 3a). In contrast, λ _abs for each compound showed no significant solvent dependence (Figure S1).

FIGURE 3.

(a) Absorption spectra of FπCM derivatives in diluted dichloromethane solution (1 × 10⁻⁵ M), (b) relative frontier orbital energies of FπCM derivatives to those of [1,1], (c) definition of atom numbers for structure parameter and frontier orbital of [2,2], calculated by ωB97X‐D/TD‐ωB97X‐D/cc‐pVDZ in vacuo.

To elucidate how the tertiary amine structure influences the absorption spectra, we performed DFT calculations to optimize the ground‐state geometries and to compute the S₀→S₁ transitions associated with absorption. For all FπCM derivatives, the S₀→S₁ transition was composed of three major orbital excitations (HOMO−1→LUMO, HOMO→LUMO, and HOMO→LUMO+1) with comparable contributions (Table S3). Moreover, the calculated λ _abs values reproduced the experimental trend well (Table S3), indicating that the observed variation in λ _abs can be rationalized primarily by differences in the optimized ground‐state geometries. The structural differences were most evident in the dihedral angles DA (C₁–N–C₃–C₄ and C₂–N–C₃–C₄), the bond length BL (N–C₃), and the bond angle BA (C₁–N–C₂) (Table 1 and S2). For [1,1], [2,2], [5], and [7], these parameters were similar; accordingly, the energies of the orbitals involved in the S₀→S₁ transition (HOMO−1, HOMO, LUMO, and LUMO+1) were essentially comparable within 0.05 eV, except for [1,1], which shows a weaker hyperconjugation effect from the dialkyl substituents (Figure 3b). In contrast, [4], [4O,4N], and [6] adopt relatively twisted conformations between the tertiary amine donor and the fluorene core (Table 1). This torsion hampers electronic delocalization and lowers the energy of the HOMO localized on the donor unit (Figures 3b, c). As a result, these compounds exhibit a hypsochromic shift of the λ _abs relative to [1,1], [2,2], [5], and [7] (Figure 3a).

TABLE 1.

Calculated dihedral angle (DA) (degree), bond length (BL) (Å), and bond angle (BA) (degree) of FπCM derivatives at ground state geometries in vacuo.

	DA (C1‐N‐C3‐C4)	DA (C2‐N‐C3‐C4)	BL (N‐C3)	BA (C1‐N‐C2)
[1, 1]	−9.40	−170.39	1.385	118.60
[2, 2]	6.92	176.33	1.386	117.71
[4]	−27.20	−153.41	1.381	93.39
[4O,4N]	−28.00	−152.46	1.384	93.10
[5]	−5.92	173.18	1.373	112.80
[6]	57.60	−165.82	1.410	112.23
[7]	8.67	164.49	1.388	114.74

2.2. Steady‐State Fluorescence Spectra

Compounds [4] and [7] exhibited dual emission in solutions of tetrahydrofuran (THF), dichloromethane (DCM), acetonitrile (MeCN), dimethyl sulfoxide (DMSO), and ethanol (EtOH). Of the two emission peaks, the short‐wavelength peak (LE band) was essentially solvent‐independent, whereas the long‐wavelength peak (ICT band) red‐shifted with increasing solvent polarity (Figure 4). Lippert–Mataga plots for the ICT band, correlating the solvent polarity parameter Δf with the Stokes shift (ν _abs‐ν _fl), showed good linearity with high R ² values for both compounds (Figure S2). The change in dipole moments between the ground state and the excited state, estimated from the slopes of these plots were large (≈ 19 D). Moreover, the intensity ratio of the ICT band (λ _obs ≈ 600 nm, where λ _obs denotes the observed wavelength) to the LE band (λ _obs ≈ 450 nm), I _ICT/I _LE, decreased with increasing medium polarity. These results support assignment of the LE and ICT bands to distinct emissive species (an LE state and an ICT state), indicating that [4] and [7] display dual emission originating from these two states. In MeCN, the excitation spectra monitored at the LE and ICT bands of [7] overlapped well with the corresponding absorption spectrum, except for a shoulder near 320 nm observed for the LE band (Figure S3). This observation indicates that both the LE and ICT states are generated from the same Franck–Condon (FC) state. Nevertheless, the fluorescence spectrum of [7] in MeCN depended on the excitation wavelength (Figure S4). The excitation wavelengths at which I _ICT/I _LE were minimized (310 and 320 nm) coincided with the shoulder observed only in the LE‐band excitation spectrum, suggesting that absorption at ∼310–320 nm populates the LE state more efficiently than the ICT state. At first glance, this appears inconsistent with the conclusion that both emissive states originate from the same FC state.

FIGURE 4.

Fluorescence spectra of FπCM derivatives in various solvents. Excitation wavelengths are the λ _abs.

However, theoretical calculations indicate that, in this wavelength region, a weakly allowed S₀→S₂ transition localized on the fluorene core occurs with low oscillator strength (f < 0.05). Thus, if one considers contributions from absorption via the S₀→S₂ transition—which is not prominent in the absorption spectrum—the above interpretation remains self‐consistent. Because the S₂ state has a low D _CT parameter ( < 1 Å) (reflecting a low degree of ICT) [76] and therefore electronic characteristics similar to those of the solvent‐insensitive LE state, it may efficiently generate the LE state. In contrast to the dual‐emissive [4] and [7] compounds [1,1], [4O,4N], and [5] exhibited only the ICT band, similar to the previously reported [2,2] and [6] [75]. In MeCN, the fluorescence maxima (λ _fl) of these compounds were comparable to those of the ICT band of [4] and [7] ([1,1] 580 nm; [4] 585 nm; [4O,4N] 575 nm; [5] 581 nm; [7] 575 nm). Therefore, it is difficult to rationalize the unique dual‐emission behavior of [4] and [7] solely in terms of the electronic nature or energy level of the ICT state in the FπCM derivatives.

No clear correlation was observed between solvent polarity and the intensity ratio of the ICT band to the LE band (IICT/ILE) for compounds [4] and [7] in DCM and acetonitrile. This apparent inconsistency is likely due to significant spectral overlap between the LE and ICT bands, particularly in DCM. To clarify this relationship, fluorescence spectra of [4] and [7] were additionally measured in 1‐butanol (BuOH). As shown in Figure 5, the IICT/ILE value in ethanol (EtOH) is lower than that in BuOH. Given that EtOH (ε = 24.3 at 25°C) is more polar than BuOH (ε = 17.8 at 25°C), while their hydrogen‐bond donor abilities are nearly identical (Kamlet–Taft parameter α: BuOH = 0.84, EtOH = 0.86), these results suggest that the IICT/ILE ratio decreases with increasing solvent polarity, rather than being governed by hydrogen‐bond donor ability.

FIGURE 5.

Fluorescence spectra of [4] and [7] in 1‐butanol (BuOH) and ethanol (EtOH).

2.3. Excited State Dynamics

To elucidate the excited‐state dynamics of the FπCM derivatives in detail, absolute fluorescence quantum yield measurements using an integrating sphere and time‐resolved fluorescence measurements were performed (Table 2). The newly synthesized FπCM derivatives exhibited low Φ _f values (0.12–0.28) in EtOH, a highly polar protic solvent, similar to the previously reported derivatives [2, 2] and [6] (Table S1). In contrast, in solvents other than EtOH, these fluorophores showed high Φ _f values exceeding 0.7 (Table 2). This behavior indicates that the ICT state of these fluorophores corresponds to a bright emissive PICT state rather than a non‐emissive (dark) TICT state.

TABLE 2.

Fluorescence quantum yield (Φ _f) and ICT lifetime (τ) for FπCM derivatives in various solvents at 20°C.

	[1, 1]			[4]			[4O,4N]			[5]			[7]
Solvent	Φ f/−	τ/ns	χ 2/−	Φ f/−	τ/ns	χ 2/−	Φ f/−	τ/ns	χ 2/−	Φ f/−	τ/ns	χ 2/−	Φ f/−	τ/ns	χ 2/−
Toluene	0.92	1.2, 1.8	1.2	0.99	0.90, 1.7	1.2	0.99	1.5	1.1	0.99	1.6	1.3	0.88	1.3, 2.0	0.9
THF	0.89	2.2	1.2	0.99	1.5, 2.4	1.2	0.99	2.1	1.2	0.94	2.2	1.2	0.87	1.3, 2.3	1.1
DCM	0.94	2.7	1.4	0.97	1.5, 2.8	1.1	0.99	2.5	1.2	0.98	2.7	1.4	0.78	1.3, 2.7	1.1
Actonitrile	0.82	2.9	1.4	0.74	1.5, 3.3	1.1	0.86	3.0	1.2	0.80	3.1	1.3	0.70	1.3, 3.0	1.2
DMSO	0.77	3.0	1.1	0.79	1.5, 2.4	1.1	0.92	3.0	1.2	0.76	3.0	1.3	0.84	1.3, 3.2	1.1
Ethanol	0.18	0.45, 3.7	1.1	0.24	0.39, 1.4	0.9	0.13	0.50	1.0	0.12	0.40 3.2	1.0	0.28	0.50, 1.3	1.0

Note: χ ², accuracy indicator for fluorescence lifetime fitting.

Time‐resolved fluorescence decay profiles R(t) were obtained by time‐correlated single‐photon counting, and the fluorescence lifetime components τ ₁ and τ ₂, together with their corresponding pre‐exponential factors B ₁ and B ₂, were determined by fitting the decay curves using the following equation:

$$R\left(t\right)=B_1\exp \left(-\frac{t}{{\tau }_1}\right)+B_2\exp \left(-\frac{t}{{\tau }_2}\right)$$

For [1,1], [4O,4N], and [5], which exhibit only a single ICT emission band, the decay profiles were well fitted with a single fluorescence lifetime τ in THF, DCM, MeCN, and DMSO (χ ² = 1.1–1.4; Table 2). As commonly observed for push–pull‐type solvatochromic fluorophores [77, 78, 79], the ICT lifetime τ increased with increasing solvent polarity and was dramatically shortened ( < 0.50 ns) in EtOH. In contrast, the decay profiles R(t) monitored at λ _obs = 600 nm for [4] and [7], which exhibit dual emission, were best fitted with two lifetime components, τ ₁ and τ ₂, in THF, DCM, MeCN, and DMSO (χ ² = 1.1–1.2; Table 2). Of these two components, the short‐lived component τ ₁ ([4], 1.5 ns; [7], 1.3 ns) was essentially solvent‐independent, whereas the long‐lived component τ ₂ was comparable to the single lifetime τ observed for the other FπCM derivatives. In MeCN, the decay profiles monitored at the LE band (λ_obs = 450 nm) and the ICT band (λ_obs = 600 nm) for [4] and [7] yielded identical lifetime components (τ₁ and τ₂; [4], 1.5 and 3.3 ns; [7], 1.3 and 3.0 ns). The relative contribution factors B₁ and B₂ strongly depended on the observation wavelength ([4], LE, B₁ = 0.1137, B₂ = 0.0027; ICT, B₁ = 0.0110, B₂ = 0.0819; [7], LE, B₁ = 0.1109, B₂ = 0.0011; ICT, B₁ = 0.0111, B₂ = 0.0798).

The relative contribution factors B ₁ and B ₂ strongly depended on the observation wavelength ([4], LE, B ₁ = 0.1137, B ₂ = 0.0027; ICT, B ₁ = 0.0110, B ₂ = 0.0819; [7], LE, B ₁, 0.1109, B ₂, 0.0011; ICT, B ₁, 0.0111, B ₂, 0.0798). These results indicate that the observed fluorescence arises from two distinct emissive states contributing differently to the LE and ICT bands. (Figures S6 and S7)

To further investigate the internal conversion process between the LE and ICT states, temperature‐dependent fluorescence measurements were carried out. However, in MeCN, the dual‐emission intensity ratio (I _ICT/I _LE) of [7] showed no significant temperature dependence over the range from −40 to 50°C (Figure S5). Consequently, no direct insight into the LE–ICT internal conversion process could be obtained, in contrast to well‐known TICT‐type dual‐emission fluorophores such as dimethylaminobenzonitrile [53] and monoaryl‐o‐carborane [80, 81].

2.4. Theoretical Study of Excited State Dynamics

To gain theoretical insight into the excited‐state dynamics, excited‐state geometry optimizations and scan calculations along reaction coordinates around the S₁ minimum (S_1min) were carried out for [4] and [4O,4N] in MeCN solution within the linear‐response SMD (LR‐SMD) solvation framework. For single‐point energy calculations, a corrected linear‐response (cLR) solvation model that accounts for strong dipole–dipole interactions between the solute and solvent was employed [82]. The optimized excited‐state structures (S_1min) of [4] and [4O,4N] exhibited planar conformations, in marked contrast to the twisted ground‐state minima (S_0min) (Table 1 and Figure 6). Moreover, the electronic structure at S_1min shows pronounced charge separation (D _CT > 4.1 Å), allowing S_1min to be assigned to a planar intramolecular charge‐transfer (PICT) state. For [4] and [4O,4N], the PESs in the vicinity of S_1min were calculated as functions of three structural parameters: the N–C₃ bond length and two dihedral angles (C₁–N–C₃–C₄ and C₅–C₆–C₇–C₈). These calculations revealed that the only locally stable structure on the S₁ PES corresponds to the aforementioned PICT state (Figure 6). Accordingly, within the computational approach employed in this study, no local minimum corresponding to an LE state could be identified.

FIGURE 6.

Relative S₁ PESs to S_1min as functions of BA (N‐C₃) and DA (C₁–N–C₃–C₄, C₅–C₆–C₇–C₈) for (a,b) [4] and (c,d) for [4O,4N], calculated by (TD)‐ωB97X‐D/def2‐SVP in acetonitrile solution within cLR/LR‐SMD solvation. Parameter definitions are given in Figure 3c.

2.5. Specific Dual‐Emission of π‐Extended Fluorene Derivatives

Burger et al. systematically investigated the correlation between the chemical structure of tertiary amines at the donor moiety and the photophysical properties of FR0 derivatives [83]—donor–acceptor‐type solvatochromic fluorophores based on a fluorene scaffold originally [84, 85, 86] developed by Klymchenko et al. [87]. Similar to the FπCM derivatives, these FR0 derivatives exhibited high Φ _f over a wide range of solvent polarities, together with pronounced solvatochromic shifts originating from the ICT band. In contrast, the FR0 derivatives did not display dual emission, irrespective of the tertiary amine structure [83]. These results are particularly intriguing when considering that the structural difference between the previously reported FR0 derivatives and the FπCM derivatives that exhibit dual emission upon donor engineering ([4] and [7]) corresponds to only one additional benzene ring in the π‐conjugated framework. To establish a systematic design strategy for bright dual‐emission fluorophores, it will therefore be necessary to identify ICT‐type fluorophores for which donor‐engineering approaches are effective in inducing dual emission, based on previously reported compounds [88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100].

3. 3D Ratiometric Polarity Sensing Utilizing Dual‐Emission

A defining feature of dual‐emission solvatochromic fluorophores is that multiple relative parameters derived from two well‐resolved emission bands—such as fluorescence wavelength ratios and fluorescence lifetime component ratios—exhibit polarity dependence. This feature enables three‐dimensional (3D) ratiometric sensing [21, 24, 101, 102] that goes beyond conventional ratiometric sensing based on a single parameter. Such an approach allows multidimensional self‐calibration, thereby minimizing the influence of environment‐dependent background signals and fluctuations in excitation intensity. Indeed, Li et al. demonstrated that polarity changes in model membranes induced by variations in cholesterol content could be resolved using 3D ratiometric analysis based on dual emission comprising prompt and delayed fluorescence [21]. As shown in Figure 7, under air‐saturated conditions in THF/acetonitrile mixed solvents, excellent linear correlations (R ² > 0.99) were observed between the solvent polarity parameter Δf of [7] and both the emission peak wavelength ratio (λ _ICT/λ _LE) and the fluorescence lifetime ratio of the ICT and LE components (τ _ICT/τ _LE). These results demonstrate that [7] functions as a 3D ratiometric fluorescence sensor capable of quantitatively evaluating polarity using both λ _ICT/λ _LE and τ _ICT/τ _LE as independent readout parameters. The compounds [4] and [7] developed in this study simultaneously exhibit dual‐emission characteristics and high Φ _f. Accordingly, the dual‐emission solvatochromic fluorophores generated via our donor‐engineering strategy are expected to substantially expand the scope of 3D ratiometric fluorescence sensing, particularly by improving signal‐to‐noise (S/N) ratios and suppressing photobleaching of samples.

FIGURE 7.

Correlation of wavelength and lifetime ratio of dual‐emission with polarity for [7]. (a) Fluorescence spectra in THF/acetonitrile mixed solution under ambient conditions (λ _ex, 370 nm). (b) Lifetime profile of ICT bands (λ _obs, 600 nm) in the mixed solution. (c) Correlation between Δf, λ _ICT/λ _LE, and τ _ICT/τ _LE. THF, tetrahydrofuran. The Δf values for the mixed solvent were calculated according to the equation provided in the Optical measurements section of the Supporting Information.

4. Conclusion

In summary, we successfully developed bright dual‐emission fluorophores [4] and [7], originating from the coexistence of locally excited (LE) and planar intramolecular charge‐transfer (PICT) states, by applying a donor‐engineering strategy involving simple modification of the tertiary amine group in donor–acceptor‐type solvatochromic fluorophores based on a 9,9‐dimethyl‐2‐phenyl‐9H‐fluorene scaffold. In these fluorophores, only the longer‐wavelength ICT band exhibits pronounced solvatochromism, with both the emission peak position and fluorescence lifetime components showing strong dependence on solvent polarity. Under air‐saturated conditions in THF/acetonitrile mixed solvents, compound [7] displayed clear linear correlations between multiple ratiometric parameters derived from the LE and ICT bands and the solvent polarity parameter, demonstrating its applicability as a three‐dimensional (3D) ratiometric polarity sensor. Accordingly, our donor‐engineering strategy provides a general molecular design principle for functional dual‐emission fluorophores that simultaneously achieve high fluorescence quantum yields (Φ _f) and compatibility with multidimensional ratiometric analysis, thereby contributing to the future development of environmentally responsive fluorescent probes [103, 104, 105, 106, 107].

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting Information: Additional supporting information can be found online in the Supporting Information section. Additional references cited within the Supporting Information [75, 108, 109, 110, 111]. Details of experimental procedures, characterization data (NMR, UV/Vis, and PL spectra), computational studies, and additional data are provided in the Supporting Information.